From cca62852590ce7ca267b7d5ad21b5f61c13bac01 Mon Sep 17 00:00:00 2001
From: adash <adash>
Date: Thu, 19 Nov 2009 21:41:34 +0000
Subject: [PATCH] add a chapters for new word repository

---
 .../Distributed/SpamFilter/emails/wordList1   | 2964 +++++++++++++++++
 1 file changed, 2964 insertions(+)
 create mode 100644 Robust/src/Benchmarks/Distributed/SpamFilter/emails/wordList1

diff --git a/Robust/src/Benchmarks/Distributed/SpamFilter/emails/wordList1 b/Robust/src/Benchmarks/Distributed/SpamFilter/emails/wordList1
new file mode 100644
index 00000000..3c318ed2
--- /dev/null
+++ b/Robust/src/Benchmarks/Distributed/SpamFilter/emails/wordList1
@@ -0,0 +1,2964 @@
+INTRODUCTION
+There is abundant evidence of a widened and deepened interest in modern
+science. How could it be otherwise when we think of the magnitude and
+the eventfulness of recent advances?
+But the interest of the general public would be even greater than it is
+if the makers of new knowledge were more willing to expound their
+discoveries in ways that could be "understanded of the people." No one
+objects very much to technicalities in a game or on board a yacht, and
+they are clearly necessary for terse and precise scientific description.
+It is certain, however, that they can be reduced to a minimum without
+sacrificing accuracy, when the object in view is to explain "the gist of
+the matter." So this OUTLINE OF SCIENCE is meant for the general reader,
+    who lacks both time and opportunity for special study, and yet would
+    take an intelligent interest in the progress of science which is making
+    the world always new.
+    The story of the triumphs of modern science is one of which Man may well
+    be proud. Science reads the secret of the distant star and anatomises
+    the atom; foretells the date of the comet's return and predicts the
+    kinds of chickens that will hatch from a dozen eggs; discovers the laws
+    of the wind that bloweth where it listeth and reduces to order the
+    disorder of disease. Science is always setting forth on Columbus
+    voyages, discovering new worlds and conquering them by understanding.
+    For Knowledge means Foresight and Foresight means Power.
+    The idea of Evolution has influenced all the sciences, forcing us to
+    think of _everything_ as with a history behind it, for we have travelled
+    far since Darwin's day. The solar system, the earth, the mountain
+    ranges, and the great deeps, the rocks and crystals, the plants and
+    animals, man himself and his social institutions--all must be seen as
+    the outcome of a long process of Becoming. There are some eighty-odd
+    chemical elements on the earth to-day, and it is now much more than a
+    suggestion that these are the outcome of an inorganic evolution, element
+    giving rise to element, going back and back to some primeval stuff, from
+    which they were all originally derived, infinitely long ago. No idea has
+    been so powerful a tool in the fashioning of New Knowledge as this
+    simple but profound idea of Evolution, that the present is the child of
+    the past and the parent of the future. And with the picture of a
+    continuity of evolution from nebula to social systems comes a promise of
+    an increasing control--a promise that Man will become not only a more
+    accurate student, but a more complete master of his world.
+    It is characteristic of modern science that the whole world is seen to
+    be more vital than before. Everywhere there has been a passage from the
+    static to the dynamic. Thus the new revelations of the constitution of
+    matter, which we owe to the discoveries of men like Professor Sir J. J.
+    Thomson, Professor Sir Ernest Rutherford, and Professor Frederick Soddy,
+    have shown the very dust to have a complexity and an activity heretofore
+    unimagined. Such phrases as "dead" matter and "inert" matter have gone
+    by the board.
+    The new theory of the atom amounts almost to a new conception of the
+    universe. It bids fair to reveal to us many of nature's hidden secrets.
+    The atom is no longer the indivisible particle of matter it was once
+    understood to be. We know now that there is an atom within the
+    atom--that what we thought was elementary can be dissociated and broken
+    up. The present-day theories of the atom and the constitution of matter
+    are the outcome of the comparatively recent discovery of such things as
+    radium, the X-rays, and the wonderful revelations of such instruments as
+    the spectroscope and other highly perfected scientific instruments.
+    The advent of the electron theory has thrown a flood of light on what
+    before was hidden or only dimly guessed at. It has given us a new
+    conception of the framework of the universe. We are beginning to know
+    and realise of what matter is made and what electric phenomena mean. We
+    can glimpse the vast stores of energy locked up in matter. The new
+    knowledge has much to tell us about the origin and phenomena, not only
+    of our own planet, but other planets, of the stars, and the sun. New
+    light is thrown on the source of the sun's heat; we can make more than
+    guesses as to its probable age. The great question to-day is: is there
+    _one_ primordial substance from which all the varying forms of matter
+    have been evolved?
+    But the discovery of electrons is only one of the revolutionary changes
+    which give modern science an entrancing interest.
+    As in chemistry and physics, so in the science of living creatures there
+    have been recent advances that have changed the whole prospect. A good
+    instance is afforded by the discovery of the "hormones," or chemical
+    messengers, which are produced by ductless glands, such as the thyroid,
+    the supra-renal, and the pituitary, and are distributed throughout the
+    body by the blood. The work of physiologists like Professor Starling and
+    Professor Bayliss has shown that these chemical messengers regulate what
+    may be called the "pace" of the body, and bring about that regulated
+    harmony and smoothness of working which we know as health. It is not too
+    much to say that the discovery of hormones has changed the whole of
+    physiology. Our knowledge of the human body far surpasses that of the
+    past generation.
+    The persistent patience of microscopists and technical improvements like
+    the "ultramicroscope" have greatly increased our knowledge of the
+    invisible world of life. To the bacteria of a past generation have been
+    added a multitude of microscopic _animal_ microbes, such as that which
+    causes Sleeping Sickness. The life-histories and the weird ways of many
+    important parasites have been unravelled; and here again knowledge means
+    mastery. To a degree which has almost surpassed expectations there has
+    been a revelation of the intricacy of the stones and mortar of the house
+    of life, and the microscopic study of germ-cells has wonderfully
+    supplemented the epoch-making experimental study of heredity which began
+    with Mendel. It goes without saying that no one can call himself
+    educated who does not understand the central and simple ideas of
+    Mendelism and other new departures in biology.
+    The procession of life through the ages and the factors in the sublime
+    movement; the peopling of the earth by plants and animals and the
+    linking of life to life in subtle inter-relations, such as those between
+    flowers and their insect-visitors; the life-histories of individual
+    types and the extraordinary results of the new inquiry called
+    "experimental embryology"--these also are among the subjects with which
+    this OUTLINE will deal.
+    The behaviour of animals is another fascinating study, leading to a
+    provisional picture of the dawn of mind. Indeed, no branch of science
+    surpasses in interest that which deals with the ways and habits--the
+    truly wonderful devices, adaptations, and instincts--of insects, birds,
+    and mammals. We no longer deny a degree of intelligence to some members
+    of the animal world--even the line between intelligence and reason is
+    sometimes difficult to find.
+    Fresh contacts between physiology and the study of man's mental life;
+    precise studies of the ways of children and wild peoples; and new
+    methods like those of the psycho-analyst must also receive the attention
+    they deserve, for they are giving us a "New Psychology" and the claims
+    of psychical research must also be recognised by the open-minded.
+    The general aim of the OUTLINE is to give the reader a clear and concise
+    view of the essentials of present-day science, so that he may follow
+    with intelligence the modern advance and share appreciatively in man's
+    continued conquest of his kingdom.
+    J. ARTHUR THOMSON.
+    I
+    THE ROMANCE OF THE HEAVENS
+    THE SCALE OF THE UNIVERSE--THE SOLAR SYSTEM
+    The story of the triumphs of modern science naturally opens with
+    Astronomy. The picture of the Universe which the astronomer offers to us
+    is imperfect; the lines he traces are often faint and uncertain. There
+    are many problems which have been solved, there are just as many about
+    which there is doubt, and notwithstanding our great increase in
+    knowledge, there remain just as many which are entirely unsolved.
+    The problem of the structure and duration of the universe [said the
+    great astronomer Simon Newcomb] is the most far-reaching with which
+    the mind has to deal. Its solution may be regarded as the ultimate
+    object of stellar astronomy, the possibility of reaching which has
+    occupied the minds of thinkers since the beginning of civilisation.
+    Before our time the problem could be considered only from the
+    imaginative or the speculative point of view. Although we can to-day
+    attack it to a limited extent by scientific methods, it must be
+    admitted that we have scarcely taken more than the first step toward
+    the actual solution.... What is the duration of the universe in
+    time? Is it fitted to last for ever in its present form, or does it
+    contain within itself the seeds of dissolution? Must it, in the
+    course of time, in we know not how many millions of ages, be
+    transformed into something very different from what it now is? This
+    question is intimately associated with the question whether the
+    stars form a system. If they do, we may suppose that system to be
+    permanent in its general features; if not, we must look further for
+    our conclusions.
+    The Heavenly Bodies
+    The heavenly bodies fall into two very distinct classes so far as their
+    relation to our Earth is concerned; the one class, a very small one,
+    comprises a sort of colony of which the Earth is a member. These bodies
+    are called _planets_, or wanderers. There are eight of them, including
+    the Earth, and they all circle round the sun. Their names, in the order
+    of their distance from the sun, are Mercury, Venus, Earth, Mars,
+    Jupiter, Saturn, Uranus, Neptune, and of these Mercury, the nearest to
+    the sun, is rarely seen by the naked eye. Uranus is practically
+    invisible, and Neptune quite so. These eight planets, together with the
+    sun, constitute, as we have said, a sort of little colony; this colony
+    is called the Solar System.
+    The second class of heavenly bodies are those which lie _outside_ the
+    solar system. Every one of those glittering points we see on a starlit
+    night is at an immensely greater distance from us than is any member of
+    the Solar System. Yet the members of this little colony of ours, judged
+    by terrestrial standards, are at enormous distances from one another. If
+    a shell were shot in a straight line from one side of Neptune's orbit to
+    the other it would take five hundred years to complete its journey. Yet
+    this distance, the greatest in the Solar System as now known (excepting
+        the far swing of some of the comets), is insignificant compared to the
+    distances of the stars. One of the nearest stars to the earth that we
+    know of is Alpha Centauri, estimated to be some twenty-five million
+    millions of miles away. Sirius, the brightest star in the firmament, is
+    double this distance from the earth.
+    We must imagine the colony of planets to which we belong as a compact
+    little family swimming in an immense void. At distances which would take
+    our shell, not hundreds, but millions of years to traverse, we reach
+    the stars--or rather, a star, for the distances between stars are as
+    great as the distance between the nearest of them and our Sun. The
+    Earth, the planet on which we live, is a mighty globe bounded by a crust
+    of rock many miles in thickness; the great volumes of water which we
+    call our oceans lie in the deeper hollows of the crust. Above the
+    surface an ocean of invisible gas, the atmosphere, rises to a height of
+    about three hundred miles, getting thinner and thinner as it ascends.
+    [Illustration: LAPLACE
+    One of the greatest mathematical astronomers of all time and the
+    originator of the nebular theory.]
+    [Illustration: _Photo: Royal Astronomical Society._
+    PROFESSOR J. C. ADAMS
+    who, anticipating the great French mathematician, Le Verrier, discovered
+    the planet Neptune by calculations based on the irregularities of the
+    orbit of Uranus. One of the most dramatic discoveries in the history of
+    Science.]
+    [Illustration: _Photo: Elliott & Fry, Ltd._
+    PROFESSOR EDDINGTON
+    Professor of Astronomy at Cambridge. The most famous of the English
+    disciples of Einstein.]
+    [Illustration: FIG. 1.--DIAGRAMS OF THE SOLAR SYSTEM
+  THE COMPARATIVE DISTANCES OF THE PLANETS
+(Drawn approximately to scale)
+  The isolation of the Solar System is very great. On the above scale the
+  _nearest_ star (at a distance of 25 trillions of miles) would be over
+  _one half mile_ away. The hours, days, and years are the measures of
+  time as we use them; that is: Jupiter's "Day" (one rotation of the
+      planet) is made in ten of _our hours_; Mercury's "Year" (one revolution
+        of the planet around the Sun) is eighty-eight of _our days_. Mercury's
+      "Day" and "Year" are the same. This planet turns always the same side to
+      the Sun.]
+      [Illustration: THE COMPARATIVE SIZES OF THE SUN AND THE PLANETS (Drawn
+          approximately to scale)
+      On this scale the Sun would be 17-1/2 inches in diameter; it is far
+      greater than all the planets put together. Jupiter, in turn, is greater
+      than all the other planets put together.]
+      Except when the winds rise to a high speed, we seem to live in a very
+      tranquil world. At night, when the glare of the sun passes out of our
+      atmosphere, the stars and planets seem to move across the heavens with a
+      stately and solemn slowness. It was one of the first discoveries of
+      modern astronomy that this movement is only apparent. The apparent
+      creeping of the stars across the heavens at night is accounted for by
+      the fact that the earth turns upon its axis once in every twenty-four
+      hours. When we remember the size of the earth we see that this implies a
+      prodigious speed.
+      In addition to this the earth revolves round the sun at a speed of more
+      than a thousand miles a minute. Its path round the sun, year in year
+      out, measures about 580,000,000 miles. The earth is held closely to this
+      path by the gravitational pull of the sun, which has a mass 333,432
+      times that of the earth. If at any moment the sun ceased to exert this
+      pull the earth would instantly fly off into space straight in the
+      direction in which it was moving at the time, that is to say, at a
+      tangent. This tendency to fly off at a tangent is continuous. It is the
+      balance between it and the sun's pull which keeps the earth to her
+      almost circular orbit. In the same way the seven other planets are held
+      to their orbits.
+      Circling round the earth, in the same way as the earth circles round the
+      sun, is our moon. Sometimes the moon passes directly between us and the
+      sun, and cuts off the light from us. We then have a total or partial
+      eclipse of the sun. At other times the earth passes directly between the
+      sun and the moon, and causes an eclipse of the moon. The great ball of
+      the earth naturally trails a mighty shadow across space, and the moon is
+      "eclipsed" when it passes into this.
+      The other seven planets, five of which have moons of their own, circle
+      round the sun as the earth does. The sun's mass is immensely larger than
+      that of all the planets put together, and all of them would be drawn
+      into it and perish if they did not travel rapidly round it in gigantic
+      orbits. So the eight planets, spinning round on their axes, follow their
+      fixed paths round the sun. The planets are secondary bodies, but they
+      are most important, because they are the only globes in which there can
+      be life, as we know life.
+      If we could be transported in some magical way to an immense distance in
+      space above the sun, we should see our Solar System as it is drawn in
+      the accompanying diagram (Fig. 1), except that the planets would be mere
+      specks, faintly visible in the light which they receive from the sun.
+      (This diagram is drawn approximately to scale.) If we moved still
+      farther away, trillions of miles away, the planets would fade entirely
+      out of view, and the sun would shrink into a point of fire, a star. And
+      here you begin to realize the nature of the universe. _The sun is a
+      star. The stars are suns._ Our sun looks big simply because of its
+      comparative nearness to us. The universe is a stupendous collection of
+      millions of stars or suns, many of which may have planetary families
+      like ours.
+      The Scale of the Universe
+      How many stars are there? A glance at a photograph of star-clouds will
+      tell at once that it is quite impossible to count them. The fine
+      photograph reproduced in Figure 2 represents a very small patch of that
+      pale-white belt, the Milky Way, which spans the sky at night. It is true
+      that this is a particularly rich area of the Milky Way, but the entire
+      belt of light has been resolved in this way into masses or clouds of
+      stars. Astronomers have counted the stars in typical districts here and
+      there, and from these partial counts we get some idea of the total
+      number of stars. There are estimated to be between two and three
+      thousand million stars.
+      Yet these stars are separated by inconceivable distances from each
+      other, and it is one of the greatest triumphs of modern astronomy to
+      have mastered, so far, the scale of the universe. For several centuries
+      astronomers have known the relative distances from each other of the sun
+      and the planets. If they could discover the actual distance of any one
+      planet from any other, they could at once tell all the distances within
+      the Solar System.
+      The sun is, on the latest measurements, at an average distance of
+      92,830,000 miles from the earth, for as the orbit of the earth is not a
+      true circle, this distance varies. This means that in six months from
+      now the earth will be right at the opposite side of its path round the
+      sun, or 185,000,000 miles away from where it is now. Viewed or
+      photographed from two positions so wide apart, the nearest stars show a
+      tiny "shift" against the background of the most distant stars, and that
+      is enough for the mathematician. He can calculate the distance of any
+      star near enough to show this "shift." We have found that the nearest
+      star to the earth, a recently discovered star, is twenty-five trillion
+      miles away. Only thirty stars are known to be within a hundred trillion
+      miles of us.
+      This way of measuring does not, however, take us very far away in the
+      heavens. There are only a few hundred stars within five hundred trillion
+      miles of the earth, and at that distance the "shift" of a star against
+      the background (parallax, the astronomer calls it) is so minute that
+      figures are very uncertain. At this point the astronomer takes up a new
+      method. He learns the different types of stars, and then he is able to
+      deduce more or less accurately the distance of a star of a known type
+      from its faintness. He, of course, has instruments for gauging their
+      light. As a result of twenty years work in this field, it is now known
+      that the more distant stars of the Milky Way are at least a hundred
+      thousand trillion (100,000,000,000,000,000) miles away from the sun.
+      Our sun is in a more or less central region of the universe, or a few
+      hundred trillion miles from the actual centre. The remainder of the
+      stars, which are all outside our Solar System, are spread out,
+      apparently, in an enormous disc-like collection, so vast that even a ray
+      of light, which travels at the rate of 186,000 miles a second, would
+      take 50,000 years to travel from one end of it to the other. This, then
+      is what we call our universe.
+      Are there other Universes?
+      Why do we say "our universe"? Why not _the_ universe? It is now believed
+      by many of our most distinguished astronomers that our colossal family
+      of stars is only one of many universes. By a universe an astronomer
+      means any collection of stars which are close enough to control each
+      other's movements by gravitation; and it is clear that there might be
+      many universes, in this sense, separated from each other by profound
+      abysses of space. Probably there are.
+      For a long time we have been familiar with certain strange objects in
+      the heavens which are called "spiral nebulæ" (Fig 4). We shall see at a
+      later stage what a nebula is, and we shall see that some astronomers
+      regard these spiral nebulæ as worlds "in the making." But some of the
+      most eminent astronomers believe that they are separate
+      universes--"island-universes" they call them--or great collections of
+      millions of stars like our universe. There are certain peculiarities in
+      the structure of the Milky Way which lead these astronomers to think
+      that our universe may be a spiral nebula, and that the other spiral
+      nebulæ are "other universes."
+      [Illustration: _Photo: Harvard College Observatory._
+      FIG. 2.--THE MILKY WAY
+      Note the cloud-like effect.]
+      [Illustration: FIG. 3--THE MOON ENTERING THE SHADOW CAST BY THE EARTH
+      The diagram shows the Moon partially eclipsed.]
+      [Illustration: _From a photograph taken at the Yerkes Observatory_
+      FIG. 4.--THE GREAT NEBULA IN ANDROMEDA, MESSIER 31]
+      Vast as is the Solar System, then, it is excessively minute in
+      comparison with the Stellar System, the universe of the Stars, which is
+      on a scale far transcending anything the human mind can apprehend.
+      THE SOLAR SYSTEM
+      THE SUN
+      But now let us turn to the Solar System, and consider the members of our
+      own little colony.
+      Within the Solar System there are a large number of problems that
+      interest us. What is the size, mass, and distance of each of the
+      planets? What satellites, like our Moon, do they possess? What are their
+      temperatures? And those other, sporadic members of our system, comets
+      and meteors, what are they? What are their movements? How do they
+      originate? And the Sun itself, what is its composition, what is the
+      source of its heat, how did it originate? Is it running down?
+      These last questions introduce us to a branch of astronomy which is
+      concerned with the physical constitution of the stars, a study which,
+      not so very many years ago, may well have appeared inconceivable. But
+      the spectroscope enables us to answer even these questions, and the
+      answer opens up questions of yet greater interest. We find that the
+      stars can be arranged in an order of development--that there are stars
+      at all stages of their life-history. The main lines of the evolution of
+      the stellar universe can be worked out. In the sun and stars we have
+      furnaces with temperatures enormously high; it is in such conditions
+      that substances are resolved into their simplest forms, and it is thus
+      we are enabled to obtain a knowledge of the most primitive forms of
+      matter. It is in this direction that the spectroscope (which we shall
+          refer to immediately) has helped us so much. It is to this wonderful
+      instrument that we owe our knowledge of the composition of the sun and
+      stars, as we shall see.
+      "That the spectroscope will detect the millionth of a milligram of
+      matter, and on that account has discovered new elements, commands
+      our admiration; but when we find in addition that it will detect the
+      nature of forms of matter trillions of miles away, and moreover,
+      that it will measure the velocities with which these forms of matter
+      are moving with an absurdly small per cent. of possible error, we
+      can easily acquiesce in the statement that it is the greatest
+      instrument ever devised by the brain and hand of man."
