The Story of Evolution
by
Joseph McCabe
Part 2 out of 6
the higher and later rocks of the series, and more of the same
comparatively high types will probably appear. In the earlier
strata, representing an earlier stage of life, we find only thick
seams of black shale, limestone, and ironstone, in which we seem
to see the ashes of primitive organisms, cremated in the
appalling fires of the volcanic age, or crushed out of
recognition by the superimposed masses. Even if some wizardry of
science were ever to restore the forms that have been reduced to
ashes in this Archaean crematorium, it would be found that they
are more or less advanced forms, far above the original level of
life. No trace will ever be found in the rocks of the first few
million years in the calendar of life.
The word impossible or unknowable is not lightly uttered in
science to-day, but there is a very plain reason for admitting it
here. The earliest living things were at least as primitive of
nature as the lowest animals and plants we know to-day, and
these, up to a fair level of organisation, are so soft of texture
that, when they die, they leave no remains which may one day be
turned into fossils. Some of them, indeed, form tiny shells of
flint or lime, or, like the corals, make for themselves a solid
bed; but this is a relatively late and higher stage of
development. Many thousands of species of animals and plants lie
below that level. We are therefore forced to conclude, from the
aspect of living nature to-day, that for ages the early organisms
had no hard and preservable parts. In thus declaring the
impotence of geology, however, we are at the same time
introducing another science, biology, which can throw appreciable
light on the evolution of life. Let us first see what geology
tells us about the infancy of the earth.
The distribution of the early rocks suggests that there was
comparatively little dry land showing above the surface of the
Archaean ocean. Our knowledge of these rocks is not at all
complete, and we must remember that some of this primitive land
may be now under the sea or buried in unsuspected regions. It is
significant, however, that, up to the present, exploration seems
to show that in those remote ages only about one-fifth of our
actual land-surface stood above the level of the waters. Apart
from a patch of some 20,000 square miles of what is now
Australia, and smaller patches in Tasmania, New Zealand, and
India, nearly the whole of this land was in the far North. A
considerable area of eastern Canada had emerged, with lesser
islands standing out to the west and south of North America.
Another large area lay round the basin of the Baltic; and as
Greenland, the Hebrides, and the extreme tip of Scotland, belong
to the same age, it is believed that a continent, of which they
are fragments, united America and Europe across the North
Atlantic. Of the rest of what is now Europe there were merely
large islands--one on the border of England and Wales, others in
France, Spain, and Southern Germany. Asia was represented by a
large area in China and Siberia, and an island or islands on the
site of India. Very little of Africa or South America existed.
It will be seen at a glance that the physical story of the earth
from that time is a record of the emergence from the waters of
larger continents and the formation of lofty chains of mountains.
Now this world-old battle of land and sea has been waged with
varying fortune from age to age, and it has been one of the most
important factors in the development of life. We are just
beginning to realise what a wonderful light it throws on the
upward advance of animals and plants. No one in the scientific
world to-day questions that, however imperfect the record may be,
there has been a continuous development of life from the lowest
level to the highest. But why there was advance at all, why the
primitive microbe climbs the scale of being, during millions of
years, until it reaches the stature of humanity, seems to many a
profound mystery. The solution of this mystery begins to break
upon us when we contemplate, in the geological record, the
prolonged series of changes in the face of the earth itself, and
try to realise how these changes must have impelled living things
to fresh and higher adaptations to their changing surroundings.
Imagine some early continent with its population of animals and
plants. Each bay, estuary, river, and lake, each forest and marsh
and solid plain, has its distinctive inhabitants. Imagine this
continent slowly sinking into the sea, until the advancing arms
of the salt water meet across it, mingling their diverse
populations in a common world, making the fresh-water lake
brackish or salt, turning the dry land into swamp, and flooding
the forest. Or suppose, on the other hand, that the land rises,
the marsh is drained, the genial climate succeeded by an icy
cold, the luscious vegetation destroyed, the whole animal
population compelled to change its habits and its food. But this
is no imaginary picture. It is the actual story of the earth
during millions of years, and it is chiefly in the light of these
vast and exacting changes in the environment that we are going to
survey the panorama of the advance of terrestrial life.
For the moment it will be enough to state two leading principles.
The first is that there is no such thing as a "law of evolution"
in the sense in which many people understand that phrase. It is
now sufficiently well known that, when science speaks of a law,
it does not mean that there is some rule that things MUST act in
such and such a way. The law is a mere general expression of the
fact that they DO act in that way. But many imagine that there is
some principle within the living organism which impels it onward
to a higher level of organisation. That is entirely an error.
There is no "law of progress." If an animal is fitted to secure
its livelihood and breed posterity in certain surroundings, it
may remain unchanged indefinitely if these surroundings do not
materially change. So the duckmole of Australia and the tuatara
of New Zealand have retained primitive features for millions of
years; so the aboriginal Australian and the Fuegian have remained
stagnant, in their isolation, for a hundred thousand years or
more; so the Chinaman, in his geographical isolation, has
remained unchanged for two thousand years. There is no more a
"conservative instinct" in Chinese than there is a "progressive
instinct" in Europeans. The difference is one of history and
geography, as we shall see.
To make this important principle still clearer, let us imagine
some primitive philosopher observing the advance of the tide over
a level beach. He must discover two things: why the water comes
onward at all, and why it advances along those particular
channels. We shall see later how men of science explain or
interpret the mechanism in a living thing which enables it to
advance, when it does advance. For the present it is enough to
say that new-born animals and plants are always tending to differ
somewhat from their parents, and we now know, by experiment, that
when some exceptional influence is brought to bear on the parent,
the young may differ considerably from her. But, if the parents
were already in harmony with their environment, these variations
on the part of the young are of no consequence. Let the
environment alter, however, and some of these variations may
chance to make the young better fitted than the parent was. The
young which happen to have the useful variation will have an
advantage over their brothers or sisters, and be more likely to
survive and breed the next generation. If the change in the
environment (in the food or climate, for instance) is prolonged
and increased for hundreds of thousands of years, we shall expect
to find a corresponding change in the animals and plants.
We shall find such changes occurring throughout the story of the
earth. At one important point in the story we shall find so grave
a revolution in the face of nature that twenty-nine out of every
thirty species of animals and plants on the earth are
annihilated. Less destructive and extreme changes have been
taking place during nearly the whole of the period we have to
cover, entailing a more gradual alteration of the structure of
animals and plants; but we shall repeatedly find them culminating
in very great changes of climate, or of the distribution of land
and water, which have subjected the living population of the
earth to the most searching tests and promoted every variation
toward a more effective organisation.*
* This is a very simple expression of "Darwinism," and will be
enlarged later. The reader should ignore the occasional statement
of non-scientific writers that Darwinism is "dead" or superseded.
The questions which are actually in dispute relate to the causes
of the variation of the young from their parents, the magnitude
of these variations' and the transmission of changes acquired by
an animal during its own life. We shall see this more fully at a
later stage. The importance of the environment as I have
described it, is admitted by all schools.
And the second guiding principle I wish to lay down in advance is
that these great changes in the face of the earth, which explain
the progress of organisms, may very largely be reduced to one
simple agency--the battle of the land and the sea. When you gaze
at some line of cliffs that is being eaten away by the waves, or
reflect on the material carried out to sea by the flooded river,
you are--paradoxical as it may seem--beholding a material process
that has had a profound influence on the development of life. The
Archaean continent that we described was being reduced constantly
by the wash of rain, the scouring of rivers, and the fretting of
the waves on the coast. It is generally thought that these
wearing agencies were more violent in early times, but that is
disputed, and we will not build on it. In any case, in the course
of time millions of tons of matter were scraped off the Archaean
continent and laid on the floor of the sea by its rivers. This
meant a very serious alteration of pressure or weight on the
surface of the globe, and was bound to entail a reaction or
restoration of the balance.
The rise of the land and formation of mountains used to be
ascribed mainly to the cooling and shrinking of the globe of the
earth. The skin (crust), it was thought, would become too large
for the globe as it shrank, and would wrinkle outwards, or pucker
up into mountain-chains. The position of our greater
mountain-chains sprawling across half the earth (the Pyrenees to
the Himalaya, and the Rocky Mountains to the Andes), seems to
confirm this, but the question of the interior of the earth is
obscure and disputed, and geologists generally conceive the rise
of land and formation of mountains in a different way. They are
due probably to the alteration of pressure on the crust in
combination with the instability of the interior. The floors of
the seas would sink still lower under their colossal burdens, and
this would cause some draining of the land-surface. At the same
time the heavy pressure below the seas and the lessening of
pressure over the land would provoke a reaction. Enormous masses
of rock would be forced toward and underneath the land-surface,
bending, crumpling, and upheaving it as if its crust were but a
leather coat. As a result, masses of land would slowly rise above
the plain, to be shaped into hills and valleys by the hand of
later time, and fresh surfaces would be dragged out of the deep,
enlarging the fringes of the primitive continents, to be warped
and crumpled in their turn at the next era of pressure.
In point of geological fact, the story of the earth has been one
prolonged series of changes in the level of land and water, and
in their respective limits. These changes have usually been very
gradual, but they have always entailed changes (in climate, etc.