+      Such are some of the questions with which modern astronomy deals. To
+      answer them requires the employment of instruments of almost incredible
+      refinement and exactitude and also the full resources of mathematical
+      genius. Whether astronomy be judged from the point of view of the
+      phenomena studied, the vast masses, the immense distances, the æons of
+      time, or whether it be judged as a monument of human ingenuity,
+      patience, and the rarest type of genius, it is certainly one of the
+      grandest, as it is also one of the oldest, of the sciences.
+      The Solar System
+      In the Solar System we include all those bodies dependent on the sun
+      which circulate round it at various distances, deriving their light and
+      heat from the sun--the planets and their moons, certain comets and a
+      multitude of meteors: in other words, all bodies whose movements in
+      space are determined by the gravitational pull of the sun.
+      The Sun
+      Thanks to our wonderful modern instruments and the ingenious methods
+      used by astronomers, we have to-day a remarkable knowledge of the sun.
+      Look at the figure of the sun in the frontispiece. The picture
+      represents an eclipse of the sun; the dark body of the moon has screened
+      the sun's shining disc and taken the glare out of our eyes; we see a
+      silvery halo surrounding the great orb on every side. It is the sun's
+      atmosphere, or "crown" (corona), stretching for millions of miles into
+      space in the form of a soft silvery-looking light; probably much of its
+      light is sunlight reflected from particles of dust, although the
+      spectroscope shows an element in the corona that has not so far been
+      detected anywhere else in the universe and which in consequence has been
+      named Coronium.
+      We next notice in the illustration that at the base of the halo there
+      are red flames peeping out from the edges of the hidden disc. When one
+      remembers that the sun is 866,000 miles in diameter, one hardly needs to
+      be told that these flames are really gigantic. We shall see what they
+      are presently.
+      Regions of the Sun
+      The astronomer has divided the sun into definite concentric regions or
+      layers. These layers envelop the nucleus or central body of the sun
+      somewhat as the atmosphere envelops our earth. It is through these
+      vapour layers that the bright white body of the sun is seen. Of the
+      innermost region, the heart or nucleus of the sun, we know almost
+      nothing. The central body or nucleus is surrounded by a brilliantly
+      luminous envelope or layer of vaporous matter which is what we see when
+      we look at the sun and which the astronomer calls the photosphere.
+      Above--that is, overlying--the photosphere there is a second layer of
+      glowing gases, which is known as the reversing layer. This layer is
+      cooler than the underlying photosphere; it forms a veil of smoke-like
+      haze and is of from 500 to 1,000 miles in thickness.
+      A third layer or envelope immediately lying over the last one is the
+      region known as the chromosphere. The chromosphere extends from 5,000
+      to 10,000 miles in thickness--a "sea" of red tumultuous surging fire.
+      Chief among the glowing gases is the vapour of hydrogen. The intense
+      white heat of the photosphere beneath shines through this layer,
+      overpowering its brilliant redness. From the uppermost portion of the
+      chromosphere great fiery tongues of glowing hydrogen and calcium vapour
+      shoot out for many thousands of miles, driven outward by some prodigious
+      expulsive force. It is these red "prominences" which are such a notable
+      feature in the picture of the eclipse of the sun already referred to.
+      During the solar eclipse of 1919 one of these red flames rose in less
+      than seven hours from a height of 130,000 miles to more than 500,000
+      miles above the sun's surface. This immense column of red-hot gas, four
+      or five times the thickness of the earth, was soaring upward at the rate
+      of 60,000 miles an hour.
+      These flaming jets or prominences shooting out from the chromosphere are
+      not to be seen every day by the naked eye; the dazzling light of the sun
+      obscures them, gigantic as they are. They can be observed, however, by
+      the spectroscope any day, and they are visible to us for a very short
+      time during an eclipse of the sun. Some extraordinary outbursts have
+      been witnessed. Thus the late Professor Young described one on September
+      7, 1871, when he had been examining a prominence by the spectroscope:
+      It had remained unchanged since noon of the previous day--a long,
+      low, quiet-looking cloud, not very dense, or brilliant, or in any
+      way remarkable except for its size. At 12:30 p.m. the Professor left
+      the spectroscope for a short time, and on returning half an hour
+      later to his observations, he was astonished to find the gigantic
+      Sun flame shattered to pieces. The solar atmosphere was filled with
+      flying debris, and some of these portions reached a height of
+      100,000 miles above the solar surface. Moving with a velocity which,
+      even at the distance of 93,000,000 miles, was almost perceptible to
+      the eye, these fragments doubled their height in ten minutes. On
+      January 30, 1885, another distinguished solar observer, the late
+      Professor Tacchini of Rome, observed one of the greatest prominences
+      ever seen by man. Its height was no less than 142,000
+      miles--eighteen times the diameter of the earth. Another mighty
+      flame was so vast that supposing the eight large planets of the
+      solar system ranged one on top of the other, the prominence would
+      still tower above them.[1]
+      [1] _The Romance of Astronomy_, by H. Macpherson.
+      [Illustration: FIG. 5.--DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN
+      Compare with frontispiece.]
+      [Illustration: _Photo: Royal Observatory, Greenwich._
+      FIG. 6.--SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, May 29, 1919.
+      TAKEN AT SOBRAL, BRAZIL.
+      The small Corona is also visible.]
+      [Illustration: FIG. 7.--THE VISIBLE SURFACE OF THE SUN
+      A photograph taken at the Mount Wilson Observatory of the Carnegie
+      Institution at Washington.]
+      [Illustration: FIG. 8.--THE SUN
+      Photographed in the light of glowing hydrogen, at the Mount Wilson
+      Observatory of the Carnegie Institution of Washington: vortex phenomena
+      near the spots are especially prominent.]
+      The fourth and uppermost layer or region is that of the corona, of
+      immense extent and fading away into the surrounding sky--this we have
+      already referred to. The diagram (Fig. 5) shows the dispositions of
+      these various layers of the sun. It is through these several transparent
+      layers that we see the white light body of the sun.
+      The Surface of the Sun
+      Here let us return to and see what more we know about the
+      photosphere--the sun's surface. It is from the photosphere that we have
+      gained most of our knowledge of the composition of the sun, which is
+      believed not to be a solid body. Examination of the photosphere shows
+      that the outer surface is never at rest. Small bright cloudlets come and
+      go in rapid succession, giving the surface, through contrasts in
+      luminosity, a granular appearance. Of course, to be visible at all at
+      92,830,000 miles the cloudlets cannot be small. They imply enormous
+      activity in the photosphere. If we might speak picturesquely the sun's
+      surface resembles a boiling ocean of white-hot metal vapours. We have
+      to-day a wonderful instrument, which will be described later, which
+      dilutes, as it were, the general glare of the sun, and enables us to
+      observe these fiery eruptions at any hour. The "oceans" of red-hot gas
+      and white-hot metal vapour at the sun's surface are constantly driven by
+      great storms. Some unimaginable energy streams out from the body or
+      muscles of the sun and blows its outer layers into gigantic shreds, as
+      it were.
+      The actual temperature at the sun's surface, or what appears to us to be
+      the surface--the photosphere--is, of course, unknown, but careful
+      calculation suggests that it is from 5,000° C. to 7,000° C. The interior
+      is vastly hotter. We can form no conception of such temperatures as must
+      exist there. Not even the most obdurate solid could resist such
+      temperatures, but would be converted almost instantaneously into gas.
+      But it would not be gas as we know gases on the earth. The enormous
+      pressures that exist on the sun must convert even gases into thick
+      treacly fluids. We can only infer this state of matter. It is beyond our
+      power to reproduce it.
+      Sun-spots
+      It is in the brilliant photosphere that the dark areas known as
+      sun-spots appear. Some of these dark spots--they are dark only by
+      contrast with the photosphere surrounding them--are of enormous size,
+      covering many thousands of square miles of surface. What they are we
+      cannot positively say. They look like great cavities in the sun's
+      surface. Some think they are giant whirlpools. Certainly they seem to be
+      great whirling streams of glowing gases with vapours above them and
+      immense upward and downward currents within them. Round the edges of the
+      sun-spots rise great tongues of flame.
+      Perhaps the most popularly known fact about sun-spots is that they are
+      somehow connected with what we call magnetic storms on earth. These
+      magnetic storms manifest themselves in interruptions of our telegraphic
+      and telephonic communications, in violent disturbances of the mariner's
+      compass, and in exceptional auroral displays. The connection between the
+      two sets of phenomena cannot be doubted, even although at times there
+      may be a great spot on the sun without any corresponding "magnetic
+      storm" effects on the earth.
+      A surprising fact about sun-spots is that they show definite periodic
+      variations in number. The best-defined period is one of about eleven
+      years. During this period the spots increase to a maximum in number and
+      then diminish to a minimum, the variation being more or less regular.
+      Now this can only mean one thing. To be periodic the spots must have
+      some deep-seated connection with the fundamental facts of the sun's
+      structure and activities. Looked at from this point of view their
+      importance becomes great.
+      [Illustration: _Reproduction from "The Forces of Nature"_ (_Messrs.
+          Macmillan_)
+      THE AURORA BOREALIS
+      The aurora borealis is one of the most beautiful spectacles in the sky.
+      The colours and shape change every instant; sometimes a fan-like cluster
+      of rays, at other times long golden draperies gliding one over the
+      other. Blue, green, yellow, red, and white combine to give a glorious
+      display of colour. The theory of its origin is still, in part, obscure,
+      but there can be no doubt that the aurora is related to the magnetic
+      phenomena of the earth and therefore is connected with the electrical
+      influence of the sun.]
+      It is from the study of sun-spots that we have learned that the sun's
+      surface does not appear to rotate all at the same speed. The
+      "equatorial" regions are rotating quicker than regions farther north or
+      south. A point forty-five degrees from the equator seems to take about
+      two and a half days longer to complete one rotation than a point on the
+      equator. This, of course, confirms our belief that the sun cannot be a
+      solid body.
+      What is its composition? We know that there are present, in a gaseous
+      state, such well-known elements as sodium, iron, copper, zinc, and
+      magnesium; indeed, we know that there is practically every element in
+      the sun that we know to be in the earth. How do we know?
+      It is from the photosphere, as has been said, that we have won most of
+      our knowledge of the sun. The instrument used for this purpose is the
+      spectroscope; and before proceeding to deal further with the sun and the
+      source of its energy it will be better to describe this instrument.
+      A WONDERFUL INSTRUMENT AND WHAT IT REVEALS
+      The spectroscope is an instrument for analysing light. So important is
+      it in the revelations it has given us that it will be best to describe
+      it fully. Every substance to be examined must first be made to glow,
+      made luminous; and as nearly everything in the heavens _is_ luminous the
+      instrument has a great range in Astronomy. And when we speak of
+      analysing light, we mean that the light may be broken up into waves of
+      different lengths. What we call light is a series of minute waves in
+      ether, and these waves are--measuring them from crest to crest, so to
+      say--of various lengths. Each wave-length corresponds to a colour of the
+      rainbow. The shortest waves give us a sensation of violet colour, and
+      the largest waves cause a sensation of red. The rainbow, in fact, is a
+      sort of natural spectrum. (The meaning of the rainbow is that the
+          moisture-laden air has sorted out these waves, in the sun's light,
+          according to their length.) Now the simplest form of spectroscope is a
+      glass prism--a triangular-shaped piece of glass. If white light
+      (sunlight, for example) passes through a glass prism, we see a series of
+      rainbow-tinted colours. Anyone can notice this effect when sunlight is
+      shining through any kind of cut glass--the stopper of a wine decanter,
+      for instance. If, instead of catching with the eye the coloured lights
+      as they emerge from the glass prism, we allow them to fall on a screen,
+      we shall find that they pass, by continuous gradations, from red at the
+      one end of the screen, through orange, yellow, green, blue, and indigo,
+      to violet at the other end. _In other words, what we call white light is
+      composed of rays of these several colours. They go to make up the effect
+      which we call white._ And now just as water can be split up into its two
+      elements, oxygen and hydrogen, so sunlight can be broken up into its
+      primary colours, which are those we have just mentioned.
+      This range of colours, produced by the spectroscope, we call the solar
+      spectrum, and these are, from the spectroscopic point of view, primary
+      colours. Each shade of colour has its definite position in the spectrum.
+      That is to say, the light of each shade of colour (corresponding to its
+          wave-length) is reflected through a certain fixed angle on passing
+      through the glass prism. Every possible kind of light has its definite
+      position, and is denoted by a number which gives the wave-length of the
+      vibrations constituting that particular kind of light.
+      Now, other kinds of light besides sunlight can be analysed. Light
+      from any substance which has been made incandescent may be observed with
+      the spectroscope in the same way, and each element can be thus
+      separated. It is found that each substance (in the same conditions of
+          pressure, etc.) gives a constant spectrum of its own. _Each metal
+      displays its own distinctive colour. It is obvious, therefore, that the
+      spectrum provides the means for identifying a particular substance._ It
+      was by this method that we discovered in the sun the presence of such
+      well-known elements as sodium, iron, copper, zinc, and magnesium.
+      [Illustration: _Yerkes Observatory._
+      FIG. 9.--THE GREAT SUN-SPOT OF JULY 17, 1905]
+      [Illustration: _From photographs taken at the Yerkes Observatory._
+      FIG. 10.--SOLAR PROMINENCES
+      These are about 60,000 miles in height. The two photographs show the
+      vast changes occurring in ten minutes. October 10, 1910.]
+      [Illustration: _Photo: Mount Wilson Observatory._
+      FIG. 11.--MARS, October 5, 1909
+      Showing the dark markings and the Polar Cap.]
+      [Illustration: FIG. 12.--JUPITER
+      Showing the belts which are probably cloud formations.]
+      [Illustration: _Photo: Professor E. E. Barnard, Yerkes Observatory._
+      FIG. 13.--SATURN, November 19, 1911
+      Showing the rings, mighty swarms of meteorites.]
+      Every chemical element known, then, has a distinctive spectrum of its
+      own when it is raised to incandescence, and this distinctive spectrum is
+      as reliable a means of identification for the element as a human face is
+      for its owner. Whether it is a substance glowing in the laboratory or in
+      a remote star makes no difference to the spectroscope; if the light of
+      any substance reaches it, that substance will be recognised and
+      identified by the characteristic set of waves.
+      The spectrum of a glowing mass of gas will consist in a number of bright
+      lines of various colours, and at various intervals; corresponding to
+      each kind of gas, there will be a peculiar and distinctive arrangement
+      of bright lines. But if the light from such a mass of glowing gas be
+      made to pass through a cool mass of the _same_ gas it will be found that
+      dark lines replace the bright lines in the spectrum, the reason for this
+      being that the cool gas absorbs the rays of light emitted by the hot
+      gas. Experiments of this kind enable us to reach the important general
+      statement that every gas, when cold, absorbs the same rays of light
+      which it emits when hot.
+      Crossing the solar spectrum are hundreds and hundreds of dark lines.
+      These could not at first be explained, because this fact of
+      discriminative absorption was not known. We understand now. The sun's
+      white light comes from the photosphere, but between us and the
+      photosphere there is, as we have seen, another solar envelope of
+      relatively cooler vapours--the reversing layer. Each constituent
+      element in this outer envelope stops its own kind of light, that is, the
+      kind of light made by incandescent atoms of the same element in the
+      photosphere. The "stoppages" register themselves in the solar spectrum
+      as dark lines placed exactly where the corresponding bright lines would
+      have been. The explanation once attained, dark lines became as
+      significant as bright lines. The secret of the sun's composition was
+      out. We have found practically every element in the sun that we know to
+      be in the earth. We have identified an element in the sun before we were
+      able to isolate it on the earth. We have been able even to point to the
+      coolest places on the sun, the centres of sun-spots, where alone the
+      temperature seems to have fallen sufficiently low to allow chemical
+      compounds to form.
+      It is thus we have been able to determine what the stars, comets, or
+      nebulæ are made of.
+      A Unique Discovery
+      In 1868 Sir Norman Lockyer detected a light coming from the prominences
+      of the sun which was not given by any substance known on earth, and
+      attributed this to an unknown gas which he called helium, from the Greek
+      _helios_, the sun. _In 1895 Sir William Ramsay discovered in certain
+      minerals the same gas identified by the spectroscope._ We can say,
+      therefore, that this gas was discovered in the sun nearly thirty years
+      before it was found on earth; this discovery of the long-lost heir is as
+      thrilling a chapter in the detective story of science as any in the
+      sensational stories of the day, and makes us feel quite certain that our
+      methods really tell us of what elements sun and stars are built up. The
+      light from the corona of the sun, as we have mentioned indicates a gas
+      still unknown on earth, which has been christened Coronium.
+      Measuring the Speed of Light
+      But this is not all; soon a new use was found for the spectroscope. We
+      found that we could measure with it the most difficult of all speeds
+      to measure, speed in the line of sight. Movement at right angles to the
+      direction in which one is looking is, if there is sufficient of it, easy
+      to detect, and, if the distance of the moving body is known, easy to
+      measure. But movement in the line of vision is both difficult to detect
+      and difficult to measure. Yet, even at the enormous distances with which
+      astronomers have to deal, the spectroscope can detect such movement and
+      furnish data for its measurement. If a luminous body containing, say,
+      sodium is moving rapidly towards the spectroscope, it will be found that
+      the sodium lines in the spectrum have moved slightly from their usual
+      definite positions towards the violet end of the spectrum, the amount of
+      the change of position increasing with the speed of the luminous body.
+      If the body is moving away from the spectroscope the shifting of the
+      spectral lines will be in the opposite direction, towards the red end of
+      the spectrum. In this way we have discovered and measured movements that
+      otherwise would probably not have revealed themselves unmistakably to us
+      for thousands of years. In the same way we have watched, and measured
+      the speed of, tremendous movements on the sun, and so gained proof that
+      the vast disturbances we should expect there actually do occur.
+      [Illustration: THE SPECTROSCOPE IS AN INSTRUMENT FOR ANALYSING LIGHT; IT
+      PROVIDES THE MEANS FOR IDENTIFYING DIFFERENT SUBSTANCES
+      This pictorial diagram illustrates the principal of Spectrum Analysis,
+      showing how sunlight is decomposed into its primary colours. What we
+      call white light is composed of seven different colours. The diagram is
+      relieved of all detail which would unduly obscure the simple process by
+      which a ray of light is broken up by a prism into different
+      wave-lengths. The spectrum rays have been greatly magnified.]
+      IS THE SUN DYING?
+      Now let us return to our consideration of the sun.
+      To us on the earth the most patent and most astonishing fact about the
+      sun is its tremendous energy. Heat and light in amazing quantities pour
+      from it without ceasing.
+      Where does this energy come from? Enormous jets of red glowing gases can
+      be seen shooting outwards from the sun, like flames from a fire, for
+      thousands of miles. Does this argue fire, as we know fire on the earth?
+      On this point the scientist is sure. The sun is not burning, and
+      combustion is not the source of its heat. Combustion is a chemical
+      reaction between atoms. The conditions that make it possible are known
+      and the results are predictable and measurable. But no chemical reaction
+      of the nature of combustion as we know it will explain the sun's energy,
+      nor indeed will any ordinary chemical reaction of any kind. If the sun
+      were composed of combustible material throughout and the conditions of
+      combustion as we understand them were always present, the sun would burn
+      itself out in some thousands of years, with marked changes in its heat
+      and light production as the process advanced. There is no evidence of
+      such changes. There is, instead, strong evidence that the sun has been
+      emitting light and heat in prodigious quantities, not for thousands, but
+      for millions of years. Every addition to our knowledge that throws light
+      on the sun's age seems to make for increase rather than decrease of its
+      years. This makes the wonder of its energy greater.
+      And we cannot avoid the issue of the source of the energy by saying
+      merely that the sun is gradually radiating away an energy that
+      originated in some unknown manner, away back at the beginning of things.
+      Reliable calculations show that the years required for the mere cooling
+      of a globe like the sun could not possibly run to millions. In other
+      words, the sun's energy must be subject to continuous and more or less
+      steady renewal. However it may have acquired its enormous energy in the
+      past, it must have some source of energy in the present.
+      The best explanation that we have to-day of this continuous accretion of
+      energy is that it is due to shrinkage of the sun's bulk under the force
+      of gravity. Gravity is one of the most mysterious forces of nature, but
+      it is an obvious fact that bodies behave as if they attracted one
+      another, and Newton worked out the law of this attraction. We may say,
+      without trying to go too deeply into things, that every particle of
+      matter attracts every other throughout the universe. If the diameter of
+      the sun were to shrink by one mile all round, this would mean that all
+      the millions of tons in the outer one-mile thickness would have a
+      straight drop of one mile towards the centre. And that is not all,
+      because obviously the layers below this outer mile would also drop
+      inwards, each to a less degree than the one above it. What a tremendous
+      movement of matter, however slowly it might take place! And what a
+      tremendous energy would be involved! Astronomers calculate that the
+      above shrinkage of one mile all round would require fifty years for its
+      completion, assuming, reasonably, that there is close and continuous
+      relationship between loss of heat by radiation and shrinkage. Even if
+      this were true we need not feel over-anxious on this theory; before the
+      sun became too cold to support life many millions of years would be
+      required.
+      It was suggested at one time that falls of meteoric matter into the sun
+      would account for the sun's heat. This position is hardly tenable now.
+      The mere bulk of the meteoric matter required by the hypothesis, apart
+      from other reasons, is against it. There is undoubtedly an enormous
+      amount of meteoric matter moving about within the bounds of the solar
+      system, but most of it seems to be following definite routes round the
+      sun like the planets. The stray erratic quantities destined to meet
+      their doom by collision with the sun can hardly be sufficient to account
+      for the sun's heat.
+      Recent study of radio-active bodies has suggested another factor that
+      may be working powerfully along with the force of gravitation to
+      maintain the sun's store of heat. In radio-active bodies certain atoms
+      seem to be undergoing disintegration. These atoms appear to be splitting
+      up into very minute and primitive constituents. But since matter may be
+      split up into such constituents, may it not be built up from them?
+      The question is whether these "radio-active" elements are undergoing
+      disintegration, or formation, in the sun. If they are undergoing
+      disintegration--and the sun itself is undoubtedly radio-active--then we
+      have another source of heat for the sun that will last indefinitely.
+      THE PLANETS
+      LIFE IN OTHER WORLDS?
+      It is quite clear that there cannot be life on the stars. Nothing solid
+      or even liquid can exist in such furnaces as they are. Life exists only
+      on planets, and even on these its possibilities are limited. Whether all
+      the stars, or how many of them, have planetary families like our sun, we
+      cannot positively say. If they have, such planets would be too faint and
+      small to be visible tens of trillions of miles away. Some astronomers
+      think that our sun may be exceptional in having planets, but their
+      reasons are speculative and unconvincing. Probably a large proportion at
+      least of the stars have planets, and we may therefore survey the globes
+      of our own solar system and in a general way extend the results to the
+      rest of the universe.
+      In considering the possibility of life as we know it we may at once rule
+      out the most distant planets from the sun, Uranus and Neptune. They are
+      probably intrinsically too hot. We may also pass over the nearest planet
+      to the sun, Mercury. We have reason to believe that it turns on its axis
+      in the same period as it revolves round the sun, and it must therefore
+      always present the same side to the sun. This means that the heat on the
+      sunlit side of Mercury is above boiling-point, while the cold on the
+      other side must be between two and three hundred degrees below
+      freezing-point.
+      The Planet Venus
+      The planet Venus, the bright globe which is known to all as the morning
+      and evening "star," seems at first sight more promising as regards the
+      possibility of life. It is of nearly the same size as the earth, and it
+      has a good atmosphere, but there are many astronomers who believe that,
+      like Mercury, it always presents the same face to the sun, and it would
+      therefore have the same disadvantage--a broiling heat on the sunny side
+      and the cold of space on the opposite side. We are not sure. The
+      surface of Venus is so bright--the light of the sun is reflected to us
+      by such dense masses of cloud and dust--that it is difficult to trace
+      any permanent markings on it, and thus ascertain how long it takes to
+      rotate on its axis. Many astronomers believe that they have succeeded,
+      and that the planet always turns the same face to the sun. If it does,
+      we can hardly conceive of life on its surface, in spite of the
+      cloud-screen.
+      [Illustration: FIG. 14.--THE MOON
+      Showing a great plain and some typical craters. There are thousands of
+      these craters, and some theories of their origin are explained on page
+      34.]
+      [Illustration: FIG. 15.--MARS
+      1} Drawings by Prof. Lowell to accompany actual photographs of Mars
+      showing many of the
+      2} canals. Taken in 1907 by Mr. E. C. Slipher of the Lowell Observatory.
+      3 Drawing by Prof. Lowell made January 6, 1914.
+      4 Drawing by Prof. Lowell made January 21, 1914.
+      Nos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4
+      depict quite different sections. Note the change in the polar snow-caps
+      in the last two.]
+      [Illustration: FIG. 16.--THE MOON, AT NINE AND THREE-QUARTER DAYS
+      Note the mysterious "rays" diverging from the almost perfectly circular
+      craters indicated by the arrows (Tycho, upper; Copernicus, lower), and
+      also the mountains to the right with the lunar dawn breaking on them.]
+      We turn to Mars; and we must first make it clear why there is so much
+      speculation about life on Mars, and why it is supposed that, if there
+      _is_ life on Mars, it must be more advanced than life on the earth.
+      Is there Life on Mars?
+      The basis of this belief is that if, as we saw, all the globes in our
+      solar system are masses of metal that are cooling down, the smaller will
+      have cooled down before the larger, and will be further ahead in their
+      development. Now Mars is very much smaller than the earth, and must have
+      cooled at its surface millions of years before the earth did. Hence, if
+      a story of life began on Mars at all, it began long before the story of
+      life on the earth. We cannot guess what sort of life-forms would be
+      evolved in a different world, but we can confidently say that they would
+      tend toward increasing intelligence; and thus we are disposed to look
+      for highly intelligent beings on Mars.
+      But this argument supposes that the conditions of life, namely air and
+      water, are found on Mars, and it is disputed whether they are found
+      there in sufficient quantity. The late Professor Percival Lowell, who
+      made a lifelong study of Mars, maintained that there are hundreds of
+      straight lines drawn across the surface of the planet, and he claimed
+      that they are beds of vegetation marking the sites of great channels or
+      pipes by means of which the "Martians" draw water from their polar
+      ocean. Professor W. H. Pickering, another high authority, thinks that
+      the lines are long, narrow marshes fed by moist winds from the poles.
+      There are certainly white polar caps on Mars. They seem to melt in the
+      spring, and the dark fringe round them grows broader.
+      Other astronomers, however, say that they find no trace of water-vapour
+      in the atmosphere of Mars, and they think that the polar caps may be
+      simply thin sheets of hoar-frost or frozen gas. They point out that, as
+      the atmosphere of Mars is certainly scanty, and the distance from the
+      sun is so great, it may be too cold for the fluid water to exist on the
+      planet.
+      If one asks why our wonderful instruments cannot settle these points,
+      one must be reminded that Mars is never nearer than 34,000,000 miles
+      from the earth, and only approaches to this distance once in fifteen or
+      seventeen years. The image of Mars on the photographic negative taken in
+      a big telescope is very small. Astronomers rely to a great extent on the
+      eye, which is more sensitive than the photographic plate. But it is easy
+      to have differences of opinion as to what the eye sees, and so there is
+      a good deal of controversy.
+      In August, 1924, the planet will again be well placed for observation,
+      and we may learn more about it. Already a few of the much-disputed
+      lines, which people wrongly call "canals," have been traced on
+      photographs. Astronomers who are sceptical about life on Mars are often
+      not fully aware of the extraordinary adaptability of life. There was a
+      time when the climate of the whole earth, from pole to pole, was
+      semi-tropical for millions of years. No animal could then endure the
+      least cold, yet now we have plenty of Arctic plants and animals. If the
+      cold came slowly on Mars, as we have reason to suppose, the population
+      could be gradually adapted to it. On the whole, it is possible that
+      there is advanced life on Mars, and it is not impossible, in spite of
+      the very great difficulties of a code of communication, that our "elder
+      brothers" may yet flash across space the solution of many of our
+      problems.
+      Jupiter and Saturn
+      Next to Mars, going outward from the sun, is Jupiter. Between Mars and
+      Jupiter, however, there are more than three hundred million miles of
+      space, and the older astronomers wondered why this was not occupied by a
+      planet. We now know that it contains about nine hundred "planetoids," or
+      small globes of from five to five hundred miles in diameter. It was at
+      one time thought that a planet might have burst into these fragments (a
+          theory which is not mathematically satisfactory), or it may be that the
+      material which is scattered in them was prevented by the nearness of the
+      great bulk of Jupiter from uniting into one globe.
+      For Jupiter is a giant planet, and its gravitational influence must
+      extend far over space. It is 1,300 times as large as the earth, and has
+      nine moons, four of which are large, in attendance on it. It is
+      interesting to note that the outermost moons of Jupiter and Saturn
+      revolve round these planets in a direction contrary to the usual
+      direction taken by moons round planets, and by planets round the sun.
+      But there is no life on Jupiter.
+      The surface which we see in photographs (Fig. 12) is a mass of cloud or
+      steam which always envelops the body of the planet. It is apparently
+      red-hot. A red tinge is seen sometimes at the edges of its cloud-belts,
+      and a large red region (the "red spot"), 23,000 miles in length, has
+      been visible on it for half a century. There may be a liquid or solid
+      core to the planet, but as a whole it is a mass of seething vapours
+      whirling round on its axis once in every ten hours. As in the case of
+      the sun, however, different latitudes appear to rotate at different
+      rates. The interior of Jupiter is very hot, but the planet is not
+      self-luminous. The planets Venus and Jupiter shine very brightly, but
+      they have no light of their own; they reflect the sunlight.
+      Saturn is in the same interesting condition. The surface in the
+      photograph (Fig. 13) is steam, and Saturn is so far away from the sun
+      that the vaporisation of its oceans must necessarily be due to its own
+      internal heat. It is too hot for water to settle on its surface. Like
+      Jupiter, the great globe turns on its axis once in ten hours--a
+      prodigious speed--and must be a swirling, seething mass of metallic
+      vapours and gases. It is instructive to compare Jupiter and Saturn in
+      this respect with the sun. They are smaller globes and have cooled down
+      more than the central fire.
+      Saturn is a beautiful object in the telescope because it has ten moons
+      (to include one which is disputed) and a wonderful system of "rings"
+      round it. The so-called rings are a mighty swarm of meteorites--pieces
+      of iron and stone of all sorts and sizes, which reflect the light of the
+      sun to us. This ocean of matter is some miles deep, and stretches from a
+      few thousand miles from the surface of the planet to 172,000 miles out
+      in space. Some astronomers think that this is volcanic material which
+      has been shot out of the planet. Others regard it as stuff which would
+      have combined to form an eleventh moon but was prevented by the nearness
+      of Saturn itself. There is no evidence of life on Saturn.
+      THE MOON
+      Mars and Venus are therefore the only planets, besides the earth, on
+      which we may look for life; and in the case of Venus, the possibility is
+      very faint. But what about the moons which attend the planets? They
+      range in size from the little ten-miles-wide moons of Mars, to Titan, a
+      moon of Saturn, and Ganymede, a satellite of Jupiter, which are about
+      3,000 miles in diameter. May there not be life on some of the larger of
+      these moons? We will take our own moon as a type of the class.
+      A Dead World
+      The moon is so very much nearer to us than any other heavenly body that
+      we have a remarkable knowledge of it. In Fig. 14 you have a photograph,
+      taken in one of our largest telescopes, of part of its surface. In a
+      sense such a telescope brings the moon to within about fifty miles of
+      us. We should see a city like London as a dark, sprawling blotch on the
+      globe. We could just detect a Zeppelin or a Diplodocus as a moving speck
+      against the surface. But we find none of these things. It is true that a
+      few astronomers believe that they see signs of some sort of feeble life
+      or movement on the moon. Professor Pickering thinks that he can trace
+      some volcanic activity. He believes that there are areas of vegetation,
+      probably of a low order, and that the soil of the moon may retain a
+      certain amount of water in it. He speaks of a very thin atmosphere, and
+      of occasional light falls of snow. He has succeeded in persuading some
+      careful observers that there probably are slight changes of some kind
+      taking place on the moon.
+      [Illustration: FIG. 17.--A MAP OF THE CHIEF PLAINS AND CRATERS OF THE
+      MOON
+      The plains were originally supposed to be seas: hence the name "Mare."]
+      [Illustration: FIG. 18.--A DIAGRAM OF A STREAM OF METEORS SHOWING THE
+      EARTH PASSING THROUGH THEM] [Illustration: _Photo: Royal Observatory,
+      Greenwich._
+      FIG. 19.--COMET, September 29, 1908
+      Notice the tendency to form a number of tails. (See photograph below.)]
+      [Illustration: _Photo: Royal Observatory, Greenwich._
+      FIG. 20.--COMET, October 3, 1908
+      The process has gone further and a number of distinct tails can now be
+      counted.]
+      But there are many things that point to absence of air on the moon. Even
+      the photographs we reproduce tell the same story. The edges of the
+      shadows are all hard and black. If there had been an appreciable
+      atmosphere it would have scattered the sun's light on to the edges and
+      produced a gradual shading off such as we see on the earth. This
+      relative absence of air must give rise to some surprising effects. There
+      will be no sounds on the moon, because sounds are merely air waves. Even
+      a meteor shattering itself to a violent end against the surface of the
+      moon would make no noise. Nor would it herald its coming by glowing into
+      a "shooting star," as it would on entering the earth's atmosphere. There
+      will be no floating dust, no scent, no twilight, no blue sky, no
+      twinkling of the stars. The sky will be always black and the stars will
+      be clearly visible by day as by night. The sun's wonderful corona, which
+      no man on earth, even by seizing every opportunity during eclipses, can
+      hope to see for more than two hours in all in a long lifetime, will be
+      visible all day. So will the great red flames of the sun. Of course,
+      there will be no life, and no landscape effects and scenery effects due
+      to vegetation.
+      The moon takes approximately twenty-seven of our days to turn once on
+      its axis. So for fourteen days there is continuous night, when the
+      temperature must sink away down towards the absolute cold of space. This
+      will be followed without an instant of twilight by full daylight. For
+      another fourteen days the sun's rays will bear straight down, with no
+      diffusion or absorption of their heat, or light, on the way. It does not
+      follow, however, that the temperature of the moon's surface must rise
+      enormously. It may not even rise to the temperature of melting ice.
+      Seeing there is no air there can be no check on radiation. The heat that
+      the moon gets will radiate away immediately. We know that amongst the
+      coldest places on the earth are the tops of very high mountains, the
+      points that have reared themselves nearest to the sun but farthest out
+      of the sheltering blanket of the earth's atmosphere. The actual
+      temperature of the moon's surface by day is a moot point. It may be
+      below the freezing-point or above the boiling-point of water.
+      The Mountains of the Moon
+      The lack of air is considered by many astronomers to furnish the
+      explanation of the enormous number of "craters" which pit the moon's
+      surface. There are about a hundred thousand of these strange rings, and
+      it is now believed by many that they are spots where very large
+      meteorites, or even planetoids, splashed into the moon when its surface
+      was still soft. Other astronomers think that they are the remains of
+      gigantic bubbles which were raised in the moon's "skin," when the globe
+      was still molten, by volcanic gases from below. A few astronomers think
+      that they are, as is popularly supposed, the craters of extinct
+      volcanoes. Our craters, on the earth, are generally deep cups, whereas
+      these ring-formations on the moon are more like very shallow and broad
+      saucers. Clavius, the largest of them, is 123 miles across the interior,
+      yet its encircling rampart is not a mile high.
+      The mountains on the moon (Fig. 16) rise to a great height, and are
+      extraordinarily gaunt and rugged. They are like fountains of lava,
+      rising in places to 26,000 and 27,000 feet. The lunar Apennines have
+      three thousand steep and weird peaks. Our terrestrial mountains are
+      continually worn down by frost acting on moisture and by ice and water,
+      but there are none of these agencies operating on the moon. Its
+      mountains are comparatively "everlasting hills."
+      The moon is interesting to us precisely because it is a dead world. It
+      seems to show how the earth, or any cooling metal globe, will evolve in
+      the remote future. We do not know if there was ever life on the moon,
+      but in any case it cannot have proceeded far in development. At the most
+      we can imagine some strange lowly forms of vegetation lingering here and
+      there in pools of heavy gas, expanding during the blaze of the sun's
+      long day, and frozen rigid during the long night.
+      METEORS AND COMETS
+      We may conclude our survey of the solar system with a word about
+      "shooting stars," or meteors, and comets. There are few now who do not
+      know that the streak of fire which suddenly lights the sky overhead at
+      night means that a piece of stone or iron has entered our atmosphere
+      from outer space, and has been burned up by friction. It was travelling
+      at, perhaps, twenty or thirty miles a second. At seventy or eighty miles
+      above our heads it began to glow, as at that height the air is thick
+      enough to offer serious friction and raise it to a white heat. By the
+      time the meteor reached about twenty miles or so from the earth's
+      surface it was entirely dissipated, as a rule in fiery vapour.
+      Millions of Meteorites
+      It is estimated that between ten and a hundred million meteorites enter
+      our atmosphere and are cremated, every day. Most of them weigh only an
+      ounce or two, and are invisible. Some of them weigh a ton or more, but
+      even against these large masses the air acts as a kind of "torpedo-net."
+      They generally burst into fragments and fall without doing damage.
+      It is clear that "empty space" is, at least within the limits of our
+      solar system, full of these things. They swarm like fishes in the seas.
+      Like the fishes, moreover, they may be either solitary or gregarious.
+      The solitary bit of cosmic rubbish is the meteorite, which we have just
+      examined. A "social" group of meteorites is the essential part of a
+      comet. The nucleus, or bright central part, of the head of a comet (Fig.
+          19) consists of a swarm, sometimes thousands of miles wide, of these
+      pieces of iron or stone. This swarm has come under the sun's
+      gravitational influence, and is forced to travel round it. From some
+      dark region of space it has moved slowly into our system. It is not then
+      a comet, for it has no tail. But as the crowded meteors approach the
+      sun, the speed increases. They give off fine vapour-like matter and the
+      fierce flood of light from the sun sweeps this vapour out in an
+      ever-lengthening tail. Whatever way the comet is travelling, the tail
+      always points away from the sun.
+      A Great Comet
+      The vapoury tail often grows to an enormous length as the comet
+      approaches the sun. The great comet of 1843 had a tail two hundred
+      million miles long. It is, however, composed of the thinnest vapours
+      imaginable. Twice during the nineteenth century the earth passed through
+      the tail of a comet, and nothing was felt. The vapours of the tail are,
+      in fact, so attenuated that we can hardly imagine them to be white-hot.
+      They may be lit by some electrical force. However that may be, the comet
+      dashes round the sun, often at three or four hundred miles a second,
+      then may pass gradually out of our system once more. It may be a
+      thousand years, or it may be fifty years, before the monarch of the
+      system will summon it again to make its fiery journey round his throne.
+      [Illustration: _Photo: Harvard College Observatory._
+      FIG. 21.--TYPICAL SPECTRA
+      Six main types of stellar spectra. Notice the lines they have in common,
+      showing what elements are met with in different types of stars. Each of
+      these spectra corresponds to a different set of physical and chemical
+      conditions.] [Illustration: _Photo: Mount Wilson Observatory._
+      FIG. 22.--A NEBULAR REGION SOUTH OF ZETA ORIONIS
+      Showing a great projection of "dark matter" cutting off the light from
+      behind.]
+      [Illustration: _Photo: Astrophysical Observatory, Victoria, British
+      Columbia._
+      FIG. 23.--STAR CLUSTER IN HERCULES
+      A wonderful cluster of stars. It has been estimated that the distance of
+      this cluster is such that it would take light more than 100,000 years to
+      reach us.]
+      THE STELLAR UNIVERSE
+      The immensity of the Stellar Universe, as we have seen, is beyond our
+      apprehension. The sun is nothing more than a very ordinary star, perhaps
+      an insignificant one. There are stars enormously greater than the sun.
+      One such, Betelgeux, has recently been measured, and its diameter is
+      more than 300 times that of the sun.
+      The Evolution of Stars
+      The proof of the similarity between our sun and the stars has come to us
+      through the spectroscope. The elements that we find by its means in the
+      sun are also found in the same way in the stars. Matter, says the
+      spectroscope, is essentially the same everywhere, in the earth and the
+      sun, in the comet that visits us once in a thousand years, in the star
+      whose distance is incalculable, and in the great clouds of "fire-mist"
+      that we call nebulæ.
+      In considering the evolution of the stars let us keep two points clearly
+      in mind. The starting-point, the nebula, is no figment of the scientific
+      imagination. Hundreds of thousands of nebulæ, besides even vaster
+      irregular stretches of nebulous matter, exist in the heavens. But the
+      stages of the evolution of this stuff into stars are very largely a
+      matter of speculation. Possibly there is more than one line of
+      evolution, and the various theories may be reconciled. And this applies
+      also to the theories of the various stages through which the stars
+      themselves pass on their way to extinction.
+      The light of about a quarter of a million stars has been analysed in the
+      spectroscope, and it is found that they fall into about a dozen classes
+      which generally correspond to stages in their evolution (Fig. 21).
+      The Age of Stars
+      In its main lines the spectrum of a star corresponds to its colour, and
+      we may roughly group the stars into red, yellow, and white. This is also
+      the order of increasing temperature, the red stars being the coolest and
+      the white stars the hottest. We might therefore imagine that the white
+      stars are the youngest, and that as they grow older and cooler they
+      become yellowish, then red, and finally become invisible--just as a
+      cooling white-hot iron would do. But a very interesting recent research
+      shows that there are two kinds of red stars; some of them are amongst
+      the oldest stars and some are amongst the youngest. The facts appear to
+      be that when a star is first formed it is not very hot. It is an immense
+      mass of diffuse gas glowing with a dull-red heat. It contracts under the
+      mutual gravitation of its particles, and as it does so it grows hotter.