) of the greatest significance in the evolution of life. What was
the swampy soil of England in the Carboniferous period is now
sometimes thousands of feet beneath us; and what was the floor of
a deep ocean over much of Europe and Asia at another time is now
to be found on the slopes of lofty Alps, or 20,000 feet above the
sea-level in Thibet. Our story of terrestrial life will be, to a
great extent, the story of how animals and plants changed their
structure in the long series of changes which this endless battle
of land and sea brought over the face of the earth.
As we have no recognisable remains of the animals and plants of
the earliest age, we will not linger over the Archaean rocks.
Starting from deep and obscure masses of volcanic matter, the
geologist, as he travels up the series of Archaean rocks, can
trace only a dim and most unsatisfactory picture of those remote
times. Between outpours of volcanic floods he finds, after a
time, traces that an ocean and rivers are wearing away the land.
He finds seams of carbon among the rocks of the second division
of the Archaean (the Keewatin), and deduces from this that a
dense sea-weed population already covered the floor of the ocean.
In the next division (the Huronian) he finds the traces of
extensive ice-action strangely lying between masses of volcanic
rock, and sees that thousands of square miles of eastern North
America were then covered with an ice-sheet. Then fresh floods of
molten matter are poured out from the depths below; then the sea
floods the land for a time; and at last it makes its final
emergence as the first definitive part of the North American
continent, to enlarge, by successive fringes, to the continent of
to-day.*
* I am quoting Professor Coleman's summary of Archaean research
in North America (Address to the Geological Section of the
British Association, 1909). Europe, as a continent, has had more
"ups and downs" than America in the course of geological time.
This meagre picture of the battle of land and sea, with
interludes of great volcanic activity and even of an ice age,
represents nearly all we know of the first half of the world's
story from geology. It is especially disappointing in regard to
the living population. The very few fossils we find in the upper
Archaean rocks are so similar to those we shall discuss in the
next chapter that we may disregard them, and the seams of
carbon-shales, iron-ore, and limestone, suggest only, at the
most, that life was already abundant. We must turn elsewhere for
some information on the origin and early development of life.
The question of the origin of life I will dismiss with a brief
account of the various speculations of recent students of
science. Broadly speaking, their views fall into three classes.
Some think that the germs of life may have come to the earth from
some other body in the universe; some think that life was evolved
out of non-living matter in the early ages of the earth, under
exceptional conditions which we do not at present know, or can
only dimly conjecture; and some think that life is being evolved
from non-life in nature to-day, and always has been so evolving.
The majority of scientific men merely assume that the earliest
living things were no exception to the general process of
evolution, but think that we have too little positive knowledge
to speculate profitably on the manner of their origin.
The first view, that the germs of life may have come to this
planet on a meteoric visitor from some other world, as a
storm-driven bird may take its parasites to some distant island,
is not without adherents to-day. It was put forward long ago by
Lord Kelvin and others; it has been revived by the distinguished
Swede, Professor Svante Arrhenius. The scientific objection to it
is that the more intense (ultra-violet) rays of the sun would
frill such germs as they pass through space. But a broader
objection, and one that may dispense us from dwelling on it, is
that we gain nothing by throwing our problems upon another
planet. We have no ground for supposing that the earth is less
capable of evolving life than other planets.
The second view is that, when the earth had passed through its
white-hot stage, great masses of very complex chemicals, produced
by the great heat, were found on its surface. There is one
complex chemical substance in particular, called cyanogen, which
is either an important constituent of living matter, or closely
akin to it. Now we need intense heat to produce this substance in
the laboratory. May we not suppose that masses of it were
produced during the incandescence of the earth, and that, when
the waters descended, they passed through a series of changes
which culminated in living plasm? Such is the "cyanogen
hypothesis" of the origin of life, advocated by able
physiologists such as Pfluger, Verworn, and others. It has the
merit of suggesting a reason why life may not be evolving from
non-life in nature to-day, although it may have so evolved in the
Archaean period.
Other students suggest other combinations of carbon-compounds and
water in the early days. Some suggest that electric action was
probably far more intense in those ages; others think that
quantities of radium may have been left at the surface. But the
most important of these speculations on the origin of life in
early times, and one that has the merit of not assuming any
essentially different conditions then than we find now, is
contained in a recent pronouncement of one of the greatest
organic chemists in Europe, Professor Armstrong. He says that
such great progress has been made in his science--the science of
the chemical processes in living things--that "their cryptic
character seems to have disappeared almost suddenly." On the
strength of this new knowledge of living matter, he ventures to
say that "a series of lucky accidents" could account for the
first formation of living things out of non-living matter in
Archaean times. Indeed, he goes further. He names certain
inorganic substances, and says that the blowing of these into
pools by the wind on the primitive planet would set afoot
chemical combinations which would issue in the production of
living matter.*
* See his address in Nature, vol. 76, p. 651. For other
speculations see Verworn's "General Physiology," Butler Burke's
"Origin of Life" (1906), and Dr. Bastian's "Origin of Life"
(1911).
It is evident that the popular notion that scientific men have
declared that life cannot be evolved from non-life is very far
astray. This blunder is usually due to a misunderstanding of the
dogmatic statement which one often reads in scientific works that
"every living thing comes from a living thing." This principle
has no reference to remote ages, when the conditions may have
been different. It means that to-day, within our experience, the
living thing is always born of a living parent. However, even
this is questioned by some scientific men of eminence, and we
come to the third view.
Professor Nageli, a distinguished botanist, and Professor
Haeckel, maintain that our experience, as well as the range of
our microscopes, is too limited to justify the current axiom.
They believe that life may be evolving constantly from inorganic
matter. Professor J. A. Thomson also warns us that our experience
is very limited, and, for all we know, protoplasm may be forming
naturally in our own time. Mr. Butler Burke has, under the action
of radium, caused the birth of certain minute specks which
strangely imitate the behaviour of bacteria. Dr. Bastian has
maintained for years that he has produced living things from
non-living matter. In his latest experiments, described in the
book quoted, purely inorganic matter is used, and it is
previously subjected, in hermetically sealed tubes, to a heat
greater than what has been found necessary to kill any germs
whatever.
Evidently the problem of the origin of life is not hopeless, but
our knowledge of the nature of living matter is still so
imperfect that we may leave detailed speculation on its origin to
a future generation. Organic chemistry is making such strides
that the day may not be far distant when living matter will be
made by the chemist, and the secret of its origin revealed. For
the present we must be content to choose the more plausible of
the best-informed speculations on the subject.
But while the origin of life is obscure, the early stages of its
evolution come fairly within the range of our knowledge. To the
inexpert it must seem strange that, whereas we must rely on pure
speculation in attempting to trace the origin of life, we can
speak with more confidence of those early developments of plants
and animals which are equally buried in the mists of the Archaean
period. Have we not said that nothing remains of the procession
of organisms during half the earth's story but a shapeless seam
of carbon or limestone?
A simple illustration will serve to justify the procedure we are
about to adopt. Suppose that the whole of our literary and
pictorial references to earlier stages in the development of the
bicycle, the locomotive, or the loom, were destroyed. We should
still be able to retrace the phases of their evolution, because
we should discover specimens belonging to those early phases
lingering in our museums, in backward regions, and elsewhere.
They might yet be useful in certain environments into which the
higher machines have not penetrated. In the same way, if all the
remains of prehistoric man and early civilisation were lost, we
could still fairly retrace the steps of the human race, by
gathering the lower tribes and races, and arranging them in the
order of their advancement. They are so many surviving
illustrations of the stages through which mankind as a whole has
passed.
Just in the same way we may marshal the countless species of
animals and plants to-day in such order that they will, in a
general way, exhibit to us the age-long procession of life. From
the very start of living evolution certain forms dropped out of
the onward march, and have remained, to our great instruction,
what their ancestors were millions of years ago. People create a
difficulty for themselves by imagining that, if evolution is
true, all animals must evolve. A glance at our own fellows will
show the error of this. Of one family of human beings, as a
French writer has said, one only becomes a Napoleon; the others
remain Lucien, Jerome, or Joseph. Of one family of animals or
trees, some advance in one or other direction; some remain at the
original level. There is no "law of progress." The accidents of
the world and hereditary endowment impel some onward, and do not
impel others. Hence at nearly every great stage in the upward
procession through the ages some regiment of plants or animals
has dropped out, and it represents to-day the stage of life at
which it ceased to progress. In other words, when we survey the
line of the hundreds of thousands of species which we find in
nature to-day, we can trace, amid their countless variations and
branches, the line of organic evolution in the past; just as we
could, from actual instances, study the evolution of a British
house, from the prehistoric remains in Devonshire to a mansion in
Park Lane or a provincial castle.
Another method of retracing the lost early chapters in the
development of life is furnished by embryology. The value of this
method is not recognised by all embryologists, but there are now
few authorities who question the substantial correctness of it,
and we shall, as we proceed, see some remarkable applications of
it. In brief, it is generally admitted that an animal or plant is
apt to reproduce, during its embryonic development, some of the
stages of its ancestry in past time. This does not mean that a
higher animal, whose ancestors were at one time worms, at another
time fishes, and at a later time reptiles, will successively take
the form of a little worm, a little fish, and a little reptile.
The embryonic life itself has been subject to evolution, and this
reproduction of ancestral forms has been proportionately
disturbed. Still, we shall find that animals will tend, in their
embryonic development, to reproduce various structural features
which can only be understood as reminiscences of ancestral
organs. In the lower animals the reproduction is much less
disturbed than in the higher, but even in the case of man this
law is most strikingly verified. We shall find it useful
sometimes at least in confirming our conclusions as to the
ancestry of a particular group.
We have, therefore, two important clues to the missing chapters
in the story of evolution. Just as the scheme of the evolution of
worlds is written broadly across the face of the heavens to-day,
so the scheme of the evolution of life is written on the face of
living nature; and it is written again, in blurred and broken
characters, in the embryonic development of each individual. With
these aids we set out to restore the lost beginning of the epic
of organic evolution.
CHAPTER VI. THE INFANCY OF THE EARTH
The long Archaean period, into which half the story of the earth
is so unsatisfactorily packed, came to a close with a
considerable uplift of the land. We have seen that the earth at
times reaches critical stages owing to the transfer of millions
of tons of matter from the land to the depths of the ocean, and
the need to readjust the pressure on the crust. Apparently this
stage is reached at the end of the Archaean, and a great rise of
the land --probably protracted during hundreds of thousands of
years--takes place. The shore-bottoms round the primitive
continent are raised above the water, their rocks crumpling like
plates of lead under the overpowering pressure. The sea retires
with its inhabitants, mingling their various provinces,
transforming their settled homes. A larger continent spans the
northern ocean of the earth.
In the shore-waters of this early continent are myriads of living
things, representing all the great families of the animal world
below the level of the fish and the insect. The mud and sand in
which their frames are entombed, as they die, will one day be the
"Cambrian" rocks of the geologist, and reveal to him their forms
and suggest their habits. No great volcanic age will reduce them
to streaks of shapeless carbon. The earth now buries its dead,
and from their petrified remains we conjure up a picture of the
swarming life of the Cambrian ocean.
A strange, sluggish population burrows in the mud, crawls over
the sand, adheres to the rocks, and swims among the thickets of
sea-weed. The strangest and most formidable, though still too
puny a thing to survive in a more strenuous age, is the familiar
Trilobite of the geological museum; a flattish animal with broad,
round head, like a shovel, its back covered with a three-lobed
shell, and a number of fine legs or swimmers below. It burrows in
the loose bottom, or lies in it with its large compound eyes
peeping out in search of prey. It is the chief representative of
the hard-cased group (Crustacea) which will later replace it with
the lobster, the shrimp, the crab, and the water-flea. Its
remains form from a third to a fourth of all the buried Cambrian
skeletons. With it, swimming in the water, are smaller members of
the same family, which come nearer to our familiar small
Crustacea.
Shell-fish are the next most conspicuous inhabitants. Molluscs
are already well represented, but the more numerous are the more
elementary Brachiopods ("lampshells"), which come next to the
Trilobites in number and variety. Worms (or Annelids) wind in and
out of the mud, leaving their tracks and tubes for later ages.
Strange ball or cup-shaped little animals, with a hard frame,
mounted on stony stalks and waving irregular arms to draw in the
food-bearing water, are the earliest representatives of the
Echinoderms. Some of these Cystids will presently blossom into
the wonderful sea-lily population of the next age, some are
already quitting their stalks, to become the free-moving
star-fish, of which a primitive specimen has been found in the
later Cambrian. Large jelly-fishes (of which casts are preserved)
swim in the water; coral-animals lay their rocky foundations, but
do not as yet form reefs; coarse sponges rise from the floor; and
myriads of tiny Radiolaria and Thalamophores, with shells of
flint and lime, float at the surface or at various depths.
This slight sketch of the Cambrian population shows us that
living things had already reached a high level of development.
Their story evidently goes back, for millions of years, deep into
those mists of the Archaean age which we were unable to
penetrate. We turn therefore to the zoologist to learn what he
can tell us of the origin and family-relations of these Cambrian
animals, and will afterwards see how they are climbing to higher
levels under the eye of the geologist.
At the basis of the living world of to-day is a vast population
of minute, generally microscopic, animals and plants, which are
popularly known as "microbes." Each consists, in scientific
language, of one cell. It is now well known that the bodies of
the larger animals and plants are made up of millions of these
units of living matter, or cells--the atoms of the organic
world--and I need not enlarge on it. But even a single cell lends
itself to infinite variety of shape, and we have to penetrate to
the very lowest level of this luxuriant world of one-celled
organisms to obtain some idea of the most primitive living
things. Properly speaking, there were no "first living things."
It cannot be doubted by any student of nature that the microbe
developed so gradually that it is as impossible to fix a precise
term for the beginning of life as it is to say when the night
ends and the day begins. In the course of time little one-celled
living units appeared in the waters of the earth, whether in the
shallow shore waters or on the surface of the deep is a matter of
conjecture.
We are justified in concluding that they were at least as
rudimentary in structure and life as the lowest inhabitants of
nature to-day. The distinction of being the lowest known living
organisms should, I think, be awarded to certain one-celled
vegetal organisms which are very common in nature. Minute simple
specks of living matter, sometimes less than the five-thousandth
of an inch in diameter, these lowly Algae are so numerous that it
is they, in their millions, which cover moist surfaces with the
familiar greenish or bluish coat. They have no visible
organisation, though, naturally, they must have some kind of
structure below the range of the microscope. Their life consists
in the absorption of food-particles, at any point of their
surface, and in dividing into two living microbes, instead of
dying, when their bulk increases. A very lowly branch of the
Bacteria (Nitrobacteria) sometimes dispute their claim to the
lowest position in the hierarchy of living nature, but there is
reason to suspect that these Bacteria may have degenerated from a
higher level.
Here we have a convenient starting-point for the story of life,
and may now trace the general lines of upward development. The
first great principle to be recognised is the early division of
these primitive organisms into two great classes, the moving and
the stationary. The clue to this important divergence is found in
diet. With exceptions on both sides, we find that the non-moving
microbes generally feed on inorganic matter, which they convert
into plasm; the moving microbes generally feed on ready-made
plasm--on the living non-movers, on each other, or on particles
of dead organic matter. Now, inorganic food is generally diffused
in the waters, so that the vegetal feeders have no incentive to
develop mobility. On the other hand, the power to move in search
of their food, which is not equally diffused, becomes a most
important advantage to the feeders on other organisms. They
therefore develop various means of locomotion. Some flow or roll
slowly along like tiny drops of oil on an inclined surface;
others develop minute outgrowths of their substance, like fine
hairs, which beat the water as oars do. Some of them have one
strong oar, like the gondolier (but in front of the boat); others
have two or more oars; while some have their little flanks
bristling with fine lashes, like the flanks of a Roman galley.
If we imagine this simple principle at work for ages among the
primitive microbes, we understand the first great division of the
living world, into plants and animals. There must have been a
long series of earlier stages below the plant and animal. In
fact, some writers insist that the first organisms were animal in
nature, feeding on the more elementary stages of living matter.
At last one type develops chlorophyll (the green matter in
leaves), and is able to build up plasm out of inorganic matter;
another type develops mobility, and becomes a parasite on the
plant world. There is no rigid distinction of the two worlds.
Many microscopic plants move about just as animals do, and many
animals live on fixed stalks; while many plants feed on organic
matter. There is so little "difference of nature" between the
plant and the animal that the experts differ in classifying some
of these minute creatures. In fact, we shall often find plants
and animals crossing the line of division. We shall find animals
rooting themselves to the floor, like plants, though they will
generally develop arms or streamers for bringing the food to
them; and we shall find plants becoming insect-catchers. All this
merely shows that the difference is a natural tendency, which
special circumstances may overrule. It remains true that the
great division of the organic world is due to a simple principle
of development; difference of diet leads to difference of
mobility.
But this simple principle will have further consequences of a
most important character. It will lead to the development of mind
in one half of living nature and leave it undeveloped in the
other. Mind, as we know it in the lower levels of life, is not
confined to the animal at all. Many even of the higher plants are
very delicately sensitive to stimulation, and at the lowest level
many plants behave just like animals. In other words, this
sensitiveness to stimuli, which is the first form of mind, is
distributed according to mobility. To the motionless organism it
is no advantage; to the pursuing and pursued organism it is an
immense advantage, and is one of the chief qualities for natural
selection to foster.
For the moment, however, we must glance at the operation of this
and other natural principles in the evolution of the one-celled
animals and plants, which we take to represent the primitive
population of the earth. As there are tens of thousands of
different species even of "microbes," it is clear that we must
deal with them in a very summary way. The evolution of the plant
I reserve for a later chapter, and I must be content to suggest
the development of one-celled animals on very broad lines. When
some of the primitive cells began to feed on each other, and
develop mobility, it is probable that at least two distinct types
were evolved, corresponding to the two lowest animal organisms in
nature to-day. One of these is a very minute and very common (in
vases of decaying flowers, for instance) speck of plasm, which
moves about by lashing the water with a single oar (flagellum),
or hair-like extension of its substance. This type, however,
which is known as the Flagellate, may be derived from the next,
which we will take as the primitive and fundamental animal type.
It is best seen in the common and familiar Amoeba, a minute sac
of liquid or viscid plasm, often not more than a hundredth of an
inch in diameter. As its "skin" is merely a finer kind of the
viscous plasm, not an impenetrable membrane, it takes in food at
any part of its surface, makes little "stomachs," or temporary
cavities, round the food at any part of its interior, ejects the
useless matter at any point, and thrusts out any part of its body
as temporary "arms" or "feet."