+      It acquires a yellowish tinge. As it continues to contract it grows
+      hotter and hotter until its temperature reaches a maximum as a white
+      star. At this point the contraction process does not stop, but the
+      heating process does. Further contraction is now accompanied by cooling,
+      and the star goes through its colour changes again, but this time in the
+      inverse order. It contracts and cools to yellow and finally to red. But
+      when it again becomes a red star it is enormously denser and smaller
+      than when it began as a red star. Consequently the red stars are divided
+      into two classes called, appropriately, Giants and Dwarfs. This theory,
+      which we owe to an American astronomer, H. N. Russell, has been
+      successful in explaining a variety of phenomena, and there is
+      consequently good reason to suppose it to be true. But the question as
+      to how the red giant stars were formed has received less satisfactory
+      and precise answers.
+      The most commonly accepted theory is the nebular theory.
+      THE NEBULAR THEORY
+      Nebulæ are dim luminous cloud-like patches in the heavens, more like
+      wisps of smoke in some cases than anything else. Both photography and
+      the telescope show that they are very numerous, hundreds of thousands
+      being already known and the number being continually added to. They are
+      not small. Most of them are immensely large. Actual dimensions cannot be
+      given, because to estimate these we must first know definitely the
+      distance of the nebulæ from the earth. The distances of some nebulæ are
+      known approximately, and we can therefore form some idea of size in
+      these cases. The results are staggering. The mere visible surface of
+      some nebulæ is so large that the whole stretch of the solar system would
+      be too small to form a convenient unit for measuring it. A ray of light
+      would require to travel for years to cross from side to side of such a
+      nebula. Its immensity is inconceivable to the human mind.
+      There appear to be two types of nebulæ, and there is evidence suggesting
+      that the one type is only an earlier form of the other; but this again
+      we do not know.
+      The more primitive nebulæ would seem to be composed of gas in an
+      extremely rarified form. It is difficult to convey an adequate idea of
+      the rarity of nebular gases. The residual gases in a vacuum tube are
+      dense by comparison. A cubic inch of air at ordinary pressure would
+      contain more matter than is contained in millions of cubic inches of the
+      gases of nebulæ. The light of even the faintest stars does not seem to
+      be dimmed by passing through a gaseous nebula, although we cannot be
+      sure on this point. The most remarkable physical fact about these gases
+      is that they are luminous. Whence they derive their luminosity we do not
+      know. It hardly seems possible to believe that extremely thin gases
+      exposed to the terrific cold of space can be so hot as to be luminous
+      and can retain their heat and their luminosity indefinitely. A cold
+      luminosity due to electrification, like that of the aurora borealis,
+      would seem to fit the case better.
+      Now the nebular theory is that out of great "fire-mists," such as we
+      have described, stars are born. We do not know whether gravitation is
+      the only or even the main force at work in a nebula, but it is supposed
+      that under the action of gravity the far-flung "fire-mists" would begin
+      to condense round centres of greatest density, heat being evolved in the
+      process. Of course the condensation would be enormously slow, although
+      the sudden irruption of a swarm of meteors or some solid body might
+      hasten matters greatly by providing large, ready-made centres of
+      condensation.
+      Spiral Nebulæ
+      It is then supposed that the contracting mass of gas would begin to
+      rotate and to throw off gigantic streamers, which would in their turn
+      form centres of condensation. The whole structure would thus form a
+      spiral, having a dense region at its centre and knots or lumps of
+      condensed matter along its spiral arms. Besides the formless gaseous
+      nebulæ there are hundreds of thousands of "spiral" nebulæ such as we
+      have just mentioned in the heavens. They are at all stages of
+      development, and they are visible to us at all angles--that is to say,
+      some of them face directly towards us, others are edge on, and some are
+      in intermediate positions. It appears, therefore, that we have here a
+      striking confirmation of the nebular hypothesis. But we must not go so
+      fast. There is much controversy as to the nature of these spiral nebulæ.
+      Some eminent astronomers think they are other stellar universes,
+      comparable in size with our own. In any case they are vast structures,
+      and if they represent stars in process of condensation, they must be
+      giving birth to huge agglomerations of stars--to star clusters at least.
+      These vast and enigmatic objects do not throw much light on the origin
+      of our own solar system. The nebular hypothesis, which was invented
+      by Laplace to explain the origin of our solar system, has not yet met
+      with universal acceptance. The explanation offers grave difficulties,
+      and it is best while the subject is still being closely investigated, to
+      hold all opinions with reserve. It may be taken as probable, however,
+      that the universe has developed from masses of incandescent gas.
+      [Illustration: _Photo: Yerkes Observatory._
+      FIG. 24.--THE GREAT NEBULA IN ORION
+      The most impressive nebula in the heavens. It is inconceivably greater
+      in dimensions than the whole solar system.]
+      [Illustration: _Photo: Lick Observatory._
+      FIG. 25--GIANT SPIRAL NEBULA, March 23, 1914
+      This spiral nebula is seen full on. Notice the central nucleus and the
+      two spiral arms emerging from its opposite directions. Is matter flowing
+      out of the nucleus into the arms or along the arms into the nucleus? In
+      either case we should get two streams in opposite directions within the
+      nucleus.]
+      THE BIRTH AND DEATH OF STARS
+      Variable, New, and Dark Stars: Dying Suns
+      Many astronomers believe that in "variable stars" we have another star,
+      following that of the dullest red star, in the dying of suns. The light
+      of these stars varies periodically in so many days, weeks, or years. It
+      is interesting to speculate that they are slowly dying suns, in which
+      the molten interior periodically bursts through the shell of thick
+      vapours that is gathering round them. What we saw about our sun seems to
+      point to some such stage in the future. That is, however, not the
+      received opinion about variable stars. It may be that they are stars
+      which periodically pass through a great swarm of meteors or a region of
+      space that is rich in cosmic dust of some sort, when, of course, a great
+      illumination would take place.
+      One class of these variable stars, which takes its name from the star
+      Algol, is of special interest. Every third night Algol has its light
+      reduced for several hours. Modern astronomy has discovered that in this
+      case there are really two stars, circulating round a common centre, and
+      that every third night the fainter of the two comes directly between us
+      and its companion and causes an "eclipse." This was until recently
+      regarded as a most interesting case in which a dead star revealed itself
+      to us by passing before the light of another star. But astronomers have
+      in recent years invented something, the "selenium-cell," which is even
+      more sensitive than the photographic plate, and on this the supposed
+      dead star registers itself as very much alive. Algol is, however,
+      interesting in another way. The pair of stars which we have discovered
+      in it are hundreds of trillions of miles away from the earth, yet we
+      know their masses and their distances from each other.
+      The Death and Birth of Stars
+      We have no positive knowledge of dead stars; which is not surprising
+      when we reflect that a dead star means an invisible star! But when we
+      see so many individual stars tending toward death, when we behold a vast
+      population of all conceivable ages, we presume that there are many
+      already dead. On the other hand, there is no reason to suppose that the
+      universe as a whole is "running down." Some writers have maintained
+      this, but their argument implies that we know a great deal more about
+      the universe than we actually do. The scientific man does not know
+      whether the universe is finite or infinite, temporal or eternal; and he
+      declines to speculate where there are no facts to guide him. He knows
+      only that the great gaseous nebulæ promise myriads of worlds in the
+      future, and he concedes the possibility that new nebulæ may be forming
+      in the ether of space.
+      The last, and not the least interesting, subject we have to notice is
+      the birth of a "new star." This is an event which astronomers now
+      announce every few years; and it is a far more portentous event than the
+      reader imagines when it is reported in his daily paper. The story is
+      much the same in all cases. We say that the star appeared in 1901, but
+      you begin to realise the magnitude of the event when you learn that the
+      distant "blaze" had really occurred about the time of the death of
+      Luther! The light of the conflagration had been speeding toward us
+      across space at 186,000 miles a second, yet it has taken nearly three
+      centuries to reach us. To be visible at all to us at that distance the
+      fiery outbreak must have been stupendous. If a mass of petroleum ten
+      times the size of the earth were suddenly fired it would not be seen at
+      such a distance. The new star had increased its light many hundredfold
+      in a few days.
+      There is a considerable fascination about the speculation that in such
+      cases we see the resurrection of a dead world, a means of renewing the
+      population of the universe. What happens is that in some region of the
+      sky where no star, or only a very faint star, had been registered on our
+      charts, we almost suddenly perceive a bright star. In a few days it may
+      rise to the highest brilliancy. By the spectroscope we learn that this
+      distant blaze means a prodigious outpour of white-hot hydrogen at
+      hundreds of miles a second. But the star sinks again after a few months,
+      and we then find a nebula round it on every side. It is natural to
+      suppose that a dead or dying sun has somehow been reconverted in whole
+      or in part into a nebula. A few astronomers think that it may have
+      partially collided with another star, or approached too closely to
+      another, with the result we described on an earlier page. The general
+      opinion now is that a faint or dead star had rushed into one of those
+      regions of space in which there are immense stretches of nebulous
+      matter, and been (at least in part) vaporised by the friction.
+      But the difficulties are considerable, and some astronomers prefer to
+      think that the blazing star may merely have lit up a dark nebula which
+      already existed. It is one of those problems on which speculation is
+      most tempting but positive knowledge is still very incomplete. We may be
+      content, even proud, that already we can take a conflagration that has
+      occurred more than a thousand trillion miles away and analyse it
+      positively into an outflame of glowing hydrogen gas at so many miles a
+      second.
+      THE SHAPE OF OUR UNIVERSE
+      Our Universe a Spiral Nebula
+      What is the shape of our universe, and what are its dimensions? This is
+      a tremendous question to ask. It is like asking an intelligent insect,
+      living on a single leaf in the midst of a great Brazilian forest, to say
+      what is the shape and size of the forest. Yet man's ingenuity has proved
+      equal to giving an answer even to this question, and by a method exactly
+      similar to that which would be adopted by the insect. Suppose, for
+      instance, that the forest was shaped as an elongated oval, and the
+      insect lived on a tree near the centre of the oval. If the trees were
+      approximately equally spaced from one another they would appear much
+      denser along the length of the oval than across its width. This is the
+      simple consideration that has guided astronomers in determining the
+      shape of our stellar universe. There is one direction in the heavens
+      along which the stars appear denser than in the directions at right
+      angles to it. That direction is the direction in which we look towards
+      the Milky Way. If we count the number of stars visible all over the
+      heavens, we find they become more and more numerous as we approach the
+      Milky Way. As we go farther and farther from the Milky Way the stars
+      thin out until they reach a maximum sparseness in directions at right
+      angles to the plane of the Milky Way. We may consider the Milky Way to
+      form, as it were, the equator of our system, and the line at right
+      angles to point to the north and south poles.
+      Our system, in fact, is shaped something like a lens, and our sun is
+      situated near the centre of this lens. In the remoter part of this lens,
+      near its edge, or possibly outside it altogether, lies the great series
+      of star clouds which make up the Milky Way. All the stars are in motion
+      within this system, but the very remarkable discovery has been made that
+      these motions are not entirely random. The great majority of the stars
+      whose motions can be measured fall into two groups drifting past one
+      another in opposite directions. The velocity of one stream relative to
+      the other is about twenty-five miles per second. The stars forming these
+      two groups are thoroughly well mixed; it is not a case of an inner
+      stream going one way and an outer stream the other. But there are not
+      quite as many stars going one way as the other. For every two stars in
+      one stream there are three in the other. Now, as we have said, some
+      eminent astronomers hold that the spiral nebulæ are universes like our
+      own, and if we look at the two photographs (Figs. 25 and 26) we see that
+      these spirals present features which, in the light of what we have just
+      said about our system, are very remarkable. The nebula in Coma Berenices
+      is a spiral edge-on to us, and we see that it has precisely the
+      lens-shaped middle and the general flattened shape that we have found in
+      our own system. The nebula in Canes Venatici is a spiral facing towards
+      us, and its shape irresistibly suggests motions along the spiral arms.
+      This motion, whether it is towards or away from the central, lens-shaped
+      portion, would cause a double streaming motion in that central portion
+      of the kind we have found in our own system. Again, and altogether apart
+      from these considerations, there are good reasons for supposing our
+      Milky Way to possess a double-armed spiral structure. And the great
+      patches of dark absorbing matter which are known to exist in the Milky
+      Way (see Fig. 22) would give very much the mottled appearance we notice
+      in the arms (which we see edge-on) of the nebula in Coma Berenices. The
+      hypothesis, therefore, that our universe is a spiral nebula has much to
+      be said for it. If it be accepted it greatly increases our estimate of
+      the size of the material universe. For our central, lens-shaped system
+      is calculated to extend towards the Milky Way for more than twenty
+      thousand times a million million miles, and about a third of this
+      distance towards what we have called the poles. If, as we suppose, each
+      spiral nebula is an independent stellar universe comparable in size with
+      our own, then, since there are hundreds of thousands of spiral nebulæ,
+      we see that the size of the whole material universe is indeed beyond our
+      comprehension.
+      Notice the lens-shaped formation of the nucleus and the arm stretching
+      as a band across it. See reference in the text to the resemblance
+      between this and our stellar universe.]
+      100-INCH TELESCOPE, MOUNT WILSON
+      A reflecting telescope: the largest in the world. The mirror is situated
+      at the base of the telescope.]
+      The above distances are merely approximate and are subject to further
+      revision. A "light-year" is the distance that light, travelling at the
+      rate of 186,000 miles per second, would cover in one year.]
+      In this simple outline we have not touched on some of the more debatable
+      questions that engage the attention of modern astronomers. Many of these
+      questions have not yet passed the controversial stage; out of these will
+      emerge the astronomy of the future. But we have seen enough to convince
+      us that, whatever advances the future holds in store, the science of the
+      heavens constitutes one of the most important stones in the wonderful
+      fabric of human knowledge.
+      ASTRONOMICAL INSTRUMENTS
+      The Telescope
+      The instruments used in modern astronomy are amongst the finest triumphs
+      of mechanical skill in the world. In a great modern observatory the
+      different instruments are to be counted by the score, but there are two
+      which stand out pre-eminent as the fundamental instruments of modern
+      astronomy. These instruments are the telescope and the spectroscope, and
+      without them astronomy, as we know it, could not exist.
+      There is still some dispute as to where and when the first telescope was
+      constructed; as an astronomical instrument, however, it dates from the
+      time of the great Italian scientist Galileo, who, with a very small and
+      imperfect telescope of his own invention, first observed the spots on
+      the sun, the mountains of the moon, and the chief four satellites of
+      Jupiter. A good pair of modern binoculars is superior to this early
+      instrument of Galileo's, and the history of telescope construction, from
+      that primitive instrument to the modern giant recently erected on Mount
+      Wilson, California, is an exciting chapter in human progress. But the
+      early instruments have only an historic interest: the era of modern
+      telescopes begins in the nineteenth century.
+      During the last century telescope construction underwent an
+      unprecedented development. An immense amount of interest was taken in
+      the construction of large telescopes, and the different countries of the
+      world entered on an exciting race to produce the most powerful possible
+      instruments. Besides this rivalry of different countries there was a
+      rivalry of methods. The telescope developed along two different lines,
+      and each of these two types has its partisans at the present day. These
+      types are known as _refractors_ and _reflectors_, and it is necessary to
+      mention, briefly, the principles employed in each. The _refractor_ is
+      the ordinary, familiar type of telescope. It consists, essentially, of a
+      large lens at one end of a tube, and a small lens, called the eye-piece,
+      at the other. The function of the large lens is to act as a sort of
+      gigantic eye. It collects a large amount of light, an amount
+      proportional to its size, and brings this light to a focus within the
+      tube of the telescope. It thus produces a small but bright image, and
+      the eye-piece magnifies this image. In the _reflector_, instead of a
+      large lens at the top of the tube, a large mirror is placed at the
+      bottom. This mirror is so shaped as to reflect the light that falls on
+      it to a focus, whence the light is again led to an eye-piece. Thus the
+      refractor and the reflector differ chiefly in their manner of gathering
+      light. The powerfulness of the telescope depends on the size of the
+      light-gatherer. A telescope with a lens four inches in diameter is four
+      times as powerful as the one with a lens two inches in diameter, for the
+      amount of light gathered obviously depends on the _area_ of the lens,
+      and the area varies as the _square_ of the diameter.
+      The largest telescopes at present in existence are _reflectors_. It is
+      much easier to construct a very large mirror than to construct a very
+      large lens; it is also cheaper. A mirror is more likely to get out of
+      order than is a lens, however, and any irregularity in the shape of a
+      mirror produces a greater distorting effect than in a lens. A refractor
+      is also more convenient to handle than is a reflector. For these reasons
+      great refractors are still made, but the largest of them, the great
+      Yerkes' refractor, is much smaller than the greatest reflector, the one
+      on Mount Wilson, California. The lens of the Yerkes' refractor measures
+      three feet four inches in diameter, whereas the Mount Wilson reflector
+      has a diameter of no less than eight feet four inches.
+      [Illustration: THE YERKES 40-INCH REFRACTOR
+      (The largest _refracting_ telescope in the world. Its big lens weighs
+       1,000 pounds, and its mammoth tube, which is 62 feet long, weighs about
+       12,000 pounds. The parts to be moved weigh approximately 22 tons.
+       The great _100-inch reflector_ of the Mount Wilson reflecting
+       telescope--the largest _reflecting_ instrument in the world--weighs
+       nearly 9,000 pounds and the moving parts of the telescope weigh about
+       100 tons.
+       The new _72-inch reflector_ at the Dominion Astrophysical Observatory,
+       near Victoria, B. C., weighs nearly 4,500 pounds, and the moving parts
+       about 35 tons.)]
+      [Illustration: _Photo: H. J. Shepstone._
+      THE DOUBLE-SLIDE PLATE HOLDER ON YERKES 40-INCH REFRACTING TELESCOPE
+      The smaller telescope at the top of the picture acts as a "finder"; the
+      field of view of the large telescope is so restricted that it is
+      difficult to recognise, as it were, the part of the heavens being
+      surveyed. The smaller telescope takes in a larger area and enables the
+      precise object to be examined to be easily selected.]
+  [Illustration: MODERN DIRECT-READING SPECTROSCOPE
+(_By A. Hilger, Ltd._)
+  The light is brought through one telescope, is split up by the prism,
+  and the resulting spectrum is observed through the other telescope.]
+  But there is a device whereby the power of these giant instruments,
+  great as it is, can be still further heightened. That device is the
+  simple one of allowing the photographic plate to take the place of the
+  human eye. Nowadays an astronomer seldom spends the night with his eye
+  glued to the great telescope. He puts a photographic plate there. The
+  photographic plate has this advantage over the eye, that it builds up
+  impressions. However long we stare at an object too faint to be seen, we
+  shall never see it. With the photographic plate, however, faint
+  impressions go on accumulating. As hour after hour passes, the star
+  which was too faint to make a perceptible impression on the plate goes
+  on affecting it until finally it makes an impression which can be made
+  visible. In this way the photographic plate reveals to us phenomena in
+  the heavens which cannot be seen even through the most powerful
+  telescopes.
+  Telescopes of the kind we have been discussing, telescopes for exploring
+  the heavens, are mounted _equatorially_; that is to say, they are
+  mounted on an inclined pillar parallel to the axis of the earth so that,
+  by rotating round this pillar, the telescope is enabled to follow the
+  apparent motion of a star due to the rotation of the earth. This motion
+  is effected by clock-work, so that, once adjusted on a star, and the
+  clock-work started, the telescope remains adjusted on that star for any
+  length of time that is desired. But a great official observatory, such
+  as Greenwich Observatory or the Observatory at Paris, also has _transit_
+  instruments, or telescopes smaller than the equatorials and without the
+  same facility of movement, but which, by a number of exquisite
+  refinements, are more adapted to accurate measurements. It is these
+  instruments which are chiefly used in the compilation of the _Nautical
+  Almanac_. They do not follow the apparent motions of the stars. Stars
+  are allowed to drift across the field of vision, and as each star
+  crosses a small group of parallel wires in the eye-piece its precise
+  time of passage is recorded. Owing to their relative fixity of position
+  these instruments can be constructed to record the _positions_ of stars
+  with much greater accuracy than is possible to the more general and
+  flexible mounting of equatorials. The recording of transit is
+  comparatively dry work; the spectacular element is entirely absent;
+  stars are treated merely as mathematical points. But these observations
+  furnish the very basis of modern mathematical astronomy, and without
+  them such publications as the _Nautical Almanac_ and the _Connaissance
+  du Temps_ would be robbed of the greater part of their importance.
+  The Spectroscope
+  We have already learnt something of the principles of the spectroscope,
+  the instrument which, by making it possible to learn the actual
+  constitution of the stars, has added a vast new domain to astronomy. In
+  the simplest form of this instrument the analysing portion consists of a
+  single prism. Unless the prism is very large, however, only a small
+  degree of dispersion is obtained. It is obviously desirable, for
+  accurate analytical work, that the dispersion--that is, the separation
+  of the different parts of the spectrum--should be as great as possible.
+  The dispersion can be increased by using a large number of prisms, the
+  light emerging from the first prism, entering the second, and so on. In
+  this way each prism produces its own dispersive effect and, when a
+  number of prisms are employed, the final dispersion is considerable. A
+  considerable amount of light is absorbed in this way, however, so that
+  unless our primary source of light is very strong, the final spectrum
+  will be very feeble and hard to decipher.