Now it is plain that in an age of increasing microbic cannibalism
the toughening of the skin would be one of the first advantages
to secure survival, and this is, in point of fact, almost the
second leading principle in early development. Naturally, as the
skin becomes firmer, the animal can no longer, like the Amoeba,
take food at, or make limbs of, any part of it. There must be
permanent pores in the membrane to receive food or let out rays
of the living substance to act as oars or arms. Thus we get an
immense variety amongst these Protozoa, as the one-celled animals
are called. Some (the Flagellates) have one or two stout oars;
some (the Ciliates) have numbers of fine hairs (or cilia). Some
have a definite mouth-funnel, but no stomach, and cilia drawing
the water into it. Some (Vorticella, etc.), shrinking from the
open battlefield, return to the plant-principle, live on stalks,
and have wreaths of cilia round the open mouth drawing the water
to them. Some (the Heliozoa) remain almost motionless, shooting
out sticky rays of their matter on every side to catch the food.
Some form tubes to live in; some (Coleps) develop horny plates
for armour; and others develop projectiles to pierce their prey
(stinging threads).
This miniature world is full of evolutionary interest, but it is
too vast for detailed study here. We will take one group, which
we know to have been already developed in the Cambrian, and let a
study of its development stand for all. In every lecture or book
on "the beauties of the microscope" we find, and are generally
greatly puzzled by, minute shells of remarkable grace and beauty
that are formed by some of these very elementary animals They are
the Radiolaria (with flinty shells, as a rule) and the
Thalamophora (with chalk frames). Evolution furnishes a simple
key to their remarkable structure.
As we saw, one of the early requirements to be fostered by
natural selection in the Archaean struggle for life was a "thick
skin," and the thick skin had to be porous to let the animal
shoot out its viscid substance in rays and earn its living. This
stage above the Amoeba is beautifully illustrated in the
sun-animalcules (Heliozoa). Now the lowest types of Radiolaria
are of this character. They have no shell or framework at all.
The next stage is for the little animal to develop fine irregular
threads of flint in its skin, a much better security against the
animal-eater. These animalcules, it must be recollected, are bits
of almost pure plasm, and, as they live in crowds, dividing and
subdividing, but never dying, make excellent mouthfuls for a
small feeder. Those with the more flint in their skins were the
more apt to survive and "breed." The threads of flint increase
until they form a sort of thorn-thicket round a little social
group, or a complete lattice round an individual body. Next,
spikes or spines jut out from the lattice, partly for additional
protection, partly to keep the little body afloat at the surface
of the sea. In this way we get a bewildering variety and
increasing complexity of forms, ascending in four divergent lines
from the naked ancestral type to the extreme grace and intricacy
of the Calocyclas monumentum or the Lychnaspis miranda. These,
however, are rare specimens in the 4000 species of Radiolaria. I
have hundreds of them, on microscopic slides, which have no
beauty and little regularity of form. We see a gradual evolution,
on utilitarian principles, as we run over the thousands of forms;
and, when we recollect the inconceivable numbers in which these
little animals have lived and struggled for
life--passively--during tens of millions of years, we are not
surprised at the elaborate protective frames of the higher types.
The Thalamophores, the sister-group of one-celled animals which
largely compose our chalk and much of our limestone, are
developed on the same principle. The earlier forms seem to have
lived in a part of the ocean where silica was scarce, and they
absorbed and built their protective frames of lime. In the
simpler types the frame is not unlike a wide-necked bottle,
turned upside-down. In later forms it takes the shape of a
spirally coiled series of chambers, sometimes amounting to
several thousand. These wonderful little houses are not difficult
to understand. The original tiny animal covers itself with a coat
of lime. It feeds, grows, and bulges out of its chamber. The new
part of its flesh must have a fresh coat, and the process goes on
until scores, or hundreds, or even thousands, of these tiny
chambers make up the spiral shell of the morsel of living matter.
With this brief indication of the mechanical principles which
have directed the evolution of two of the most remarkable groups
of the one-celled animals we must be content, or the dimensions
of this volume will not enable us even to reach the higher and
more interesting types. We must advance at once to the larger
animals, whose bodies are composed of myriads of cells.
The social tendency which pervades the animal world, and the
evident use of that tendency, prepare us to understand that the
primitive microbes would naturally come in time to live in
clusters. Union means effectiveness in many ways, even when it
does not mean strength. We have still many loose associations of
one-celled animals in nature, illustrating the approach to a
community life. Numbers of the Protozoa are social; they live
either in a common jelly-like matrix, or on a common stalk. In
fact, we have a singularly instructive illustration of the
process in the evolution of the sponges.
It is well known that the horny texture to which we commonly give
the name of sponge is the former tenement and shelter of a colony
of one-celled animals, which are the real Sponges. In other
groups the structure is of lime; in others, again, of flinty
material. Now, the Sponges, as we have them to-day, are so
varied, and start from so low a level, that no other group of
animals "illustrates so strikingly the theory of evolution," as
Professor Minchin says. We begin with colonies in which the
individuals are (as in Proterospongia) irregularly distributed in
their jelly-like common bed, each animal lashing the water, as
stalked Flagellates do, and bringing the food to it. Such a
colony would be admirable food for an early carnivore, and we
soon find the protective principle making it less pleasant for
the devourer. The first stage may be--at least there are such
Sponges even now--that the common bed is strewn or sown with the
cast shells of Radiolaria. However that may be, the Sponges soon
begin to absorb the silica or lime of the sea-water, and deposit
it in needles or fragments in their bed. The deposit goes on
until at last an elaborate framework of thorny, or limy, or
flinty material is constructed by the one-celled citizens. In the
higher types a system of pores or canals lets the food-bearing
water pass through, as the animals draw it in with their lashes;
in the highest types the animals come still closer together,
lining the walls of little chambers in the interior.
Here we have a very clear evolutionary transition from the
solitary microbe to a higher level, but, unfortunately, it does
not take us far. The Sponges are a side-issue, or cul de sac,
from the Protozoic world, and do not lead on to the higher. Each
one-celled unit remains an animal; it is a colony of
unicellulars, not a many-celled body. We may admire it as an
instructive approach toward the formation of a many-celled body,
but we must look elsewhere for the true upward advance.
The next stage is best illustrated in certain spherical colonies
of cells like the tiny green Volvox (now generally regarded as
vegetal) of our ponds, or Magosphoera. Here the constituent cells
merge their individuality in the common action. We have the first
definite many-celled body. It is the type to which a moving close
colony of one-celled microbes would soon come. The round surface
is well adapted for rolling or spinning along in the water, and,
as each little cell earns its own living, it must be at the
surface, in contact with the water. Thus a hollow, or
fluid-filled, little sphere, like the Volvox, is the natural
connecting-link between the microbe and the many-celled body, and
may be taken to represent the first important stage in its
development.
The next important stage is also very clearly exhibited in
nature, and is more or less clearly reproduced in the embryonic
development of all animals. We may imagine that the age of
microbes was succeeded by an age of these many-celled larger
bodies, and the struggle for life entered upon a new phase. The
great principle we have already recognised came into play once
more. Large numbers of the many-celled bodies shrank from the
field of battle, and adopted the method of the plant. They rooted
themselves to the floor of the ocean, and developed long arms or
lashes for creating a whirlpool movement in the water, and thus
bringing the food into their open mouths. Forfeiting mobility,
they have, like the plant, forfeited the greater possibilities of
progress, and they remain flowering to-day on the floors of our
waters, recalling the next phase in the evolution of early life.
Such are the hydra, the polyp, the coral, and the sea-anemone. It
is not singular that earlier observers could not detect that they
were animals, and they were long known in science as
"animal-plants" (Zoophytes).
When we look to the common structure of these animals, to find
the ancestral type, we must ignore the nerve and muscle-cells
which they have developed in some degree. Fundamentally, their
body consists of a pouch, with an open mouth, the sides of the
pouch consisting of a double layer of cells. In this we have a
clue to the next stage of animal development. Take a soft
india-rubber ball to represent the first many-celled animal.
Press in one half of the ball close upon the other, narrow the
mouth, and you have something like the body-structure of the
coral and hydra. As this is the course of embryonic development,
and as it is so well retained in the lowest groups of the
many-celled animals, we take it to be the next stage. The reason
for it will become clear on reflection. Division of labour
naturally takes place in a colony, and in that way certain cells
in the primitive body were confined to the work of digestion. It
would be an obvious advantage for these to retire into the
interior, leaving the whole external surface free for the
adjustment of the animal's relations to the outer world.
Again we must refrain from following in detail the development of
this new world of life which branches off in the Archaean ocean.
The evolution of the Corals alone would be a lengthy and
interesting story. But a word must be said about the jelly-fish,
partly because the inexpert will be puzzled at the inclusion of
so active an animal, and partly because its story admirably
illustrates the principle we are studying. The Medusa really
descends from one of the plant-like animals of the early Archaean
period, but it has abandoned the ancestral stalk, turned upside
down, and developed muscular swimming organs. Its past is
betrayed in its embryonic development. As a rule the germ
develops into a stalked polyp, out of which the free-swimming
Medusa is formed. This return to active and free life must have
occurred early, as we find casts of large Medusae in the Cambrian
beds. In complete harmony with the principle we laid down, the
jelly-fish has gained in nerve and sensitiveness in proportion to
its return to an active career.
But this principle is best illustrated in the other branch of the
early many-celled animals, which continued to move about in
search of food. Here, as will be expected, we have the main stem
of the animal world, and, although the successive stages of
development are obscure, certain broad lines that it followed are
clear and interesting.
It is evident that in a swarming population of such animals the
most valuable qualities will be speed and perception. The
sluggish Coral needs only sensitiveness enough, and mobility
enough, to shrink behind its protecting scales at the approach of
danger. In the open water the most speedy and most sensitive will
be apt to escape destruction, and have the larger share in
breeding the next generation. Imagine a selection on this
principle going on for millions of years, and the general result
can be conjectured. A very interesting analogy is found in the
evolution of the boat. From the clumsy hollowed tree of Neolithic
man natural selection, or the need of increasing speed, has
developed the elongated, evenly balanced modern boat, with its
distinct stem and stern. So in the Archaean ocean the struggle to
overtake food, or escape feeders, evolved an elongated two-sided
body, with head and tail, and with the oars (cilia) of the one-
celled ancestor spread thickly along its flanks. In other words,
a body akin to that of the lower water-worms would be the natural
result; and this is, in point of fact, the next stage we find in
the hierarchy of living nature.
Probably myriads of different types of this worm-like
organisation were developed, but such animals leave no trace in
the rocks, and we can only follow the development by broad
analogies. The lowest flat-worms of to-day may represent some of
these early types, and as we ascend the scale of what is loosely
called "worm" organisation, we get some instructive suggestions
of the way in which the various organs develop. Division of
labour continues among the colony of cells which make up the
body, and we get distinct nerve-cells, muscle-cells, and
digestive cells. The nerve-cells are most useful at the head of
an organism which moves through the water, just as the look-out
peers from the head of the ship, and there they develop most
thickly. By a fresh division of labour some of these cells become
especially sensitive to light, some to the chemical qualities of
matter, some to movements of the water; we have the beginning of
the eyes, the nose, and the ears, as simple little depressions in
the skin of the head, lined with these sensitive cells. A
muscular gullet arises to protect the digestive tube; a simple
drainage channel for waste matter forms under the skin; other
channels permit the passage of the fluid food, become (in the
higher worms) muscular blood-vessels, and begin to
contract--somewhat erratically at first-- and drive the blood
through the system.
Here, perhaps, are millions of years of development compressed
into a paragraph. But the purpose of this work is chiefly to
describe the material record of the advance of life in the
earth's strata, and show how it is related to great geological
changes. We must therefore abstain from endeavouring to trace the
genealogy of the innumerable types of animals which were, until
recently, collected in zoology under the heading "Worms." It is
more pertinent to inquire how the higher classes of animals,
which we found in the Cambrian seas, can have arisen from this
primitive worm-like population.
The struggle for life in the Archaean ocean would become keener
and more exacting with the appearance of each new and more
effective type. That is a familiar principle in our industrial
world to-day, and we shall find it illustrated throughout our
story. We therefore find the various processes of evolution,
which we have already seen, now actively at work among the
swarming Archaean population, and producing several very distinct
types. In some of these struggling organisms speed is developed,
together with offensive and defensive weapons, and a line slowly
ascends toward the fish, which we will consider later. In others
defensive armour is chiefly developed, and we get the lines of
the heavy sluggish shell-fish, the Molluscs and Brachiopods, and,
by a later compromise between speed and armour, the more active
tough-coated Arthropods. In others the plant-principle reappears;
the worm-like creature retires from the free-moving life,
attaches itself to a fixed base, and becomes the Bryozoan or the
Echinoderm. To trace the development of these types in any detail
is impossible. The early remains are not preserved. But some
clues are found in nature or in embryonic development, and, when
the types do begin to be preserved in the rocks, we find the
process of evolution plainly at work in them. We will therefore
say a few words about the general evolution of each type, and
then return to the geological record in the Cambrian rocks.
The starfish, the most familiar representative of the
Echinoderms, seems very far removed from the kind of worm-like
ancestor we have been imagining, but, fortunately, the very
interesting story of the starfish is easily learned from the
geological chronicle. Reflect on the flower-like expansion of its
arms, and then imagine it mounted on a stalk, mouth side upward,
with those arms--more tapering than they now are--waving round
the mouth. That, apparently, was the past of the starfish and its
cousins. We shall see that the earliest Echinoderms we know are
cup-shaped structures on stalks, with a stiff, limy frame and (as
in all sessile animals) a number of waving arms round the mouth.
In the next geological age the stalk will become a long and
flexible arrangement of muscles and plates of chalk, the cup will
be more perfectly compacted of chalky plates, and the five arms
will taper and branch until they have an almost feathery
appearance; and the animal will be considered a "sea-lily" by the
early geologist.
The evidence suggests that both the free-moving and the stalked
Echinoderms descend from a common stalked Archaean ancestor. Some
primitive animal abandoned the worm-like habit, and attached
itself, like a polyp, to the floor. Like all such sessile
animals, it developed a wreath of arms round the open mouth. The
"sea-cucumber" (Holothurian) seems to be a type that left the
stalk, retaining the little wreath of arms, before the body was
heavily protected and deformed. In the others a strong limy
skeleton was developed, and the nerves and other organs were
modified in adaptation to the bud-like or flower-like structure.
Another branch of the family then abandoned the stalk, and,
spreading its arms flat, and gradually developing in them numbers
of little "feet" (water-tubes), became the starfish. In the
living Comatula we find a star passing through the stalked stage
in its early development, when it looks like a tiny sea-lily. The
sea-urchin has evolved from the star by folding the arms into a
ball.*
* See the section on Echinoderms, by Professor MacBride, in the
"Cambridge Natural History," I.
The Bryozoa (sea-mats, etc.) are another and lower branch of the
primitive active organisms which have adopted a sessile life. In
the shell-fish, on the other hand, the principle of
armour-plating has its greatest development. It is assuredly a
long and obscure way that leads from the ancestral type of animal
we have been describing to the headless and shapeless mussel or
oyster. Such a degeneration is, however, precisely what we should
expect to find in the circumstances. Indeed, the larva, of many
of the headless Molluscs have a mouth and eyes, and there is a
very common type of larva--the trochosphere--in the Mollusc world
which approaches the earlier form of some of the higher worms.
The Molluscs, as we shall see, provide some admirable
illustrations of the process of evolution. In some of the later
fossilised specimens (Planorbis, Paludina, etc.) we can trace the
animal as it gradually passes from one species to another. The
freshening of the Caspian Sea, which was an outlying part of the
Mediterranean quite late in the geological record, seems to have
evolved several new genera of Molluscs.
Although, therefore, the remains are not preserved of those
primitive Molluscs in which we might see the protecting shell
gradually thickening, and deforming the worm-like body, we are
not without indications of the process. Two unequal branches of
the early wormlike organisms shrank into strong protective
shells. The lower branch became the Brachiopods; the more
advanced branch the Molluscs. In the Mollusc world, in turn,
there are several early types developed. In the Pelecypods (or
Lamellibranchs--the mussel, oyster, etc.) the animal retires
wholly within its fortress, and degenerates. The Gastropods
(snails, etc.) compromise, and retain a certain amount of
freedom, so that they degenerate less. The highest group, the
Cephalopods, "keep their heads," in the literal sense, and we
shall find them advancing from form to form until, in the octopus
of a later age, they discard the ancestral shell, and become the
aristocrats of the Mollusc kingdom.
The last and most important line that led upward from the chaos
of Archaean worms is that of the Arthropods. Its early
characteristic was the acquisition of a chitinous coat over the
body. Embryonic indications show that this was at first a
continuous shield, but a type arose in which the coat broke into
sections covering each segment of the body, giving greater
freedom of movement. The shield, in fact, became a fine coat of
mail. The Trilobite is an early and imperfect experiment of the
class, and the larva of the modern king-crab bears witness that
it has not perished without leaving descendants. How later
Crustacea increase the toughness of the coat by deposits of lime,
and lead on to the crab and lobster, and how one early branch
invades the land, develops air-breathing apparatus, and
culminates in the spiders and insects, will be considered later.
We shall see that there is most remarkable evidence connecting
the highest of the Arthropods, the insect, with a remote Annelid
ancestor.
We are thus not entirely without clues to the origin of the more
advanced animals we find when the fuller geological record
begins. Further embryological study, and possibly the discovery
of surviving primitive forms, of which Central Africa may yet
yield a number, may enlarge our knowledge, but it is likely to
remain very imperfect. The fossil records of the long ages during
which the Mollusc, the Crustacean, and the Echinoderm slowly
assumed their characteristic forms are hopelessly lost. But we
are now prepared to return to the record which survives, and we
shall find the remaining story of the earth a very ample and
interesting chronicle of evolution.