+  Another way of obtaining considerable dispersion is by using a
+  _diffraction grating_ instead of a prism. This consists essentially of a
+  piece of glass on which lines are ruled by a diamond point. When the
+  lines are sufficiently close together they split up light falling on
+  them into its constituents and produce a spectrum. The modern
+  diffraction grating is a truly wonderful piece of work. It contains
+  several thousands of lines to the inch, and these lines have to be
+  spaced with the greatest accuracy. But in this instrument, again, there
+  is a considerable loss of light.
+  We have said that every substance has its own distinctive spectrum, and
+  it might be thought that, when a list of the spectra of different
+  substances has been prepared, spectrum analysis would become perfectly
+  straightforward. In practice, however, things are not quite so simple.
+  The spectrum emitted by a substance is influenced by a variety of
+  conditions. The pressure, the temperature, the state of motion of the
+  object we are observing, all make a difference, and one of the most
+  laborious tasks of the modern spectroscopist is to disentangle these
+  effects from one another. Simple as it is in its broad outlines,
+  spectroscopy is, in reality, one of the most intricate branches of
+  modern science.
+  BIBLIOGRAPHY
+  (The following list of books may be useful to readers wishing to pursue
+   further the study of Astronomy.)
+  BALL, _The Story of the Heavens_.
+  BALL, _The Story of the Sun_.
+  FORBES, _History of Astronomy_.
+  HINCKS, _Astronomy_.
+  KIPPAX, _Call of the Stars_.
+  LOWELL, _Mars and Its Canals_.
+  LOWELL, _Evolution of Worlds_.
+  MCKREADY, _A Beginner's Star-Book_.
+  NEWCOMB, _Popular Astronomy_.
+  NEWCOMB, _The Stars: A Study of the Universe_.
+  OLCOTT, _Field Book of the Stars_.
+  PRICE, _Essence of Astronomy_.
+  SERVISS, _Curiosities of the Skies_.
+  WEBB, _Celestial Objects for Common Telescopes_.
+  YOUNG, _Text-Book of General Astronomy_.
+  II
+  THE STORY OF EVOLUTION
+  INTRODUCTORY
+  THE BEGINNING OF THE EARTH--MAKING A HOME FOR LIFE--THE FIRST LIVING
+  CREATURES
+  The Evolution-idea is a master-key that opens many doors. It is a
+  luminous interpretation of the world, throwing the light of the past
+  upon the present. Everything is seen to be an antiquity, with a history
+  behind it--a _natural history_, which enables us to understand in some
+  measure how it has come to be as it is. We cannot say more than
+  "understand in some measure," for while the _fact_ of evolution is
+  certain, we are only beginning to discern the _factors_ that have been
+  at work.
+  The evolution-idea is very old, going back to some of the Greek
+  philosophers, but it is only in modern times that it has become an
+  essential part of our mental equipment. It is now an everyday
+  intellectual tool. It was applied to the origin of the solar system and
+  to the making of the earth before it was applied to plants and animals;
+  it was extended from these to man himself; it spread to language, to
+  folk-ways, to institutions. Within recent years the evolution-idea has
+  been applied to the chemical elements, for it appears that uranium may
+  change into radium, that radium may produce helium, and that lead is the
+  final stable result when the changes of uranium are complete. Perhaps
+  all the elements may be the outcome of an inorganic evolution. Not less
+  important is the extension of the evolution-idea to the world within as
+  well as to the world without. For alongside of the evolution of bodies
+  and brains is the evolution of feelings and emotions, ideas and
+  imagination.
+  Organic evolution means that the present is the child of the past and
+  the parent of the future. It is not a power or a principle; it is a
+  process--a process of becoming. It means that the present-day animals
+  and plants and all the subtle inter-relations between them have arisen
+  in a natural knowable way from a preceding state of affairs on the whole
+  somewhat simpler, and that again from forms and inter-relations simpler
+  still, and so on backwards and backwards for millions of years till we
+  lose all clues in the thick mist that hangs over life's beginnings.
+  Our solar system was once represented by a nebula of some sort, and we
+  may speak of the evolution of the sun and the planets. But since it has
+  been _the same material throughout_ that has changed in its distribution
+  and forms, it might be clearer to use some word like genesis. Similarly,
+  our human institutions were once very different from what they are now,
+  and we may speak of the evolution of government or of cities. But Man
+  works with a purpose, with ideas and ideals in some measure controlling
+  his actions and guiding his achievements, so that it is probably clearer
+  to keep the good old word history for all processes of social becoming
+  in which man has been a conscious agent. Now between the genesis of the
+  solar system and the history of civilisation there comes the vast
+  process of organic evolution. The word development should be kept for
+  the becoming of the individual, the chick out of the egg, for instance.
+  Organic evolution is a continuous natural process of racial change, by
+  successive steps in a definite direction, whereby distinctively new
+  individualities arise, take root, and flourish, sometimes alongside of,
+  and sometimes, sooner or later, in place of, the originative stock. Our
+  domesticated breeds of pigeons and poultry are the results of
+  evolutionary change whose origins are still with us in the Rock Dove and
+  the Jungle Fowl; but in most cases in Wild Nature the ancestral stocks
+  of present-day forms are long since extinct, and in many cases they are
+  unknown. Evolution is a long process of coming and going, appearing and
+  disappearing, a long-drawn-out sublime process like a great piece of
+  music.
+  [Illustration: _Photo: Rischgitz Collection._
+  CHARLES DARWIN
+  Greatest of naturalists, who made the idea of evolution current
+  intellectual coin, and in his _Origin of Species_ (1859) made the whole
+  world new.]
+  [Illustration: _Photo: Rischgitz Collection._
+  LORD KELVIN
+  One of the greatest physicists of the nineteenth century. He estimated
+  the age of the earth at 20,000,000 years. He had not at his disposal,
+  however, the knowledge of recent discoveries, which have resulted in
+  this estimate being very greatly increased.]
+  [Illustration: _Photo: Lick Observatory._
+  A GIANT SPIRAL NEBULA
+  Laplace's famous theory was that the planets and the earth were formed
+  from great whirling nebulæ.]
+  [Illustration: _Photo: Natural History Museum._
+  METEORITE WHICH FELL NEAR SCARBOROUGH, AND IS NOW TO BE SEEN IN THE
+  NATURAL HISTORY MUSEUM
+  It weighs about 56 lb., and is a "stony" meteorite, i.e., an aerolite.]
+  The Beginning of the Earth
+  When we speak the language of science we cannot say "In the beginning,"
+  for we do not know of and cannot think of any condition of things that
+  did not arise from something that went before. But we may qualify the
+  phrase, and legitimately inquire into the beginning of the earth within
+  the solar system. If the result of this inquiry is to trace the sun and
+  the planets back to a nebula we reach only a relative beginning. The
+  nebula has to be accounted for. And even before matter there may have
+  been a pre-material world. If we say, as was said long ago, "In the
+  beginning was Mind," we may be expressing or trying to express a great
+  truth, but we have gone BEYOND SCIENCE.
+  The Nebular Hypothesis
+  One of the grandest pictures that the scientific mind has ever thrown
+  upon the screen is that of the Nebular Hypothesis. According to
+  Laplace's famous form of this theory (1796), the solar system was once a
+  gigantic glowing mass, spinning slowly and uniformly around its centre.
+  As the incandescent world-cloud of gas cooled and its speed of rotation
+  increased the shrinking mass gave off a separate whirling ring, which
+  broke up and gathered together again as the first and most distant
+  planet. The main mass gave off another ring and another till all the
+  planets, including the earth, were formed. The central mass persisted as
+  the sun.
+  Laplace spoke of his theory, which Kant had anticipated forty-one years
+  before, with scientific caution: "conjectures which I present with all
+  the distrust which everything not the result of observation or of
+  calculation ought to inspire." Subsequent research justified his
+  distrust, for it has been shown that the original nebula need not have
+  been hot and need not have been gaseous. Moreover, there are great
+  difficulties in Laplace's theory of the separation of successive rings
+  from the main mass, and of the condensation of a whirling gaseous ring
+  into a planet.
+  So it has come about that the picture of a hot gaseous nebula revolving
+  as a unit body has given place to other pictures. Thus Sir Norman
+  Lockyer pointed out (1890) that the earth is gathering to itself
+  millions of meteorites every day; this has been going on for millions of
+  years; in distant ages the accretion may have been vastly more rapid and
+  voluminous; and so the earth has grown! Now the meteoritic contributions
+  are undoubted, but they require a centre to attract them, and the
+  difficulty is to account for the beginning of a collecting centre or
+  planetary nucleus. Moreover, meteorites are sporadic and erratic,
+  scattered hither and thither rather than collecting into unit-bodies. As
+  Professor Chamberlin says, "meteorites have rather the characteristics
+  of the wreckage of some earlier organisation than of the parentage of
+  our planetary system." Several other theories have been propounded to
+  account for the origin of the earth, but the one that has found most
+  favour in the eyes of authorities is that of Chamberlin and Moulton.
+  According to this theory a great nebular mass condensed to form the sun,
+  from which under the attraction of passing stars planet after planet,
+  the earth included, was heaved off in the form of knotted spiral nebulæ,
+  like many of those now observed in the heavens.
+  Of great importance were the "knots," for they served as collecting
+  centres drawing flying matter into their clutches. Whatever part of the
+  primitive bolt escaped and scattered was drawn out into independent
+  orbits round the sun, forming the "planetesimals" which behave like
+  minute planets. These planetesimals formed the food on which the knots
+  subsequently fed.
+  The Growth of the Earth
+  It has been calculated that the newborn earth--the "earth-knot" of
+  Chamberlin's theory--had a diameter of about 5,500 miles. But it grew
+  by drawing planetesimals into itself until it had a diameter of over
+  8,100 miles at the end of its growing period. Since then it has shrunk,
+  by periodic shrinkages which have meant the buckling up of successive
+  series of mountains, and it has now a diameter of 7,918 miles. But
+  during the shrinking the earth became more varied.
+  A sort of slow boiling of the internally hot earth often forced molten
+  matter through the cold outer crust, and there came about a gradual
+  assortment of lighter materials nearer the surface and heavier materials
+  deeper down. The continents are built of the lighter materials, such as
+  granites, while the beds of the great oceans are made of the heavier
+  materials such as basalts. In limited areas land has often become sea,
+  and sea has often given place to land, but the probability is that the
+  distinction of the areas corresponding to the great continents and
+  oceans goes back to a very early stage.
+  The lithosphere is the more or less stable crust of the earth, which may
+  have been, to begin with, about fifty miles in thickness. It seems that
+  the young earth had no atmosphere, and that ages passed before water
+  began to accumulate on its surface--before, in other words, there was
+  any hydrosphere. The water came from the earth itself, to begin with,
+  and it was long before there was any rain dissolving out saline matter
+  from the exposed rocks and making the sea salt. The weathering of the
+  high grounds of the ancient crust by air and water furnished the
+  material which formed the sandstones and mudstones and other sedimentary
+  rocks, which are said to amount to a thickness of over fifty miles in
+  all.
+  Making a Home for Life
+  It is interesting to inquire how the callous, rough-and-tumble
+  conditions of the outer world in early days were replaced by others that
+  allowed of the germination and growth of that tender plant we call
+  LIFE. There are very tough living creatures, but the average organism is
+  ill suited for violence. Most living creatures are adapted to mild
+  temperatures and gentle reactions. Hence the fundamental importance of
+  the early atmosphere, heavy with planetesimal dust, in blanketing the
+  earth against intensities of radiance from without, as Chamberlin says,
+  and inequalities of radiance from within. This was the first preparation
+  for life, but it was an atmosphere without free oxygen. Not less
+  important was the appearance of pools and lakelets, of lakes and seas.
+  Perhaps the early waters covered the earth. And water was the second
+  preparation for life--water, that can dissolve a larger variety of
+  substances in greater concentration than any other liquid; water, that
+  in summer does not readily evaporate altogether from a pond, nor in
+  winter freeze throughout its whole extent; water, that is such a mobile
+  vehicle and such a subtle cleaver of substances; water, that forms over
+  80 per cent. of living matter itself.
+  Of great significance was the abundance of carbon, hydrogen, and oxygen
+  (in the form of carbonic acid and water) in the atmosphere of the
+  cooling earth, for these three wonderful elements have a unique
+  _ensemble_ of properties--ready to enter into reactions and relations,
+  making great diversity and complexity possible, favouring the formation
+  of the plastic and permeable materials that build up living creatures.
+  We must not pursue the idea, but it is clear that the stones and mortar
+  of the inanimate world are such that they built a friendly home for
+  life.
+  Origin of Living Creatures upon the Earth
+  During the early chapters of the earth's history, no living creature
+  that we can imagine could possibly have lived there. The temperature was
+  too high; there was neither atmosphere nor surface water. Therefore it
+  follows that at some uncertain, but inconceivably distant date, living
+  creatures appeared upon the earth. No one knows how, but it is
+  interesting to consider possibilities.
+  [Illustration: _Reproduced from the Smithsonian Report, 1915._
+  A LIMESTONE CANYON
+  Many fossils of extinct animals have been found in such rock
+  formations.]
+  [Illustration: GENEALOGICAL TREE OF ANIMALS
+  Showing in order of evolution the general relations of the chief classes
+  into which the world of living things is divided. This scheme represents
+  the present stage of our knowledge, but is admittedly provisional.]
+  [Illustration: DIAGRAM OF AMOEBA
+(Greatly magnified.)
+  The amoeba is one of the simplest of all animals, and gives us a hint
+  of the original ancestors. It looks like a tiny irregular speck of
+  greyish jelly, about 1/100th of an inch in diameter. It is commonly
+  found gliding on the mud or weeds in ponds, where it engulfs its
+  microscopic food by means of out-flowing lobes (PS). The food vacuole
+  (FV) contains ingested food. From the contractile vacuole (CV) the waste
+  matter is discharged. N is the nucleus, GR, granules.]
+  From ancient times it has been a favourite answer that the dust of the
+  earth may have become living in a way which is outside scientific
+  description. This answer forecloses the question, and it is far too soon
+  to do that. Science must often say "Ignoramus": Science should be slow
+  to say "Ignorabimus."
+  A second position held by Helmholtz, Lord Kelvin, and others, suggests
+  that minute living creatures may have come to the earth from elsewhere,
+  in the cracks of a meteorite or among cosmic dust. It must be remembered
+  that seeds can survive prolonged exposure to very low temperatures; that
+  spores of bacteria can survive high temperature; that seeds of plants
+  and germs of animals in a state of "latent life" can survive prolonged
+  drought and absence of oxygen. It is possible, according to Berthelot,
+  that as long as there is not molecular disintegration vital activities
+  may be suspended for a time, and may afterwards recommence when
+  appropriate conditions are restored. Therefore, one should be slow to
+  say that a long journey through space is impossible. The obvious
+  limitation of Lord Kelvin's theory is that it only shifts the problem of
+  the origin of organisms (i.e. living creatures) from the earth to
+  elsewhere.
+  The third answer is that living creatures of a very simple sort may have
+  emerged on the earth's surface from not-living material, e.g. from some
+  semi-fluid carbon compounds activated by ferments. The tenability of
+  this view is suggested by the achievements of the synthetic chemists,
+  who are able artificially to build up substances such as oxalic acid,
+  indigo, salicylic acid, caffeine, and grape-sugar. We do not know,
+  indeed, what in Nature's laboratory would take the place of the clever
+  synthetic chemist, but there seems to be a tendency to complexity.
+  Corpuscles form atoms, atoms form molecules, small molecules large
+  ones.
+  Various concrete suggestions have been made in regard to the possible
+  origin of living matter, which will be dealt with in a later chapter. So
+  far as we know of what goes on to-day, there is no evidence of
+  spontaneous generation; organisms seem always to arise from pre-existing
+  organisms of the same kind; where any suggestion of the contrary has
+  been fancied, there have been flaws in the experimenting. But it is one
+  thing to accept the verdict "omne vivum e vivo" as a fact to which
+  experiment has not yet discovered an exception and another thing to
+  maintain that this must always have been true or must always remain
+  true.
+  If the synthetic chemists should go on surpassing themselves, if
+  substances like white of egg should be made artificially, and if we
+  should get more light on possible steps by which simple living creatures
+  may have arisen from not-living materials, this would not greatly affect
+  our general outlook on life, though it would increase our appreciation
+  of what is often libelled as "inert" matter. If the dust of the earth
+  did naturally give rise very long ago to living creatures, if they are
+  in a real sense born of her and of the sunshine, then the whole world
+  becomes more continuous and more vital, and all the inorganic groaning
+  and travailing becomes more intelligible.
+  The First Organisms upon the Earth
+  We cannot have more than a speculative picture of the first living
+  creatures upon the earth or, rather, in the waters that covered the
+  earth. A basis for speculation is to be found, however, in the simplest
+  creatures living to-day, such as some of the bacteria and one-celled
+  animalcules, especially those called Protists, which have not taken any
+  very definite step towards becoming either plants or animals. No one can
+  be sure, but there is much to be said for the theory that the first
+  creatures were microscopic globules of living matter, not unlike the
+  simplest bacteria of to-day, but able to live on air, water, and
+  dissolved salts. From such a source may have originated a race of
+  one-celled marine organisms which were able to manufacture chlorophyll,
+  or something like chlorophyll, that is to say, the green pigment which
+  makes it possible for plants to utilise the energy of the sunlight in
+  breaking up carbon dioxide and in building up (photosynthesis) carbon
+  compounds like sugars and starch. These little units were probably
+  encased in a cell-wall of cellulose, but their boxed-in energy expressed
+  itself in the undulatory movement of a lash or flagellum, by means of
+  which they propelled themselves energetically through the water. There
+  are many similar organisms to-day, mostly in water, but some of
+  them--simple one-celled plants--paint the tree-stems and even the
+  paving-stones green in wet weather. According to Prof. A. H. Church
+  there was a long chapter in the history of the earth when the sea that
+  covered everything teemed with these green flagellates--the originators
+  of the Vegetable Kingdom.
+  On another tack, however, there probably evolved a series of simple
+  predatory creatures, not able to build up organic matter from air,
+  water, and salts, but devouring their neighbours. These units were not
+  closed in with cellulose, but remained naked, with their living matter
+  or protoplasm flowing out in changeful processes, such as we see in the
+  Amoebæ in the ditch or in our own white blood corpuscles and other
+  amoeboid cells. These were the originators of the animal kingdom. Thus
+  from very simple Protists the first animals and the first plants may
+  have arisen. All were still very minute, and it is worth remembering
+  that had there been any scientific spectator after our kind upon the
+  earth during these long ages, he would have lamented the entire absence
+  of life, although the seas were teeming. The simplest forms of life and
+  the protoplasm which Huxley called the physical basis of life will be
+  dealt with in the chapter on Biology in a later section of this work.
+  FIRST GREAT STEPS IN EVOLUTION
+  THE FIRST PLANTS--THE FIRST ANIMALS--BEGINNINGS OF BODIES--EVOLUTION OF
+  SEX--BEGINNING OF NATURAL DEATH
+  The Contrast between Plants and Animals
+  However it may have come about, there is no doubt at all that one of the
+  first great steps in Organic Evolution was the forking of the
+  genealogical tree into Plants and Animals--the most important parting of
+  the ways in the whole history of Nature.
+  Typical plants have chlorophyll; they are able to feed at a low chemical
+  level on air, water, and salts, using the energy of the sunlight in
+  their photosynthesis. They have their cells boxed in by cellulose walls,
+  so that their opportunities for motility are greatly restricted. They
+  manufacture much more nutritive material than they need, and live far
+  below their income. They have no ready way of getting rid of any
+  nitrogenous waste matter that they may form, and this probably helps to
+  keep them sluggish.
+  Animals, on the other hand, feed at a high chemical level, on the
+  carbohydrates (e.g. starch and sugar), fats, and proteins (e.g. gluten,
+      albumin, casein) which are manufactured by other animals, or to begin
+  with, by plants. Their cells have not cellulose walls, nor in most cases
+  much wall of any kind, and motility in the majority is unrestricted.
+  Animals live much more nearly up to their income. If we could make for
+  an animal and a plant of equal weight two fractions showing the ratio of
+  the upbuilding, constructive, chemical processes to the down-breaking,
+  disruptive, chemical processes that go on in their respective bodies,
+  the ratio for the plant would be much greater than the corresponding
+  ratio for the animal. In other words, animals take the munitions which
+  plants laboriously manufacture and explode them in locomotion and
+  work; and the entire system of animate nature depends upon the
+  photosynthesis that goes on in green plants.
+  [Illustration: _From the Smithsonian Report, 1917_
+  A PIECE OF A REEF-BUILDING CORAL, BUILT UP BY A LARGE COLONY OF SMALL
+  SEA-ANEMONE-LIKE POLYPS, EACH OF WHICH FORMS FROM THE SALTS OF THE SEA A
+  SKELETON OR SHELL OF LIME
+  The wonderful mass of corals, which are very beautiful, are the skeleton
+  remains of hundreds of these little creatures.]
+  [Illustration: _Photo: J. J. Ward, F.E.S._
+  THE INSET CIRCLE SHOWS A GROUP OF CHALK-FORMING ANIMALS, OR
+  FORAMINIFERA, EACH ABOUT THE SIZE OF A VERY SMALL PIN'S HEAD
+  They form a great part of the chalk cliffs of Dover and similar deposits
+  which have been raised from the floor of an ancient sea.