CHAPTER VII. THE PASSAGE TO THE LAND
Slender as our knowledge is of the earlier evolution of the
Invertebrate animals, we return to our Cambrian population with
greater interest. The uncouth Trilobite and its livelier cousins,
the sluggish, skulking Brachiopod and Mollusc, the squirming
Annelids, and the plant-like Cystids, Corals, and Sponges are the
outcome of millions of years of struggle. Just as men, when their
culture and their warfare advanced, clothed themselves with
armour, and the most completely mailed survived the battle, so,
generation after generation, the thicker and harder-skinned
animals survived in the Archaean battlefield, and the Cambrian
age opened upon the various fashions of armour that we there
described. But, although half the story of life is over,
organisation is still imperfect and sluggish. We have now to see
how it advances to higher levels, and how the drama is
transferred from the ocean to a new and more stimulating
environment.
The Cambrian age begins with a vigorous move on the part of the
land. The seas roll back from the shores of the "lost Atlantis,"
and vast regions are laid bare to the sun and the rains. In the
bays and hollows of the distant shores the animal survivors of
the great upheaval adapt themselves to their fresh homes and
continue the struggle. But the rivers and the waves are at work
once more upon the land, and, as the Cambrian age proceeds, the
fringes of the continents are sheared, and the shore-life
steadily advances upon the low-lying land. By the end of the
Cambrian age a very large proportion of the land is covered with
a shallow sea, in which the debris of its surface is deposited.
The levelling continues through the next (Ordovician) period.
Before its close nearly the whole of the United States and the
greater part of Canada are under water, and the new land that had
appeared on the site of Europe is also for the most part
submerged. The present British Isles are almost reduced to a
strip of north-eastern Ireland, the northern extremity of
Scotland, and large islands in the south-west and centre of
England.
We have already seen that these victories of the sea are just as
stimulating, in a different way, to animals as the victories of
the land. American geologists are tracing, in a very instructive
way, the effect on that early population of the encroachment of
the sea. In each arm of the sea is a distinctive fauna. Life is
still very parochial; the great cosmopolitans, the fishes, have
not yet arrived. As the land is revelled, the arms of the sea
approach each other, and at last mingle their waters and their
populations, with stimulating effect. Provincial characters are
modified, and cosmopolitan characters increase in the great
central sea of America. The vast shallow waters provide a greatly
enlarged theatre for the life of the time, and it flourishes
enormously. Then, at the end of the Ordovician, the land begins
to rise once more. Whether it was due to a fresh shrinking of the
crust, or to the simple process we have described, or both, we
need not attempt to determine; but both in Europe and America
there is a great emergence of land. The shore-tracts and the
shallow water are narrowed, the struggle is intensified in them,
and we pass into the Silurian age with a greatly reduced number
but more advanced variety of animals. In the Silurian age the sea
advances once more, and the shore-waters expand. There is another
great "expansive evolution" of life. But the Silurian age closes
with a fresh and very extensive emergence of the land, and this
time it will have the most important consequences. For two new
things have meantime appeared on the earth. The fish has evolved
in the waters, and the plant, at least, has found a footing on
the land.
These geological changes which we have summarised and which have
been too little noticed until recently in evolutionary studies,
occupied 7,000,000 years, on the lowest estimate, and probably
twice that period. The impatient critic of evolutionary
hypotheses is apt to forget the length of these early periods. We
shall see that in the last two or three million years of the
earth's story most extraordinary progress has been made in plant
and animal development, and can be very fairly traced. How much
advance should we allow for these seven or fourteen million years
of swarming life and changing environments?
We cannot nearly cover the whole ground of paleontology for the
period, and must be content to notice some of the more
interesting advances, and then deal more fully with the evolution
of the fish, the forerunner of the great land animals.
The Trilobite was the most arresting figure in the Cambrian sea,
and its fortunes deserve a paragraph. It reaches its climax in
the Ordovician sea, and then begins to decline, as more powerful
animals come upon the scene. At first (apparently) an eyeless
organism, it gradually develops compound eyes, and in some
species the experts have calculated that there were 15,000 facets
to each eye. As time goes on, also, the eye stands out from the
head on a kind of stalk, giving a wider range of vision. Some of
the more sluggish species seem to have been able to roll
themselves up, like hedgehogs, in their shells, when an enemy
approached. But another branch of the same group (Crustacea) has
meantime advanced, and it gradually supersedes the dwindling
Trilobites. Toward the close of the Silurian great scorpion-like
Crustaceans (Pterygotus, Eurypterus, etc.) make their appearance.
Their development is obscure, but it must be remembered that the
rocks only give the record of shore-life, and only a part of that
is as yet opened by geology. Some experts think that they were
developed in inland waters. Reaching sometimes a length of five
or six feet, with two large compound eyes and some smaller
eye-spots (ocelli), they must have been the giants of the
Silurian ocean until the great sharks and other fishes appeared.
The quaint stalked Echinoderm which also we noticed in the
Cambrian shallows has now evolved into a more handsome creature,
the sea-lily. The cup-shaped body is now composed of a large
number of limy plates, clothed with flesh; the arms are long,
tapering, symmetrical, and richly fringed; the stalk advances
higher and higher, until the flower-like animal sometimes waves
its feathery arms from the top of a flexible pedestal composed of
millions of tiny chalk disks. Small forests of these sea-lilies
adorn the floor of the Silurian ocean, and their broken and dead
frames form whole beds of limestone. The primitive Cystids
dwindle and die out in the presence of such powerful competitors.
Of 250 species only a dozen linger in the Silurian strata, though
a new and more advanced type--the Blastoid--holds the field for a
time. It is the age of the Crinoids or sea-lilies. The starfish,
which has abandoned the stalk, does not seem to prosper as yet,
and the brittle-star appears. Their age will come later. No
sea-urchins or sea-cucumbers (which would hardly be preserved)
are found as yet. It is precisely the order of appearance which
our theory of their evolution demands.
The Brachiopods have passed into entirely new and more advanced
species in the many advances and retreats of the shores, but the
Molluscs show more interesting progress. The commanding group
from the start is that of the Molluscs which have "kept their
head," the Cephalopods, and their large shells show a most
instructive evolution. The first great representative of the
tribe is a straight-shelled Cephalopod, which becomes "the tyrant
and scavenger of the Silurian ocean" (Chamberlin). Its tapering,
conical shell sometimes runs to a length of fifteen feet, and a
diameter of one foot. It would of itself be an important
evolutionary factor in the primitive seas, and might explain more
than one advance in protective armour or retreat into heavy
shells. As the period advances the shell begins to curve, and at
last it forms a close spiral coil. This would be so great an
advantage that we are not surprised to find the coiled type
(Goniatites) gain upon and gradually replace the straight-shelled
types (Orthoceratites). The Silurian ocean swarms with these
great shelled Cephalopods, of which the little Nautilus is now
the only survivor.
We will not enlarge on the Sponges and Corals, which are slowly
advancing toward the higher modern types. Two new and very
powerful organisms have appeared, and merit the closest
attention. One is the fish, the remote ancestor of the birds and
mammals that will one day rule the earth. The other may be the
ancestor of the fish itself, or it may be one of the many
abortive outcomes and unsuccessful experiments of the stirring
life of the time. And while these new types are themselves a
result of the great and stimulating changes which we have
reviewed and the incessant struggle for food and safety, they in
turn enormously quicken the pace of development. The Dreadnought
appears in the primitive seas; the effect on the fleets of the
world of the evolution of our latest type of battleship gives us
a faint idea of the effect, on all the moving population, of the
coming of these monsters of the deep. The age had not lacked
incentives to progress; it now obtains a more terrible and
far-reaching stimulus.
To understand the situation let us see how the battle of land and
sea had proceeded. The Devonian Period had opened with a fresh
emergence of the land, especially in Europe, and great inland
seas or lakes were left in the hollows. The tincture of iron
which gives a red colour to our characteristic Devonian rocks,
the Old Red Sandstone, shows us that the sand was deposited in
inland waters. The fish had already been developed, and the
Devonian rocks show it swarming, in great numbers and variety, in
the enclosed seas and round the fringe of the continents.
The first generation was a group of strange creatures, half fish
and half Crustacean, which are known as the Ostracoderms. They
had large armour-plated heads, which recall the Trilobite, and
suggest that they too burrowed in the mud of the sea or (as many
think) of the inland lakes, making havoc among the shell-fish,
worms, and small Crustacea. The hind-part of their bodies was
remarkably fish-like in structure. But they had no
backbone--though we cannot say whether they may not have had a
rod of cartilage along the back-- and no articulated jaws like
the fish. Some regard them as a connecting link between the
Crustacea and the fishes, but the general feeling is that they
were an abortive development in the direction of the fish. The
sharks and other large fishes, which have appeared in the
Silurian, easily displace these clumsy and poor-mouthed
competitors One almost thinks of the aeroplane superseding the
navigable balloon.
Of the fishes the Arthrodirans dominated the inland seas
(apparently), while the sharks commanded the ocean. One of the
Arthrodirans, the Dinichthys ("terrible fish"), is the most
formidable fish known to science. It measured twenty feet from
snout to tail. Its monstrous head, three feet in width, was
heavily armoured, and, instead of teeth, its great jaws, two feet
in length, were sharpened, and closed over the victim like a
gigantic pair of clippers. The strongly plated heads of these
fishes were commonly a foot or two feet in width. Life in the
waters became more exacting than ever. But the Arthrodirans were
unwieldy and sluggish, and had to give way before more
progressive types. The toothed shark gradually became the lord of
the waters.
The early shark ate, amongst other things, quantities of Molluscs
and Brachiopods. Possibly he began with Crustacea; in any case
the practice of crunching shellfish led to a stronger and
stronger development of the hard plate which lined his mouth. The
prickles of the plate grew larger and harder, until--as may be
seen to-day in the mouth of a young shark--the cavity was lined
with teeth. In the bulk of the Devonian sharks these developed
into what are significantly called "pavement teeth." They were
solid plates of enamel, an inch or an inch and a half in width,
with which the monster ground its enormous meals of Molluscs,
Crustacea, sea-weed, etc. A new and stimulating element had come
into the life of the invertebrate world. Other sharks snapped
larger victims, and developed the teeth on the edges of their
jaws, to the sacrifice of the others, until we find these teeth
in the course of time solid triangular masses of enamel, four or
five inches long, with saw-like edges. Imagine these terrible
mouths--the shears of the Arthrodiran, and the grindstones and
terrible crescents of the giant sharks--moving speedily amongst
the crowded inhabitants of the waters, and it is easy to see what
a stimulus to the attainment of speed and of protective devices
was given to the whole world of the time.
What was the origin of the fish? Here we are in much the same
position as we were in regard to the origin of the higher
Invertebrates. Once the fish plainly appears upon the scene it is
found to be undergoing a process of evolution like all other
animals. The vast majority of our fishes have bony frames (or are
Teleosts); the fishes of the Devonian age nearly all have frames
of cartilage, and we know from embryonic development that
cartilage is the first stage in the formation of bone. In the
teeth and tails, also, we find a gradual evolution toward the
higher types. But the earlier record is, for reasons I have
already given, obscure; and as my purpose is rather to discover
the agencies of evolution than to strain slender evidence in
drawing up pedigrees, I need only make brief reference to the
state of the problem.
Until comparatively recent times the animal world fell into two
clearly distinct halves, the Vertebrates and the Invertebrates.
There were several anatomical differences between the two
provinces, but the most conspicuous and most puzzling was the
backbone. Nowhere in living nature or in the rocks was any
intermediate type known between the backboned and the
non-backboned animal. In the course of the nineteenth century,
however, several animals of an intermediate type were found. The
sea-squirt has in its early youth the line of cartilage through
the body which, in embryonic development, represents the first
stage of the backbone; the lancelet and the Appendicularia have a
rod of cartilage throughout life; the "acorn-headed worm" shows
traces of it. These are regarded as surviving specimens of
various groups of animals which, in early times, fell between the
Invertebrate and Vertebrate worlds, and illustrate the
transition.
With their aid a genealogical tree was constructed for the fish.
It was assumed that some Cambrian or Silurian Annelid obtained
this stiffening rod of cartilage. The next advantage--we have
seen it in many cases-- was to combine flexibility with support.
The rod was divided into connected sections (vertebrae), and
hardened into bone. Besides stiffening the body, it provided a
valuable shelter for the spinal cord, and its upper part expanded
into a box to enclose the brain. The fins were formed of folds of
skin which were thrown off at the sides and on the back, as the
animal wriggled through the water. They were of use in swimming,
and sections of them were stiffened with rods of cartilage, and
became the pairs of fins. Gill slits (as in some of the highest
worms) appeared in the throat, the mouth was improved by the
formation of jaws, and--the worm culminated in the shark.
Some experts think, however, that the fish developed directly
from a Crustacean, and hold that the Ostracoderms are the
connecting link. A close discussion of the anatomical details
would be out of place here,* and the question remains open for
the present. Directly or indirectly, the fish is a descendant of
some Archaean Annelid. It is most probable that the shark was the
first true fish-type. There are unrecognisable fragments of
fishes in the Ordovician and Silurian rocks, but the first
complete skeletons (Lanarkia, etc.) are of small shark-like
creatures, and the low organisation of the group to which the
shark belongs, the Elasmobranchs, makes it probable that they are
the most primitive. Other remains (Palaeospondylus) show that the
fish-like lampreys had already developed.
* See, especially, Dr. Gaskell's "Origin of Vertebrates" (1908).
Two groups were developed from the primitive fish, which have
great interest for us. Our next step, in fact, is to trace the
passage of the fish from the water to the land, one of the most
momentous chapters in the story of life. To that incident or
accident of primitive life we owe our own existence and the whole
development of the higher types of animals. The advance of
natural history in modern times has made this passage to the land
easy to understand. Not only does every frog reenact it in the
course of its development, but we know many fishes that can live
out of water. There is an Indian perch--called the "climbing
perch," but it has only once been seen by a European to climb a
tree--which crosses the fields in search of another pool, when
its own pool is evaporating. An Indian marine fish
(Periophthalmus) remains hunting on the shore when the tide goes
out. More important still, several fishes have lungs as well as
gills. The Ceratodus of certain Queensland rivers has one lung;
though, I was told by the experts in Queensland, it is not a
"mud-fish," and never lives in dry mud. However, the Protopterus
of Africa and the Lepidosiren of South America have two lungs, as
well as gills, and can live either in water or, in the dry
season, on land.
When the skeletons of fishes of the Ceratodus type were
discovered in the Devonian rocks, it was felt that we had found
the fish-ancestor of the land Vertebrates, but a closer
anatomical examination has made this doubtful. The Devonian
lung-fish has characters which do not seem to lead on to the
Amphibia. The same general cause probably led many groups to
leave the water, or adapt themselves to living on land as well as
in water, and the abundant Dipoi or Dipneusts
("double-breathers") of the Devonian lakes are one of the chief
of these groups, which have luckily left descendants to our time.
The ancestors of the Amphibia are generally sought amongst the
Crossopterygii, a very large group of fishes in Devonian times,
with very few representatives to-day.
It is more profitable to investigate the process itself than to
make a precarious search for the actual fish, and, fortunately,
this inquiry is more hopeful. The remains that we find make it
probable that the fish left the water about the beginning of the
Devonian or the end of the Silurian. Now this period coincides
with two circumstances which throw a complete light on the step;
one is the great rise of the land, catching myriads of fishes in
enclosed inland seas, and the other is the appearance of
formidable carnivores in the waters. As the seas evaporated* and
the great carnage proceeded, the land, which was already covered
with plants and inhabited by insects, offered a safe retreat for
such as could adopt it. Emigration to the land had been going on
for ages, as we shall see. Curious as it must seem to the
inexpert, the fishes, or some of them, were better prepared than
most other animals to leave the water. The chief requirement was
a lung, or interior bag, by which the air could be brought into
close contact with the absorbing blood vessels. Such a bag,
broadly speaking, most of the fishes possess in their
floating-bladder: a bag of gas, by compressing or expanding which
they alter their specific gravity in the water. In some fishes it
is double; in some it is supplied with blood-vessels; in some it
is connected by a tube with the gullet, and therefore with the
atmosphere.
* It is now usually thought that the inland seas were the theatre
of the passage to land. I must point out, however, that the wide
distribution of our Dipneusts, in Australia, tropical Africa, and
South America, suggests that they were marine though they now
live in fresh water. But we shall see that a continent united the
three regions at one time, and it may afford some explanation.
Thus we get very clear suggestions of the transition from water
to land. We must, of course, conceive it as a slow and gradual
adaptation. At first there may have been a rough contrivance for
deriving oxygen directly and partially from the atmosphere, as
the water of the lake became impure. So important an advantage
would be fostered, and, as the inland sea became smaller, or its
population larger or fiercer, the fishes with a sufficiently
developed air-breathing apparatus passed to the land, where, as
yet, they would find no serious enemy. The fact is beyond
dispute; the theory of how it occurred is plausible enough; the
consequences were momentous. Great changes were preparing on the
land, and in a comparatively short time we shall find its new
inhabitant subjected to a fierce test of circumstances that will
carry it to an enormously higher level than life had yet reached.
I have said that the fact of this transition to the land is
beyond dispute. The evidence is very varied, but need not all be
enlarged upon here. The widespread Dipneust fishes of the
Devonian rocks bear strong witness to it, and the appearance of
the Amphibian immediately afterwards makes it certain. The
development of the frog is a reminiscence of it, on the lines of
the embryonic law which we saw earlier. An animal, in its
individual development, more or less reproduces the past phases
of its ancestry. So the free-swimming jelly-fish begins life as a
fixed polyp; a kind of star-fish (Comatula) opens its career as a
stalked sea-lily; the gorgeous dragon-fly is at first an uncouth
aquatic animal, and the ethereal butterfly a worm-like creature.
But the most singular and instructive of all these embryonic
reminiscences of the past is found in the fact that all the
higher land-animals of to-day clearly reproduce a fish-stage in
their embryonic development.