+  THE ENORMOUSLY ENLARGED ILLUSTRATION IS THAT OF A COMMON FORAMINIFER
+  (POLYSTOMELLA) SHOWING THE SHELL IN THE CENTRE AND THE OUTFLOWING
+  NETWORK OF LIVING MATTER, ALONG WHICH GRANULES ARE CONTINUALLY
+  TRAVELLING, AND BY WHICH FOOD PARTICLES ARE ENTANGLED AND DRAWN IN
+  _Reproduced by permission of the Natural History Museum_ (_after Max
+      Schultze_).]
+  As the result of much more explosive life, animals have to deal with
+  much in the way of nitrogenous waste products, the ashes of the living
+  fire, but these are usually got rid of very effectively, e.g. in the
+  kidney filters, and do not clog the system by being deposited as
+  crystals and the like, as happens in plants. Sluggish animals like
+  sea-squirts which have no kidneys are exceptions that prove the rule,
+  and it need hardly be said that the statements that have been made in
+  regard to the contrasts between plants and animals are general
+  statements. There is often a good deal of the plant about the animal, as
+  in sedentary sponges, zoophytes, corals, and sea-squirts, and there is
+  often a little of the animal about the plant, as we see in the movements
+  of all shoots and roots and leaves, and occasionally in the parts of the
+  flower. But the important fact is that on the early forking of the
+  genealogical tree, i.e. the divergence of plants and animals, there
+  depended and depends all the higher life of the animal kingdom, not to
+  speak of mankind. The continuance of civilisation, the upkeep of the
+  human and animal population of the globe, and even the supply of oxygen
+  to the air we breathe, depend on the silent laboratories of the green
+  leaves, which are able with the help of the sunlight to use carbonic
+  acid, water, and salts to build up the bread of life.
+  The Beginnings of Land Plants
+  It is highly probable that for long ages the waters covered the earth,
+  and that all the primeval vegetation consisted of simple Flagellates in
+  the universal Open Sea. But contraction of the earth's crust brought
+  about elevations and depressions of the sea-floor, and in places the
+  solid substratum was brought near enough the surface to allow the
+  floating plants to begin to settle down without getting out of the
+  light. This is how Professor Church pictures the beginning of a fixed
+  vegetation--a very momentous step in evolution. It was perhaps among
+  this early vegetation that animals had their first successes. As the
+  floor of the sea in these shallow areas was raised higher and higher
+  there was a beginning of dry land. The sedentary plants already spoken
+  of were the ancestors of the shore seaweeds, and there is no doubt that
+  when we go down at the lowest tide and wade cautiously out among the
+  jungle of vegetation only exposed on such occasions we are getting a
+  glimpse of very ancient days. _This_ is the forest primeval.
+  The Protozoa
+  Animals below the level of zoophytes and sponges are called Protozoa.
+  The word obviously means "First Animals," but all that we can say is
+  that the very simplest of them may give us some hint of the simplicity
+  of the original first animals. For it is quite certain that the vast
+  majority of the Protozoa to-day are far too complicated to be thought of
+  as primitive. Though most of them are microscopic, each is an animal
+  complete in itself, with the same fundamental bodily attributes as are
+  manifested in ourselves. They differ from animals of higher degree in
+  not being built up of the unit areas or corpuscles called cells. They
+  have no cells, no tissues, no organs, in the ordinary acceptation of
+  these words, but many of them show a great complexity of internal
+  structure, far exceeding that of the ordinary cells that build up the
+  tissues of higher animals. They are complete living creatures which have
+  not gone in for body-making.
+  In the dim and distant past there was a time when the only animals were
+  of the nature of Protozoa, and it is safe to say that one of the great
+  steps in evolution was the establishment of three great types of
+  Protozoa: (_a_) Some were very active, the Infusorians, like the slipper
+  animalcule, the night-light (Noctiluca), which makes the seas
+  phosphorescent at night, and the deadly Trypanosome, which causes
+  Sleeping Sickness. (_b_) Others were very sluggish, the parasitic
+  Sporozoa, like the malaria organism which the mosquito introduces into
+  man's body. (_c_) Others were neither very active nor very passive, the
+  Rhizopods, with out-flowing processes of living matter. This amoeboid
+  line of evolution has been very successful; it is represented by the
+  Rhizopods, such as Amoebæ and the chalk-forming Foraminifera and the
+  exquisitely beautiful flint-shelled Radiolarians of the open sea. They
+  have their counterparts in the amoeboid cells of most multicellular
+  animals, such as the phagocytes which migrate about in the body,
+  engulfing and digesting intruding bacteria, serving as sappers and
+  miners when something has to be broken down and built up again, and
+  performing other useful offices.
+  The Making of a Body
+  The great naturalist Louis Agassiz once said that the biggest gulf in
+  Organic Nature was that between the unicellular and the multicellular
+  animals (Protozoa and Metazoa). But the gulf was bridged very long ago
+  when sponges, stinging animals, and simple worms were evolved, and
+  showed, for the first time, a "body." What would one not give to be able
+  to account for the making of a body, one of the great steps in
+  evolution! No one knows, but the problem is not altogether obscure.
+  When an ordinary Protozoon or one-celled animal divides into two or
+  more, which is its way of multiplying, the daughter-units thus formed
+  float apart and live independent lives. But there are a few Protozoa in
+  which the daughter-units are not quite separated off from one another,
+  but remain coherent. Thus Volvox, a beautiful green ball, found in some
+  canals and the like, is a colony of a thousand or even ten thousand
+  cells. It has almost formed a body! But in this "colony-making"
+  Protozoon, and in others like it, the component cells are all of one
+  kind, whereas in true multicellular animals there are different kinds
+  of cells, showing division of labour. There are some other Protozoa in
+  which the nucleus or kernel divides into many nuclei within the cell.
+  This is seen in the Giant Amoeba (Pelomyxa), sometimes found in
+  duck-ponds, or the beautiful Opalina, which always lives in the hind
+  part of the frog's food-canal. If a portion of the living matter of
+  these Protozoa should gather round each of the nuclei, then _that would
+  be the beginning of a body_. It would be still nearer the beginning of a
+  body if division of labour set in, and if there was a setting apart of
+  egg-cells and sperm-cells distinct from body-cells.
+  It was possibly in some such way that animals and plants with a body
+  were first evolved. Two points should be noticed, that body-making is
+  not essentially a matter of size, though it made large size possible.
+  For the body of a many-celled Wheel Animalcule or Rotifer is no bigger
+  than many a Protozoon. Yet the Rotifer--we are thinking of Hydatina--has
+  nine hundred odd cells, whereas the Protozoon has only one, except in
+  forms like Volvox. Secondly, it is a luminous fact that _every
+  many-celled animal from sponge to man that multiplies in the ordinary
+  way begins at the beginning again as a "single cell,"_ the fertilised
+  egg-cell. It is, of course, not an ordinary single cell that develops
+  into an earthworm or a butterfly, an eagle, or a man; it is a cell in
+  which a rich inheritance, the fruition of ages, is somehow condensed;
+  but it is interesting to bear in mind the elementary fact that every
+  many-celled creature, reproduced in the ordinary way and not by budding
+  or the like, starts as a fertilised egg-cell. The coherence of the
+  daughter-cells into which the fertilised egg-cell divides is a
+  reminiscence, as it were, of the primeval coherence of daughter-units
+  that made the first body possible.
+  The Beginning of Sexual Reproduction
+  A freshwater Hydra, growing on the duckweed usually multiplies by
+  budding. It forms daughter-buds, living images of itself; a check comes
+  to nutrition and these daughter-buds go free. A big sea-anemone may
+  divide in two or more parts, which become separate animals. This is
+  asexual reproduction, which means that the multiplication takes place by
+  dividing into two or many portions, and not by liberating egg-cells and
+  sperm-cells. Among animals as among plants, asexual reproduction is very
+  common. But it has great disadvantages, for it is apt to be
+  physiologically expensive, and it is beset with difficulties when the
+  body shows great division of labour, and is very intimately bound into
+  unity. Thus, no one can think of a bee or a bird multiplying by division
+  or by budding. Moreover, if the body of the parent has suffered from
+  injury or deterioration, the result of this is bound to be handed on to
+  the next generation if asexual reproduction is the only method.
+  [Illustration: _Photos: J. J. Ward, F.E.S._
+  A PLANT-LIKE ANIMAL, OR ZOOPHYTE, CALLED OBELIA
+  Consisting of a colony of small polyps, whose stinging tentacles are
+  well shown greatly enlarged in the lower photograph.]
+  [Illustration: _Reproduced by permission of "The Quart. Journ. Mic.
+  Sci."_
+  TRYPANOSOMA GAMBIENSE
+(Very highly magnified.)
+  The microscopic animal Trypanosome, which causes Sleeping Sickness. The
+  study of these organisms has of late years acquired an immense
+  importance on account of the widespread and dangerous maladies to which
+  some of them give rise. It lives in the blood of man, who is infected by
+  the bite of a Tse-tse fly which carries the parasite from some other
+  host.]
+  [Illustration: VOLVOX
+  The Volvox is found in some canals and the like. It is one of the first
+  animals to suggest the beginning of a body. It is a colony of a thousand
+  or even ten thousand cells, but they are all cells of one kind. In
+  _multicellular_ animals the cells are of _different_ kinds with
+  different functions. Each of the ordinary cells (marked 5) has two
+  lashes or flagella. Daughter colonies inside the Parent colony are being
+  formed at 3, 4, and 2. The development of germ-cells is shown at 1.]
+  [Illustration: PROTEROSPONGIA
+  One of the simplest multicellular animals, illustrating the beginning of
+  a body. There is a setting apart of egg-cells and sperm-cells, distinct
+  from body-cells; the collared lashed cells on the margin are different
+  in kind from those farther in. Thus, as in indubitable multicellular
+  animals, division of labour has begun.]
+  Splitting into two or many parts was the old-fashioned way of
+  multiplying, but one of the great steps in evolution was the discovery
+  of a better method, namely, sexual reproduction. The gist of this is
+  simply that during the process of body-building (by the development of
+      the fertilised egg-cell) certain units, _the germ-cells_, do not share
+  in forming ordinary tissues or organs, but remain apart, continuing the
+  full inheritance which was condensed in the fertilised egg-cell. _These
+  cells kept by themselves are the originators of the future reproductive
+  cells of the mature animal_; they give rise to the egg-cells and the
+  sperm-cells.
+  The advantages of this method are great. (1) The new generation is
+  started less expensively, for it is easier to shed germ-cells into the
+  cradle of the water than to separate off half of the body. (2) It is
+  possible to start a great many new lives at once, and this may be of
+  vital importance when the struggle for existence is very keen, and when
+  parental care is impossible. (3) The germ-cells are little likely to be
+  prejudicially affected by disadvantageous dints impressed on the body of
+  the parent--little likely unless the dints have peculiarly penetrating
+  consequences, as in the case of poisons. (4) A further advantage is
+  implied in the formation of two kinds of germ-cells--the ovum or
+  egg-cell, with a considerable amount of building material and often with
+  a legacy of nutritive yolk; the spermatozoon or sperm-cell, adapted to
+  move in fluids and to find the ovum from a distance, thus securing
+  change-provoking cross-fertilisation.
+  The Evolution of Sex
+  Another of the great steps in organic evolution was the differentiation
+  of two different physiological types, the male or sperm-producer and the
+  female or egg-producer. It seems to be a deep-seated difference in
+  constitution, which leads one egg to develop into a male, and another,
+  lying beside it in the nest, into a female. In the case of pigeons it
+  seems almost certain, from the work of Professor Oscar Riddle, that
+  there are two kinds of egg, a male-producing egg and a female-producing
+  egg, which differ in their yolk-forming and other physiological
+  characters.
+  In sea-urchins we often find two creatures superficially
+  indistinguishable, but the one is a female with large ovaries and the
+  other is a male with equally large testes. Here the physiological
+  difference does not affect the body as a whole, but the reproductive
+  organs or gonads only, though more intimate physiology would doubtless
+  discover differences in the blood or in the chemical routine
+  (metabolism). In a large number of cases, however, there are marked
+  superficial differences between the sexes, and everyone is familiar with
+  such contrasts as peacock and peahen, stag and hind. In such cases the
+  physiological difference between the sperm-producer and the
+  ovum-producer, for this is the essential difference, saturates through
+  the body and expresses itself in masculine and feminine structures and
+  modes of behaviour. The expression of the masculine and feminine
+  characters is in some cases under the control of hormones or chemical
+  messengers which are carried by the blood from the reproductive organs
+  throughout the body, and pull the trigger which brings about the
+  development of an antler or a wattle or a decorative plume or a capacity
+  for vocal and saltatory display. In some cases it is certain that the
+  female carries in a latent state the masculine features, but these are
+  kept from expressing themselves by other chemical messengers from the
+  ovary. Of these chemical messengers more must be said later on.
+  Recent research has shown that while the difference between male and
+  female is very deep-rooted, corresponding to a difference in gearing, it
+  is not always clear-cut. Thus a hen-pigeon may be very masculine, and a
+  cock-pigeon very feminine. The difference is in degree, not in kind.
+  What is the meaning of the universal or almost universal inevitableness
+  of death? A Sequoia or "Big Tree" of California has been known to live
+  for over two thousand years, but eventually it died. A centenarian
+  tortoise has been known, and a sea-anemone sixty years of age; but
+  eventually they die. What is the meaning of this apparently inevitable
+  stoppage of bodily life?
+  The Beginning of Natural Death
+  There are three chief kinds of death, (_a_) The great majority of
+  animals come to a violent end, being devoured by others or killed by
+  sudden and extreme changes in their surroundings. (_b_) When an animal
+  enters a new habitat, or comes into new associations with other
+  organisms, it may be invaded by a microbe or by some larger parasite to
+  which it is unaccustomed and to which it can offer no resistance. With
+  many parasites a "live-and-let-live" compromise is arrived at, but new
+  parasites are apt to be fatal, as man knows to his cost when he is
+  bitten by a tse-tse fly which infects him with the microscopic animal (a
+      Trypanosome) that causes Sleeping Sickness. In many animals the
+  parasites are not troublesome as long as the host is vigorous, but if
+  the host is out of condition the parasites may get the upper hand, as in
+  the so-called "grouse disease," and become fatal. (_c_) But besides
+  violent death and microbic (or parasitic) death, there is natural death.
+  This is in great part to be regarded as the price paid for a body. A
+  body worth having implies complexity or division of labour, and this
+  implies certain internal furnishings of a more or less stable kind in
+  which the effects of wear and tear are apt to accumulate. It is not the
+  living matter itself that grows old so much as the framework in which it
+  works--the furnishings of the vital laboratory. There are various
+  processes of rejuvenescence, e.g. rest, repair, change, reorganisation,
+  which work against the inevitable processes of senescence, but sooner or
+  later the victory is with ageing. Another deep reason for natural death
+  is to be found in the physiological expensiveness of reproduction, for
+  many animals, from worms to eels, illustrate natural death as the
+  nemesis of starting new lives. Now it is a very striking fact that to a
+  large degree the simplest animals or Protozoa are exempt from natural
+  death. They are so relatively simple that they can continually
+  recuperate by rest and repair; they do not accumulate any bad debts.
+  Moreover, their modes of multiplying, by dividing into two or many
+  units, are very inexpensive physiologically. It seems that in some
+  measure this bodily immortality of the Protozoa is shared by some simple
+  many-celled animals like the freshwater Hydra and Planarian worms. Here
+  is an interesting chapter in evolution, the evolution of means of
+  evading or staving off natural death. Thus there is the well-known case
+  of the Paloloworm of the coral-reefs where the body breaks up in
+  liberating the germ-cells, but the head-end remains fixed in a crevice
+  of the coral, and buds out a new body at leisure.
+  Along with the evolution of the ways of avoiding death should be
+  considered also the gradual establishment of the length of life best
+  suited to the welfare of the species, and the punctuation of the
+  life-history to suit various conditions.
+  [Illustration: _Photo: J. J. Ward, F.E.S._
+  GREEN HYDRA
+  A little freshwater polyp, about half an inch long, with a crown of
+  tentacles round the mouth. It is seen giving off a bud, a clear
+  illustration of asexual reproduction. When a tentacle touches some small
+  organism the latter is paralysed and drawn into the mouth.]
+  [Illustration: _Photo: J. J. Ward, F.E.S._
+  EARTHWORM
+  Earthworms began the profitable habit of moving with one end of the body
+  always in front, and from worms to man the great majority of animals
+  have bilateral symmetry.]
+  [Illustration: DIAGRAM ILLUSTRATING THE BEGINNING OF INDIVIDUAL LIFE
+1. An immature _sperm_-cell, with 4 chromosomes (nuclear bodies)
+  represented as rods.
+  2. A mature sperm-cell, with 2 chromosomes.
+  3. An immature _egg_-cell, with 4 chromosomes represented as curved
+  bodies.
+  4. A mature egg-cell, with 2 chromosomes.
+  5. The spermatozoon fertilises the ovum, introducing 2 chromosomes.
+  6. The fertilised ovum, with 4 chromosomes, 2 of paternal origin and 2
+  of maternal origin.
+  7. The chromosomes lie at the equator, and each is split longitudinally.
+  The centrosome introduced by the spermatozoon has divided into two
+  centrosomes, one at each pole of the nucleus. These play an important
+  part in the division or segmentation of the egg.
+  8. The fertilised egg has divided into two cells. Each cell has 2
+  paternal and 2 maternal chromosomes.]
+  [Illustration: _Reproduced from the Smithsonian Report, 1917._
+  GLASS MODEL OF A SEA-ANEMONE
+  A long tubular sea-anemone, with a fine crown of tentacles around the
+  mouth. The suggestion of a flower is very obvious. By means of stinging
+  lassoes on the tentacles minute animals on which it feeds are paralysed
+  and captured for food.]
+  [Illustration: THIS DRAWING SHOWS THE EVOLUTION OF THE BRAIN FROM FISH
+  TO MAN
+  The Cerebrum, the seat of intelligence, increases in proportion to the
+  other parts. In mammals it becomes more and more convoluted. The brain,
+  which lies in one plane in fishes, becomes gradually curved on itself.
+  In birds it is more curved than the drawing shows.]
+  Great Acquisitions
+  In animals like sea-anemones and jellyfishes the general symmetry of the
+  body is radial; that is to say, there is no right or left, and the body
+  might be halved along many planes. It is a kind of symmetry well suited
+  for sedentary or for drifting life. But worms began the profitable habit
+  of moving with one end of the body always in front, and from worms to
+  man the great majority of animals have bilateral symmetry. They have a
+  right and a left side, and there is only one cut that halves the body.
+  This kind of symmetry is suited for a more strenuous life than radial
+  animals show; it is suited for pursuing food, for avoiding enemies, for
+  chasing mates. And _with the establishment of bilateral symmetry must be
+  associated the establishment of head-brains_, the beginning of which is
+  to be found in some simple worm-types.
+  Among the other great acquisitions gradually evolved we may notice: a
+  well-developed head with sense-organs, the establishment of large
+  internal surfaces such as the digestive and absorptive wall of the
+  food-canal, the origin of quickly contracting striped muscle and of
+  muscular appendages, the formation of blood as a distributing medium
+  throughout the body, from which all the parts take what they need and to
+  which they also contribute.
+Another very important acquisition, almost confined (so far as is known)
+  to backboned animals, was the evolution of what are called glands of
+  internal secretion, such as the thyroid and the supra-renal. These
+  manufacture subtle chemical substances which are distributed by the
+  blood throughout the body, and have a manifold influence in regulating
+  and harmonising the vital processes. Some of these chemical messengers
+  are called hormones, which stimulate organs and tissues to greater
+  activity; others are called chalones, which put on a brake. Some
+  regulate growth and others rapidly alter the pressure and composition
+  of the blood. Some of them call into active development certain parts of
+  the body which have been, as it were, waiting for an appropriate
+  trigger-pulling. Thus, at the proper time, the milk-glands of a
+  mammalian mother are awakened from their dormancy. This very interesting
+  outcome of evolution will be dealt with in another portion of this work.
+  THE INCLINED PLANE OF ANIMAL BEHAVIOUR
+  Before passing to a connected story of the gradual emergence of higher
+  and higher forms of life in the course of the successive ages--the
+  procession of life, as it may be called--it will be useful to consider
+  the evolution of animal behaviour.
+  Evolution of Mind
+  A human being begins as a microscopic fertilised egg-cell, within which
+  there is condensed the long result of time--Man's inheritance. The long
+  period of nine months before birth, with its intimate partnership
+  between mother and offspring, is passed as it were in sleep, and no one
+  can make any statement in regard to the mind of the unborn child. Even
+  after birth the dawn of mind is as slow as it is wonderful. To begin
+  with, there is in the ovum and early embryo no nervous system at all,
+  and it develops very gradually from simple beginnings. Yet as mentality
+  cannot come in from outside, we seem bound to conclude that the
+  potentiality of it--whatever that means--resides in the individual from
+  the very first. The particular kind of activity known to us as thinking,
+  feeling, and willing is the most intimate part of our experience, known
+  to us directly apart from our senses, and the possibility of that must
+  be implicit in the germ-cell just as the genius of Newton was implicit
+  in a very miserable specimen of an infant. Now what is true of the
+  individual is true also of the race--there is a gradual evolution of
+  that aspect of the living creature's activity which we call mind. We
+  cannot put our finger on any point and say: Before this stage there was
+  no mind. Indeed, many facts suggest the conclusion that wherever there
+  is life there is some degree of mind--even in the plants. Or it might be
+  more accurate to put the conclusion in another way, that the activity we
+  call life has always in some degree an inner or mental aspect.
+  [Illustration: OKAPI AND GIRAFFE
+  The Okapi is one of the great zoölogical discoveries. It gives a good
+  idea of what the Giraffe's ancestors were like. The Okapi was unknown
+  until discovered in 1900 by Sir Harry Johnston in Central Africa, where
+  these strange animals have probably lived in dense forests from time
+  immemorial.]
+  In another part of this book there is an account of the dawn of mind in
+  backboned animals; what we aim at here is an outline of what may be
+  called the inclined plane of animal behaviour.
+  A very simple animal accumulates a little store of potential energy, and
+  it proceeds to expend this, like an explosive, by acting on its
+  environment. It does so in a very characteristic self-preservative
+  fashion, so that it burns without being consumed and explodes without
+  being blown to bits. It is characteristic of the organism that it
+  remains a going concern for a longer or shorter period--its length of
+  life. Living creatures that expended their energy ineffectively or
+  self-destructively would be eliminated in the struggle for existence.
+  When a simple one-celled organism explores a corner of the field seen
+  under a microscope, behaving to all appearance very like a dog scouring
+  a field seen through a telescope, it seems permissible to think of
+  something corresponding to mental endeavour associated with its
+  activity. This impression is strengthened when an amoeba pursues
+  another amoeba, overtakes it, engulfs it, loses it, pursues it again,
+  recaptures it, and so on. What is quite certain is that the behaviour of
+  the animalcule is not like that of a potassium pill fizzing about in a
+  basin of water, nor like the lurching movements of a gun that has got
+  loose and "taken charge" on board ship. Another feature is that the
+  locomotor activity of an animalcule often shows a distinct
+  individuality: it may swim, for instance, in a loose spiral.
+  But there is another side to vital activity besides acting upon the
+  surrounding world; the living creature is acted on by influences from
+  without. The organism acts on its environment; that is the one side of
+  the shield: the environment acts upon the organism; that is the other
+  side. If we are to see life whole we must recognise these two sides of
+  what we call living, and it is missing an important part of the history
+  of animal life if we fail to see that evolution implies becoming more
+  advantageously sensitive to the environment, making more of its
+  influences, shutting out profitless stimuli, and opening more gateways
+  to knowledge. The bird's world is a larger and finer world than an
+  earthworm's; the world means more to the bird than to the worm.
+  The Trial and Error Method
+  Simple creatures act with a certain degree of spontaneity on their
+  environment, and they likewise react effectively to surrounding stimuli.
+  Animals come to have definite "answers back," sometimes several,
+  sometimes only one, as in the case of the Slipper Animalcule, which
+  reverses its cilia when it comes within the sphere of some disturbing
+  influence, retreats, and, turning upon itself tentatively, sets off
+  again in the same general direction as before, but at an angle to the
+  previous line. If it misses the disturbing influence, well and good; if
+  it strikes it again, the tactics are repeated until a satisfactory way
+  out is discovered or the stimulation proves fatal.
+  It may be said that the Slipper Animalcule has but one answer to every
+  question, but there are many Protozoa which have several enregistered
+  reactions. When there are alternative reactions which are tried one
+  after another, the animal is pursuing what is called the trial-and-error
+  method, and a higher note is struck.
+  There is an endeavour after satisfaction, and a trial of answers. When
+  the creature profits by experience to the extent of giving the right
+  answer first, there is the beginning of learning.
+  [Illustration: DIAGRAM OF A SIMPLE REFLEX ARC IN A BACKBONELESS ANIMAL
+  LIKE AN EARTHWORM
+  1. A sensory nerve-cell (S.C.) on the surface receives a stimulus.
+2. The stimulus travels along the sensatory nerve-fibre (S.F.)
+  3. The sensory nerve-fibre branches in the nerve-cord.
+  4. Its branches come into close contact (SY^{1}) with those of an
+  associative or communicating nerve-cell (A.C.).
+  5. Other branches of the associative cell come into close contact
+  (SY^{2}) with the branches or dendrites of a motor nerve-cell (M.C.).
+  6. An impulse or command travels along the motor nerve-fibre or axis
+  cylinder of the motor nerve-cell.
+  7. The motor nerve-fibre ends on a muscle-fibre (M.F.) near the surface.
+  This moves and the reflex action is complete.]
+  [Illustration: _Photo: British Museum_ (_Natural History_).
+  THE YUCCA MOTH
+  The Yucca Moth, emerging from her cocoon, flies at night to a Yucca
+  flower and collects pollen from the stamens, holding a little ball of it
+  in her mouth-parts. She then visits another flower and lays an egg in
+  the seed-box. After this she applies the pollen to the tip of the
+  pistil, thus securing the fertilisation of the flower and the growth of
+  the ovules in the pod. Yucca flowers in Britain do not produce seeds
+  because there are no Yucca Moths.]
+  [Illustration: INCLINED PLANE OF ANIMAL BEHAVIOUR
+  Diagram illustrating animal behaviour. The main line represents the
+  general life of the creature. On the upper side are activities implying
+  initiative; on the lower side actions which are almost automatic.
+  _Upper Side._--I. Energetic actions. II. Simple tentatives. III.
+  Trial-and-error methods. IV. Non-intelligent experiments. V.
+  Experiential "learning." VI. Associative "learning." VII. Intelligent
+  behaviour. VIII. Rational conduct (man).
+  _Lower Side._--1. Reactions to environment. 2. Enregistered reactions.
+  3. Simple reflex actions. 4. Compound reflex actions. 5. Tropisms. 6.
+  Enregistered rhythms. 7. Simple instincts. 8. Chain instincts. 9.
+  Instinctive activities influenced by intelligence. 10. Subconscious
+  cerebration at a high level (man).]
+  [Illustration: _Photo: J. J. Ward, F.E.S._
+  VENUS' FLY-TRAP
+  One of the most remarkable plants in the world, which captures its prey
+  by means of a trap formed from part of its leaf. It has been induced to
+  snap at and hold a bristle. If an insect lighting on the leaf touches
+  one of six very sensitive hairs, which pull the trigger of the movement,
+  the two halves of the leaf close rapidly and the fringing teeth on the
+  margin interlock, preventing the insect's escape. Then follows an
+  exudation of digestive juice.]
+  [Illustration: _Reproduced by permission from "The Wonders of Instinct"
+  by J. H. Fabre._
+  A SPIDER SUNNING HER EGGS
+  A kind of spider, called Lycosa, lying head downwards at the edge of her
+  nest, and holding her silken cocoon--the bag containing the eggs--up
+  towards the sun in her hindmost pair of legs. This extraordinary
+  proceeding is believed to assist in the hatching.]
+  Reflex Actions
+  Among simple multicellular animals, such as sea-anemones, we find the
+  beginnings of reflex actions, and a considerable part of the behaviour
+  of the lower animals is reflex. That is to say, there are laid down in
+  the animal in the course of its development certain pre-arrangements of
+  nerve-cells and muscle-cells which secure that a fit and proper answer
+  is given to a frequently recurrent stimulus. An earthworm half out of
+  its burrow becomes aware of the light tread of a thrush's foot, and
+  jerks itself back into its hole before anyone can say "reflex action."
+  What is it that happens?
+  Certain sensory nerve-cells in the earthworm's skin are stimulated by
+  vibrations in the earth; the message travels down a sensory nerve-fibre
+  from each of the stimulated cells and enters the nerve-cord. The sensory
+  fibres come into vital connection with branches of intermediary,
+  associative, or communicating cells, which are likewise connected with
+  motor nerve-cells. To these the message is thus shunted. From the motor
+  nerve-cells an impulse or command travels by motor nerve-fibres, one
+  from each cell, to the muscles, which contract. If this took as long to
+  happen as it takes to describe, even in outline, it would not be of much
+  use to the earthworm. But the motor answer follows the sensory stimulus
+  almost instantaneously. The great advantage of establishing or
+  enregistering these reflex chains is that the answers are practically
+  ready-made or inborn, not requiring to be learned. It is not necessary
+  that the brain should be stimulated if there is a brain; nor does the
+  animal will to act, though in certain cases it may by means of higher
+  controlling nerve-centres keep the natural reflex response from being
+  given, as happens, for instance, when we control a cough or a sneeze on
+  some solemn occasion. The evolutionary method, if we may use the
+  expression, has been to enregister ready-made responses; and as we
+  ascend the animal kingdom, we find reflex actions becoming complicated
+  and often linked together, so that the occurrence of one pulls the
+  trigger of another, and so on in a chain. The behaviour of the
+  insectivorous plant called Venus's fly-trap when it shuts on an insect
+  is like a reflex action in an animal, but plants have no definite
+  nervous system.
+  What are Called Tropisms
+  A somewhat higher level on the inclined plane is illustrated by what are
+  called "tropisms," obligatory movements which the animal makes,
+  adjusting its whole body so that physiological equilibrium results in
+  relation to gravity, pressure, currents, moisture, heat, light,
+  electricity, and surfaces of contact. A moth is flying past a candle;
+the eye next the light is more illumined than the other; a physiological
+inequilibrium results, affecting nerve-cells and muscle-cells; the
+outcome is that the moth automatically adjusts its flight so that both
+eyes become equally illumined; in doing this it often flies into the
+candle.
+It may seem bad business that the moth should fly into the candle, but
+the flame is an utterly artificial item in its environment to which no
+one can expect it to be adapted. These tropisms play an important rôle
+in animal behaviour.
+Instinctive Behaviour
+On a higher level is instinctive behaviour, which reaches such
+remarkable perfection in ants, bees, and wasps. In its typical
+expression instinctive behaviour depends on inborn capacities; it does
+not require to be learned; it is independent of practice or experience,
+    though it may be improved by both; it is shared equally by all members
+    of the species of the same sex (for the female's instincts are often
+        different from the male's); it refers to particular conditions of life
+    that are of vital importance, though they may occur only once in a
+    lifetime. The female Yucca Moth emerges from the cocoon when the Yucca
+    flower puts forth its bell-like blossoms. She flies to a flower,
+    collects some pollen from the stamens, kneads it into a pill-like ball,
+    and stows this away under her chin. She flies to an older Yucca flower
+    and lays her eggs in some of the ovules within the seed-box, but before
+    she does so she has to deposit on the stigma the ball of pollen. From
+    this the pollen-tubes grow down and the pollen-nucleus of a tube
+    fertilises the egg-cell in an ovule, so that the possible seeds become
+    real seeds, for it is only a fraction of them that the Yucca Moth has
+    destroyed by using them as cradles for her eggs. Now it is plain that
+    the Yucca Moth has no individual experience of Yucca flowers, yet she
+    secures the continuance of her race by a concatenation of actions which
+    form part of her instinctive repertory.
+    From a physiological point of view instinctive behaviour is like a chain
+    of compound reflex actions, but in some cases, at least, there is reason
+    to believe that the behaviour is suffused with awareness and backed by
+    endeavour. This is suggested in exceptional cases where the stereotyped
+    routine is departed from to meet exceptional conditions. It should also
+    be noted that just as ants, hive bees, and wasps exhibit in most cases
+    purely instinctive behaviour, but move on occasion on the main line of
+    trial and error or of experimental initiative, so among birds and
+    mammals the intelligent behaviour is sometimes replaced by instinctive
+    routine. Perhaps there is no instinctive behaviour without a spice of
+    intelligence, and no intelligent behaviour without an instinctive
+    element. The old view that instinctive behaviour was originally
+    intelligent, and that instinct is "lapsed intelligence," is a tempting
+    one, and is suggested by the way in which habitual intelligent actions
+    cease in the individual to require intelligent control, but it rests on
+    the unproved hypothesis that the acquisitions of the individual can be
+    entailed on the race. It is almost certain that instinct is on a line of
+    evolution quite different from intelligence, and that it is nearer to
+    the inborn inspirations of the calculating boy or the musical genius
+    than to the plodding methods of intelligent learning.
+    Animal Intelligence
+    The higher reaches of the inclined plane of behaviour show intelligence
+    in the strict sense. They include those kinds of behaviour which cannot
+    be described without the suggestion that the animal makes some sort of
+    perceptual inference, not only profiting by experience but learning by
+    ideas. Such intelligent actions show great individual variability; they
+    are plastic and adjustable in a manner rarely hinted at in connection
+    with instincts where routine cannot be departed from without the
+    creature being nonplussed; they are not bound up with particular
+    circumstances as instinctive actions are, but imply an appreciative
+    awareness of relations.
+    When there is an experimenting with general ideas, when there is
+    _conceptual_ as contrasted with _perceptual_ inference, we speak of
+    Reason, but there is no evidence of this below the level of man. It is
+    not, indeed, always that we can credit man with rational conduct, but he
+    has the possibility of it ever within his reach.
+    Animal instinct and intelligence will be illustrated in another part of
+    this work. We are here concerned simply with the general question of the
+    evolution of behaviour. There is a main line of tentative experimental
+    behaviour both below and above the level of intelligence, and it has
+    been part of the tactics of evolution to bring about the hereditary
+    enregistration of capacities of effective response, the advantages being
+    that the answers come more rapidly and that the creature is left free,
+    if it chooses, for higher adventures.
+    There is no doubt as to the big fact that in the course of evolution
+    animals have shown an increasing complexity and masterfulness of
+    behaviour, that they have become at once more controlled and more
+    definitely free agents, and that the inner aspect of the
+    behaviour--experimenting, learning, thinking, feeling, and willing--has
+    come to count for more and more.
+    Evolution of Parental Care
+    Mammals furnish a crowning instance of a trend of evolution which
+    expresses itself at many levels--the tendency to bring forth the young
+    at a well-advanced stage and to an increase of parental care associated
+    with a decrease in the number of offspring. There is a British starfish
+    called _Luidia_ which has two hundred millions of eggs in a year, and
+    there are said to be several millions of eggs in conger-eels and some
+    other fishes. These illustrate the spawning method of solving the
+    problem of survival. Some animals are naturally prolific, and the number
+    of eggs which they sow broadcast in the waters allows for enormous
+    infantile mortality and obviates any necessity for parental care.
+    But some other creatures, by nature less prolific, have found an
+    entirely different solution of the problem. They practise parental care
+    and they secure survival with greatly economised reproduction. This is a
+    trend of evolution particularly characteristic of the higher animals. So
+    much so that Herbert Spencer formulated the generalisation that the size
+    and frequency of the animal family is inverse ratio to the degree of
+    evolution to which the animal has attained.
+    Now there are many different methods of parental care which secure the
+    safety of the young, and one of these is called viviparity. The young
+    ones are not liberated from the parent until they are relatively well
+    advanced and more or less able to look after themselves. This gives the
+    young a good send-off in life, and their chances of death are greatly
+    reduced. In other words, the animals that have varied in the direction
+    of economised reproduction may keep their foothold in the struggle for
+    existence if they have varied at the same time in the direction of
+    parental care. In other cases it may have worked the other way round.
+    In the interesting archaic animal called _Peripatus_, which has to face
+    a modern world too severe for it, one of the methods of meeting the
+    environing difficulties is the retention of the offspring for many
+    months within the mother, so that it is born a fully-formed creature.
+    There are only a few offspring at a time, and, although there are
+    exceptional cases like the summer green-flies, which are very prolific
+    though viviparous, the general rule is that viviparity is associated
+    with a very small family. The case of flowering plants stands by itself,
+    for although they illustrate a kind of viviparity, the seed being
+    embryos, an individual plant may have a large number of flowers and
+    therefore a huge family.
+    Viviparity naturally finds its best illustrations among terrestrial
+    animals, where the risks to the young life are many, and it finds its
+    climax among mammals.
+    Now it is an interesting fact that the three lowest mammals, the
+    Duckmole and two Spiny Ant-eaters, lay eggs, i.e. are oviparous; that
+    the Marsupials, on the next grade, bring forth their young, as it were,
+    prematurely, and in most cases stow them away in an external pouch;
+while all the others--the Placentals--show a more prolonged ante-natal
+life and an intimate partnership between the mother and the unborn
+young.
+There is another way of looking at the sublime process of evolution. It
+has implied a mastery of all the possible haunts of life; it has been a
+progressive conquest of the environment.
+1. It is highly probable that living organisms found their foothold in
+the stimulating conditions of the shore of the sea--the shallow water,
+    brightly illumined, seaweed-growing shelf fringing the Continents. This
+    littoral zone was a propitious environment where sea and fresh water,
+    earth and air all meet, where there is stimulating change, abundant
+    oxygenation and a copious supply of nutritive material in what the
+    streams bring down and in the rich seaweed vegetation.
+    [Illustration: THE HOATZIN INHABITS BRITISH GUIANA
+    The newly hatched bird has claws on its thumb and first finger and so is
+    enabled to climb on the branches of trees with great dexterity until
+    such time as the wings are strong enough to sustain it in flight.]
+    [Illustration: _Photograph, from the British Museum (Natural History),
+    of a drawing by Mr. E. Wilson._
+    PERIPATUS
+    A widely distributed old-fashioned type of animal, somewhat like a
+    permanent caterpillar. It has affinities both with worms and with
+    insects. It has a velvety skin, minute diamond-like eyes, and short
+    stump-like legs. A defenceless, weaponless animal, it comes out at
+    night, and is said to capture small insects by squirting jets of slime
+    from its mouth.]
+    [Illustration: _Photo: W. S. Berridge, F.Z.S._
+    ROCK KANGAROO CARRYING ITS YOUNG IN A POUCH
+    The young are born so helpless that they cannot even suck. The mother
+    places them in the external pouch, and fitting their mouths on the teats
+    injects the milk. After a time the young ones go out and in as they
+    please.]
+    It is not an easy haunt of life, but none the worse for that, and it is
+    tenanted to-day by representatives of practically every class of animals
+    from infusorians to seashore birds and mammals.
+    The Cradle of the Open Sea
+    2. The open-sea or pelagic haunt includes all the brightly illumined
+    surface waters beyond the shallow water of the shore area.
+    It is perhaps the easiest of all the haunts of life, for there is no
+    crowding, there is considerable uniformity, and an abundance of food for
+    animals is afforded by the inexhaustible floating "sea-meadows" of
+    microscopic Algæ. These are reincarnated in minute animals like the
+    open-sea crustaceans, which again are utilised by fishes, these in turn
+    making life possible for higher forms like carnivorous turtles and
+    toothed whales. It is quite possible that the open sea was the original
+    cradle of life and perhaps Professor Church is right in picturing a long
+    period of pelagic life before there was any sufficiently shallow water
+    to allow the floating plants to anchor. It is rather in favour of this
+    view that many shore animals such as crabs and starfishes, spend their
+    youthful stages in the relatively safe cradle of the open sea, and only
+    return to the more strenuous conditions of their birthplace after they
+    have gained considerable strength of body. It is probably safe to say
+    that the honour of being the original cradle of life lies between the
+    shore of the sea and the open sea.
+    The Great Deeps
+    3. A third haunt of life is the floor of the Deep Sea, the abyssal area,
+    which occupies more than a half of the surface of the globe. It is a
+    region of extreme cold--an eternal winter; of utter darkness--an eternal
+    night--relieved only by the fitful gleams of "phosphorescent" animals;
+    of enormous pressure--2-1/2 tons on the square inch at a depth of 2,500
+    fathoms; of profound calm, unbroken silence, immense monotony. And as
+    there are no plants in the great abysses, the animals must live on one
+    another, and, in the long run, on the rain of moribund animalcules which
+    sink from the surface through the miles of water. It seems a very
+    unpromising haunt of life, but it is abundantly tenanted, and it gives
+    us a glimpse of the insurgent nature of the living creature that the
+    difficulties of the Deep Sea should have been so effectively conquered.
+    It is probable that the colonising of the great abysses took place in
+    relatively recent times, for the fauna does not include many very
+    antique types. It is practically certain that the colonisation was due
+    to littoral animals which followed the food-débris, millennium after
+    millennium, further and further down the long slope from the shore.
+    The Freshwaters
+    4. A fourth haunt of life is that of the freshwaters, including river
+    and lake, pond and pool, swamp and marsh. It may have been colonised by
+    gradual migration up estuaries and rivers, or by more direct passage
+    from the seashore into the brackish swamp. Or it may have been in some
+    cases that partially landlocked corners of ancient seas became gradually
+    turned into freshwater basins. The animal population of the freshwaters
+    is very representative, and is diversely adapted to meet the
+    characteristic contingencies--the risk of being dried up, the risk of
+    being frozen hard in winter, and the risk of being left high and dry
+    after floods or of being swept down to the sea.
+    Conquest of the Dry Land
+    5. The terrestrial haunt has been invaded age after age by contingents
+    from the sea or from the freshwaters. We must recognise the worm
+    invasion, which led eventually to the making of the fertile soil, the
+    invasion due to air-breathing Arthropods, which led eventually to the
+    important linkage between flowers and their insect visitors, and the
+    invasion due to air-breathing Amphibians, which led eventually to the
+    higher terrestrial animals and to the development of intelligence and
+    family affection. Besides these three great invasions, there were minor
+    ones such as that leading to land-snails, for there has been a
+    widespread and persistent tendency among aquatic animals to try to
+    possess the dry land.
+    Getting on to dry land had a manifold significance.
+    It implied getting into a medium with a much larger supply of oxygen
+    than there is dissolved in the water. But the oxygen of the air is more
+    difficult to capture, especially when the skin becomes hard or well
+    protected, as it is almost bound to become in animals living on dry
+    ground. Thus this leads to the development of _internal surfaces_, such
+    as those of lungs, where the oxygen taken into the body may be absorbed
+    by the blood. In most animals the blood goes to the surface of
+    oxygen-capture; but in insects and their relatives there is a different
+    idea--of taking the air to the blood or in greater part to the area of
+    oxygen-combustion, the living tissues. A system of branching air-tubes
+    takes air into every hole and corner of the insect's body, and this
+    thorough aeration is doubtless in part the secret of the insect's
+    intense activity. The blood never becomes impure.
+    The conquest of the dry land also implied a predominance of that kind of
+    locomotion which may be compared to punting, when the body is pushed
+    along by pressing a lever against a hard substratum. And it also
+    followed that with few exceptions the body of the terrestrial animal
+    tended to be compact, readily lifted off the ground by the limbs or
+    adjusted in some other way so that there may not be too large a surface
+    trailing on the ground. An animal like a jellyfish, easily supported in
+    the water, would be impossible on land. Such apparent exceptions as
+    earthworms, centipedes, and snakes are not difficult to explain, for the
+    earthworm is a burrower which eats its way through the soil, the
+    centipede's long body is supported by numerous hard legs, and the snake
+    pushes itself along by means of the large ventral scales to which the
+    lower ends of very numerous ribs are attached.
+    Methods of Mastering the Difficulties of Terrestrial Life
+    A great restriction attendant on the invasion of the dry land is that
+    locomotion becomes limited to one plane, namely, the surface of the
+    earth. This is in great contrast to what is true in the water, where the
+    animal can move up or down, to right or to left, at any angle and in
+    three dimensions. It surely follows from this that the movements of land
+    animals must be rapid and precise, unless, indeed, safety is secured in
+    some other way. Hence it is easy to understand why most land animals
+    have very finely developed striped muscles, and why a beetle running on
+    the ground has far more numerous muscles than a lobster swimming in the
+    sea.
+    Land animals were also handicapped by the risks of drought and of frost,
+    but these were met by defences of the most diverse description, from the
+    hairs of woolly caterpillars to the fur of mammals, from the carapace of
+    tortoises to the armour of armadillos. In other cases, it is hardly
+    necessary to say, the difficulties may be met in other ways, as frogs
+    meet the winter by falling into a lethargic state in some secluded
+    retreat.
+    Another consequence of getting on to dry land is that the eggs or young
+    can no longer be set free anyhow, as is possible when the animal is
+    surrounded by water, which is in itself more or less of a cradle. If the
+    eggs were laid or the young liberated on dry ground, the chances are
+    many that they would be dried up or devoured. So there are numerous ways
+    in which land animals secure the safety of their young, e.g. by burying
+    them in the ground, or by hiding them in nests, or by carrying them
+    about for a prolonged period either before or after birth. This may mean
+    great safety for the young, this may make it possible to have only a
+    small family, and this may tend to the evolution of parental care and
+    the kindly emotions. Thus it may be understood that from the conquest of
+    the land many far-reaching consequences have followed.
+  [Illustration: _Photo: Rischgitz._
+PROFESSOR THOMAS HENRY HUXLEY (1825-95)
+  One of the most distinguished of zoologists, with unsurpassed gifts as a
+  teacher and expositor. He did great service in gaining a place for
+  science in ordinary education and in popular estimation. No one
+  championed Evolutionism with more courage and skill.]
+  [Illustration: BARON CUVIER, 1769-1832
+  One of the founders of modern Comparative Anatomy. A man of gigantic
+  intellect, who came to Paris as a youth from the provinces, and became
+  the director of the higher education of France and a peer of the Empire.
+  He was opposed to Evolutionist ideas, but he had anatomical genius.]
+  [Illustration: AN ILLUSTRATION SHOWING VARIOUS METHODS OF FLYING AND
+  SWOOPING
+  Gull, with a feather-wing, a true flier. Fox-bat, with a skin-wing, a
+  true flier. Flying Squirrel, with a parachute of skin, able to swoop
+  from tree to tree, but not to fly. Flying Fish, with pectoral fins used
+  as volplanes in a great leap due to the tail. To some extent able to
+  sail in albatros fashion.]
+  Finally, it is worth dwelling on the risks of terrestrial life, because
+  they enable us better to understand why so many land animals have become
+  burrowers and others climbers of trees, why some have returned to the
+  water and others have taken to the air. It may be asked, perhaps, why
+  the land should have been colonised at all when the risks and
+  difficulties are so great. The answer must be that necessity and
+  curiosity are the mother and father of invention. Animals left the water
+  because the pools dried up, or because they were overcrowded, or because
+  of inveterate enemies, but also because of that curiosity and spirit of
+  adventure which, from first to last, has been one of the spurs of
+  progress.
+  Conquering the Air
+  6. The last great haunt of life is the air, a mastery of which must be
+  placed to the credit of insects, Pterodactyls, birds, and bats. These
+  have been the successes, but it should be noted that there have been
+  many brilliant failures, which have not attained to much more than
+  parachuting. These include the Flying Fishes, which take leaps from the
+  water and are carried for many yards and to considerable heights,
+  holding their enlarged pectoral fins taut or with little more than a
+  slight fluttering. There is a so-called Flying Frog (_Rhacophorus_) that
+  skims from branch to branch, and the much more effective Flying Dragon
+  (_Draco volans_) of the Far East, which has been mentioned already.
+  Among mammals there are Flying Phalangers, Flying Lemurs, and more
+  besides, all attaining to great skill as parachutists, and illustrating
+  the endeavour to master the air which man has realised in a way of his
+  own.
+  The power of flight brings obvious advantages. A bird feeding on the
+  ground is able to evade the stalking carnivore by suddenly rising into
+  the air; food and water can be followed rapidly and to great distances;
+  the eggs or the young can be placed in safe situations; and birds in
+  their migrations have made a brilliant conquest both of time and space.
+  Many of them know no winter in their year, and the migratory flight of
+  the Pacific Golden Plover from Hawaii to Alaska and back again does not
+  stand alone.
+  THE PROCESSION OF LIFE THROUGH THE AGES
+  The Rock Record
+  How do we know when the various classes of animals and plants were
+  established on the earth? How do we know the order of their appearance
+  and the succession of their advances? The answer is: by reading the Rock
+  Record. In the course of time the crust of the earth has been elevated
+  into continents and depressed into ocean-troughs, and the surface of the
+  land has been buckled up into mountain ranges and folded in gentler
+  hills and valleys. The high places of the land have been weathered by
+  air and water in many forms, and the results of the weathering have been
+  borne away by rivers and seas, to be laid down again elsewhere as
+  deposits which eventually formed sandstones, mudstones, and similar
+  sedimentary rocks. Much of the material of the original crust has thus
+  been broken down and worked up again many times over, and if the total
+  thickness of the sedimentary rocks is added up it amounts, according to
+  some geologists, to a total of 67 miles. In most cases, however, only a
+  small part of this thickness is to be seen in one place, for the
+  deposits were usually formed in limited areas at any one time.
+  The Use of Fossils
+  When the sediments were accumulating age after age, it naturally came
+  about that remains of the plants and animals living at the time were
+  buried, and these formed the fossils by the aid of which it is possible
+  to read the story of the past. By careful piecing together of evidence
+  the geologist is able to determine the order in which the different
+  sedimentary rocks were laid down, and thus to say, for instance, that
+  the Devonian period was the time of the origin of Amphibians. In other
+  cases the geologist utilises the fossils in his attempt to work out the
+  order of the strata when these have been much disarranged. For the
+  simpler fossil forms of any type must be older than those that are more
+  complex. There is no vicious circle here, for the general succession of
+  strata is clear, and it is quite certain that there were fishes before
+  there were amphibians, and amphibians before there were reptiles, and
+  reptiles before there were birds and mammals. In certain cases, e.g. of
+  fossil horses and elephants, the actual historical succession has been
+  clearly worked out.
+  If the successive strata contained good samples of all the plants and
+  animals living at the time when the beds were formed, then it would be
+  easy to read the record of the rocks, but many animals were too soft to
+  become satisfactory fossils, many were eaten or dissolved away, many
+  were destroyed by heat and pressure, so that the rock record is like a
+  library very much damaged by fire and looting and decay.
+  The Geological Time-table
+  The long history of the earth and its inhabitants is conveniently
+  divided into eras. Thus, just as we speak of the ancient, mediæval, and
+  modern history of mankind, so we may speak of Palæozoic, Mesozoic and
+  Cenozoic eras in the history of the earth as a whole.
+  Geologists cannot tell us except in an approximate way how long the
+  process of evolution has taken. One of the methods is to estimate how
+  long has been required for the accumulation of the salts of the sea,
+  for all these have been dissolved out of the rocks since rain began to
+  fall on the earth. Dividing the total amount of saline matter by what is
+  contributed every year in modern times, we get about a hundred million
+  years as the age of the sea. But as the present rate of
+  salt-accumulation is probably much greater than it was during many of
+  the geological periods, the prodigious age just mentioned is in all
+  likelihood far below the mark. Another method is to calculate how long
+  it would take to form the sedimentary rocks, like sandstones and
+  mudstones, which have a _total_ thickness of over fifty miles, though
+  the _local_ thickness is rarely over a mile. As most of the materials
+  have come from the weathering of the earth's crust, and as the annual
+  amount of weathering now going on can be estimated, the time required
+  for the formation of the sedimentary rocks of the world can be
+  approximately calculated. There are some other ways of trying to tell
+  the earth's age and the length of the successive periods, but no
+  certainty has been reached.
+  The eras marked on the table (page 92) as _before the Cambrian_
+  correspond to about thirty-two miles of thickness of strata; and all the
+  subsequent eras with fossil-bearing rocks to a thickness of about
+  twenty-one miles--in itself an astounding fact. Perhaps thirty million
+  years must be allotted to the Pre-Cambrian eras, eighteen to the
+  Palæozoic, nine to the Mesozoic, three to the Cenozoic, making a grand
+  total of sixty millions.
+  The Establishment of Invertebrate Stocks
+  It is an astounding fact that at least half of geological time (the
+      Archæozoic and Proterozoic eras) passed before there were living
+  creatures with parts sufficiently hard to form fossils. In the latter
+  part of the Proterozoic era there are traces of one-celled marine
+  animals (Radiolarians) with shells of flint, and of worms that wallowed
+  in the primal mud. It is plain that as regards the most primitive
+  creatures the rock record tells us little.
+  [Illustration: _From Knipe's "Nebula to Man."_
+  ANIMALS OF THE CAMBRIAN PERIOD e.g. Sponges, Jellyfish, Starfish,
+  Sea-lilies, Water-fleas, and Trilobites]
+  [Illustration: _Photo: J. J. Ward, F.E.S._
+  A TRILOBITE
+  Trilobites were ancient seashore animals, abundant from the Upper
+  Cambrian to the Carboniferous eras. They have no direct descendants
+  to-day. They were jointed-footed animals, allied to Crustaceans and
+  perhaps also to King-crabs. They were able to roll themselves up in
+  their ring-armour.]
+  [Illustration: _Photo: British Museum (Natural History)._
+  THE GAMBIAN MUD-FISH, PROTOPTERUS
+  It can breathe oxygen dissolved in water by its gills; it can also
+  breathe dry air by means of its swim-bladder, which has become a lung.
+  It is a _double-breather_, showing evolution in process. For seven
+  months of the year, the dry season, it can remain inert in the mud,
+  getting air through an open pipe to the surface. When water fills the
+  pools it can use its gills again. Mud-nests or mud encasements with the
+  lung-fish inside have often been brought to Britain and the fish when
+  liberated were quite lively.]
+  [Illustration: THE ARCHÆOPTERYX
+(_After William Leche of Stockholm._)
+  A good restoration of the oldest known bird, Archæopteryx (Jurassic
+      Era). It was about the size of a crow; it had teeth on both jaws; it had
+  claws on the thumb and two fingers; and it had a long lizard-like tail.
+  But it had feathers, proving itself a true bird.]
+  [Illustration: WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS
+  The longest feathers or primaries (PR) are borne by the two fingers (2
+      and 3), and their palm-bones (CMC); the second longest or secondaries
+  are borne by the ulna bone (U) of the fore-arm; there is a separate tuft
+  (AS) on the thumb (TH).]
+  The rarity of direct traces of life in the oldest rocks is partly due to
+  the fact that the primitive animals would be of delicate build, but it
+  must also be remembered that the ancient rocks have been profoundly and
+  repeatedly changed by pressure and heat, so that the traces which did
+  exist would be very liable to obliteration. And if it be asked what
+  right we have to suppose the presence of living creatures in the absence
+  or extreme rarity of fossils, we must point to great accumulations of
+  limestone which indicate the existence of calcareous algæ, and to
+  deposits of iron which probably indicate the activity of iron-forming
+  Bacteria. Ancient beds of graphite similarly suggest that green plants
+  flourished in these ancient days.
+The Era of Ancient Life (Palæozoic)
+  The _Cambrian_ period was the time of the establishment of the chief
+  stocks of backboneless animals such as sponges, jellyfishes, worms,
+  sea-cucumbers, lamp-shells, trilobites, crustaceans, and molluscs. There
+  is something very eloquent in the broad fact that the peopling of the
+  seas had definitely begun some thirty million years ago, for Professor
+  H. F. Osborn points out that in the Cambrian period there was already a
+  colonisation of the shore of the sea, the open sea, and the deep waters.
+  The _Ordovician_ period was marked by abundant representation of the
+  once very successful class of Trilobites--jointed-footed,
+  antenna-bearing, segmented marine animals, with numerous appendages and
+  a covering of chitin. They died away entirely with the end of the
+  Palæozoic era. Also very notable was the abundance of predatory
+  cuttlefishes, the bullies of the ancient seas. But it was in this period
+  that the first backboned animals made their appearance--an epoch-making
+  step in evolution. In other words, true fishes were evolved--destined in
+the course of ages to replace the cuttlefishes (which are mere molluscs)
+  in dominating the seas.
+  In the _Silurian_ period in which the peopling of the seas went on
+  apace, there was the first known attempt at colonising the dry land. For
+  in Silurian rocks there are fossil scorpions, and that implies ability
+  to breathe dry air--by means of internal surfaces, in this case known as
+  lungbooks. It was also towards the end of the Silurian, when a period of
+  great aridity set in, that fishes appeared related to our mud-fishes or
+  double-breathers (Dipnoi), which have lungs as well as gills. This,
+  again, meant utilising dry air, just as the present-day mud-fishes do
+  when the water disappears from the pools in hot weather. The lung-fishes
+  or mud-fishes of to-day are but three in number, one in Queensland, one
+  in South America, and one in Africa, but they are extremely
+  interesting "living fossils," binding the class of fishes to that of
+  amphibians. It is highly probable that the first invasion of the dry
+  land should be put to the credit of some adventurous worms, but the
+  second great invasion was certainly due to air-breathing Arthropods,
+  like the pioneer scorpion we mentioned.
+  [Illustration: PICTORIAL REPRESENTATION OF THE SUCCESSIVE STRATA OF THE
+  EARTH'S CRUST, WITH SUGGESTIONS OF CHARACTERISTIC FOSSILS
+  E.g. Fish and Trilobite in the Devonian (red), a large Amphibian in the
+  Carboniferous (blue), Reptiles in Permian (light red), the first Mammal
+  in the Triassic (blue), the first Bird in the Jurassic (yellow), Giant
+  Reptiles in the Cretaceous (white), then follow the Tertiary strata with
+  progressive mammals, and Quaternary at the top with man and mammoth.]
+  The _Devonian_ period, including that of the Old Red Sandstone, was one
+  of the most significant periods in the earth's history. For it was the
+  time of the establishment of flowering plants upon the earth and of
+  terrestrial backboned animals. One would like to have been the
+  discoverer of the Devonian foot-print of _Thinopus_, the first known
+  Amphibian foot-print--an eloquent vestige of the third great invasion of
+  the dry land. It was probably from a stock of Devonian lung-fishes that
+  the first Amphibians sprang, but it was not till the next period that
+  they came to their own. While they were still feeling their way, there
+  was a remarkable exuberance of shark-like and heavily armoured fishes in
+  the Devonian seas.
+  EVOLUTION OF LAND ANIMALS
+  Giant Amphibians and Coal-measures
+  The _Carboniferous_ period was marked by a mild moist climate and a
+  luxuriant vegetation in the swampy low grounds. It was a much less
+  strenuous time than the Devonian period; it was like a very long summer.
+  There were no trees of the type we see now, but there were forests of
+  club-mosses and horsetails which grew to a gigantic size compared with
+  their pigmy representatives of to-day. In these forests the
+  jointed-footed invaders of the dry land ran riot in the form of
+  centipedes, spiders, scorpions, and insects, and on these the primeval
+  Amphibians fed. The appearance of insects made possible a new linkage of
+  far-reaching importance, namely, the cross-fertilisation of flowering
+  plants by their insect visitors, and from this time onwards it may be
+  said that flowers and their visitors have evolved hand in hand.
+  Cross-fertilisation is much surer by insects than by the wind, and
+  cross-fertilisation is more advantageous than self-fertilisation because
+  it promotes both fertility and plasticity. It was probably in this
+  period that _coloured_ flowers--attractive to insect-visitors--began to
+  justify themselves as beauty became useful, and began to relieve the
+  monotonous green of the horsetail and club-moss forests, which covered
+  great tracts of the earth for millions of years. In the Carboniferous
+  forests there were also land-snails, representing one of the minor
+  invasions of the dry land, tending on the whole to check vegetation.
+  They, too, were probably preyed upon by the Amphibians, some of which
+  attained a large size. Each age has had its giants, and those of the
+  Carboniferous were Amphibians called Labyrinthodonts, some of which were
+  almost as big as donkeys. It need hardly be said that it was in this
+  period that most of the Coal-measures were laid down by the immense
+  accumulation of the spores and debris of the club-moss forests. Ages
+  afterwards, it was given to man to tap this great source of
+  energy--traceable back to the sunshine of millions of years ago. Even
+  then it was true that no plant or animal lives or dies to itself!
+  The Acquisitions of Amphibians.
+  As Amphibians had their Golden Age in the Carboniferous period we may
+  fitly use this opportunity of indicating the advances in evolution which
+  the emergence of Amphibians implied. (1) In the first place the passage
+  from water to dry land was the beginning of a higher and more promiseful
+  life, taxed no doubt by increased difficulties. The natural question
+  rises why animals should have migrated from water to dry land at all
+  when great difficulties were involved in the transition. The answers
+  must be: (_a_) that local drying up of water-basins or elevations of the
+  land surface often made the old haunts untenable; (_b_) that there may
+  have been great congestion and competition in the old quarters; and
+  (_c_) that there has been an undeniable endeavour after well-being
+  throughout the history of animal life. In the same way with mankind,
+  migrations were prompted by the setting in of prolonged drought, by
+  over-population, and by the spirit of adventure. (2) In Amphibians for
+  the first time the non-digitate paired fins of fishes were replaced by
+  limbs with fingers and toes. This implied an advantageous power of
+  grasping, of holding firm, of putting food into the mouth, of feeling
+  things in three dimensions. (3) We cannot be positive in regard to the
+  soft parts of the ancient Amphibians known only as fossils, but if they
+  were in a general way like the frogs and toads, newts and salamanders of
+  the present day, we may say that they made among other acquisitions the
+  following: true ventral lungs, a three-chambered heart, a movable
+  tongue, a drum to the ear, and lids to the eyes. It is very interesting
+  to find that though the tongue of the tadpole has some muscle-fibres in
+  it, they are not strong enough to effect movement, recalling the tongue
+  of fishes, which has not any muscles at all. Gradually, as the tadpole
+  becomes a frog, the muscle-fibres grow in strength, and make it possible
+  for the full-grown creature to shoot out its tongue upon insects. This
+  is probably a recapitulation of what was accomplished in the course of
+  millennia in the history of the Amphibian race. (4) Another acquisition
+  made by Amphibians was a voice, due, as in ourselves, to the rapid
+  passage of air over taut membranes (vocal cords) stretched in the
+  larynx. It is an interesting fact that for millions of years there was
+  upon the earth no sound of life at all, only the noise of wind and wave,
+  thunder and avalanche. Apart from the instrumental music of some
+  insects, perhaps beginning in the Carboniferous, the first vital sounds
+  were due to Amphibians, and theirs certainly was the first voice--surely
+  one of the great steps in organic evolution.
-- 
2.34.1