In the third and fourth weeks of development the human embryo
shows four (closed) slits under the head, with corresponding
arches. The bird, the dog, the horse--all the higher land
animals, in a word, pass through the same phase. The suggestion
has been made that these structures do not recall the gill-slits
and gill-arches of the fish, but are folds due to the packing of
the embryo in the womb. In point of fact, they appear just at the
time when the human embryo is only a fifth of an inch long, and
there is no such compression. But all doubt as to their
interpretation is dispelled when we remove the skin and examine
the heart and blood-vessels. The heart is up in the throat, as in
the fish, and has only two chambers, as in the fish (not four, as
in the bird and mammal); and the arteries rise in five pairs of
arches over the swellings in the throat, as they do in the lower
fish, but do not in the bird and mammal. The arrangement is
purely temporary--lasting only a couple of weeks in the human
embryo--and purposeless. Half these arteries will disappear
again. They quite plainly exist to supply fine blood-vessels for
breathing at the gill-clefts, and are never used, for the embryo
does not breathe, except through the mother. They are a most
instructive reminder of the Devonian fish which quitted its
element and became the ancestor of all the birds and mammals of a
later age.
Several other features of man's embryonic development--the
budding of the hind limbs high up, instead of at the base of, the
vertebral column, the development of the ears, the nose, the
jaws, etc.--have the same lesson, but the one detailed
illustration will suffice. The millions of years of stimulating
change and struggle which we have summarised have resulted in the
production of a fish which walks on four limbs (as the South
American mud-fish does to-day), and breathes the atmosphere.
We have been quite unable to follow the vast changes which have
meantime taken place in its organisation. The eyes, which were
mere pits in the skin, lined with pigment cells, in the early
worm, now have a crystalline lens to concentrate the light and
define objects on the nerve. The ears, which were at first
similar sensitive pits in the skin, on which lay a little stone
whose movements gave the animal some sense of direction, are now
closed vesicles in the skull, and begin to be sensitive to waves
of sound. The nose, which was at first two blind, sensitive pits
in the skin of the head, now consists of two nostrils opening
into the mouth, with an olfactory nerve spreading richly over the
passages. The brain, which was a mere clump of nerve-cells
connecting the rough sense-impressions, is now a large and
intricate structure, and already exhibits a little of that
important region (the cerebrum) in which the varied images of the
outside world are combined. The heart, which was formerly was a
mere swelling of a part of one of the blood-vessels, now has two
chambers.
We cannot pursue these detailed improvements of the mechanism, as
we might, through the ascending types of animals. Enough if we
see more or less clearly how the changes in the face of the earth
and the rise of its successive dynasties of carnivores have
stimulated living things to higher and higher levels in the
primitive ocean. We pass to the clearer and far more important
story of life on land, pursuing the fish through its continuous
adaptations to new conditions until, throwing out side-branches
as it progresses, it reaches the height of bird and mammal life.
CHAPTER VIII. THE COAL-FOREST
With the beginning of life on land we open a new and more
important volume of the story of life, and we may take the
opportunity to make clearer certain principles or processes of
development which we may seem hitherto to have taken for granted.
The evolutionary work is too often a mere superficial description
of the strange and advancing classes of plants and animals which
cross the stage of geology. Why they change and advance is not
explained. I have endeavoured to supply this explanation by
putting the successive populations of the earth in their
respective environments, and showing the continuous and
stimulating effect on them of changes in those environments. We
have thus learned to decipher some lines of the decalogue of
living nature. "Thou shalt have a thick armour," "Thou shalt be
speedy," "Thou shalt shelter from the more powerful," are some of
the laws of primeval life. The appearance of each higher and more
destructive type enforces them with more severity; and in their
observance animals branch outward and upward into myriads of
temporary or permanent forms.
But there is no consciousness of law and no idea of evading
danger. There is not even some mysterious instinct "telling" the
animal, as it used to be said, to do certain things. It is, in
fact, not strictly accurate to say that a certain change in the
environment stimulates animals to advance. Generally speaking, it
does not act on the advancing at all, but on the non-advancing,
which it exterminates. The procedure is simple, tangible, and
unconscious. Two invading arms of the sea meet and pour together
their different waters and populations. The habits, the foods,
and the enemies of many types of animals are changed; the less
fit for the new environment die first, the more fit survive
longest and breed most of the new generation. It is so with men
when they migrate to a more exacting environment, whether a
dangerous trade or a foreign clime. Again, take the case of the
introduction of a giant Cephalopod or fish amongst a population
of Molluscs and Crustacea. The toughest, the speediest, the most
alert, the most retiring, or the least conspicuous, will be the
most apt to survive and breed. In hundreds or thousands of
generations there will be an enormous improvement in the armour,
the speed, the sensitiveness, the hiding practices, and the
protective colours, of the animals which are devoured. The
"natural selection of the fittest" really means the "natural
destruction of the less fit."
The only point assumed in this is that the young of an animal or
plant tend to differ from each other and from their parents.
Darwin was content to take this as a fact of common observation,
as it obviously is, but later science has thrown some light on
the causes of these variations. In the first place, the germs in
the parent's body may themselves be subject to struggle and
natural selection, and not share equally in the food-supply.
Then, in the case of the higher animals (or the majority of
animals), there is a clear source of variation in the fact that
the mature germ is formed of certain elements from two different
parents, four grandparents, and so on. In the case of the lower
animals the germs and larvae float independently in the water,
and are exposed to many influences. Modern embryologists have
found, by experiment, that an alteration of the temperature or
the chemical considerable effect on eggs and larvae. Some recent
experiments have shown that such changes may even affect the eggs
in the mother's ovary. These discoveries are very important and
suggestive, because the geological changes which we are studying
are especially apt to bring about changes of temperature and
changes in the freshness or saltiness of water.
Evolution is, therefore, not a "mere description" of the
procession of living things; it is to a great extent an
explanation of the procession. When, however, we come to apply
these general principles to certain aspects of the advance in
organisation we find fundamental differences of opinion among
biologists, which must be noted. As Sir E. Ray Lankester recently
said, it is not at all true that Darwinism is questioned in
zoology to-day. It is true only that Darwin was not omniscient or
infallible, and some of his opinions are disputed.
Let me introduce the subject with a particular instance of
evolution, the flat-fish. This animal has been fitted to survive
the terrible struggle in the seas by acquiring such a form that
it can lie almost unseen upon the floor of the ocean. The eye on
the under side of the body would thus be useless, but a glance at
a sole or plaice in a fishmonger's shop will show that this eye
has worked upward to the top of the head. Was the eye shifted by
the effort and straining of the fish, inherited and increased
slightly in each generation? Is the explanation rather that those
fishes in each generation survived and bred which happened from
birth to have a slight variation in that direction, though they
did not inherit the effect of the parent's effort to strain the
eye? Or ought we to regard this change of structure as brought
about by a few abrupt and considerable variations on the part of
the young? There you have the three great schools which divide
modern evolutionists: Lamarckism, Weismannism, and Mendelism (or
Mutationism). All are Darwinians. No one doubts that the
flat-fish was evolved from an ordinary fish--the flat-fish is an
ordinary fish in its youth--or that natural selection (enemies)
killed off the old and transitional types and overlooked (and so
favoured) the new. It will be seen that the language used in this
volume is not the particular language of any one of these
schools. This is partly because I wish to leave seriously
controverted questions open, and partly from a feeling of
compromise, which I may explain.*
* Of recent years another compromise has been proposed between
the Lamarckians and Weismannists. It would say that the efforts
of the parent and their effect on the position of the eye--in our
case--are not inherited, but might be of use in sheltering an
embryonic variation in the direction of a displaced eye.
First, the plain issue between the Mendelians and the other two
schools--whether the passage from species to species is brought
about by a series of small variations during a long period or by
a few large variations (or "mutations") in a short period--is
open to an obvious compromise. It is quite possible that both
views are correct, in different cases, and quite impossible to
find the proportion of each class of cases. We shall see later
that in certain instances where the conditions of preservation
were good we can sometimes trace a perfectly gradual advance from
species to species. Several shellfish have been traced in this
way, and a sea-urchin in the chalk has been followed, quite
gradually, from one end of a genus to the other. It is
significant that the advance of research is multiplying these
cases. There is no reason why we may not assume most of the
changes of species we have yet seen to have occurred in this way.
In fact, in some of the lower branches of the animal world
(Radiolaria, Sponges, etc.) there is often no sharp division of
species at all, but a gradual series of living varieties.
On the other hand we know many instances of very considerable
sudden changes. The cases quoted by Mendelists generally belong
to the plant world, but instances are not unknown in the animal
world. A shrimp (Artemia) was made to undergo considerable
modification, by altering the proportion of salt in the water in
which it was kept. Butterflies have been made to produce young
quite different from their normal young by subjecting them to
abnormal temperature, electric currents, and so on; and, as I
said, the most remarkable effects have been produced on eggs and
embryos by altering the chemical and physical conditions. Rats--I
was informed by the engineer in charge of the refrigerating room
on an Australian liner--very quickly became adapted to the
freezing temperature by developing long hair. All that we have
seen of the past changes in the environment of animals makes it
probable that these larger variations often occur. I would
conclude, therefore, that evolution has proceeded continuously
(though by no means universally) through the ages, but there were
at times periods of more acute change with correspondingly larger
changes in the animal and plant worlds.
In regard to the issue between the Lamarckians and
Weismannists--whether changes acquired by the parent are
inherited by the young--recent experiments again suggest
something of a compromise. Weismann says that the body of the
parent is but the case containing the germ-plasm, so that all
modifications of the living parent body perish with it, and do
not affect the germ, which builds the next generation. Certainly,
when we reflect that the 70,000 ova in the human mother's ovary
seem to have been all formed in the first year of her life, it is
difficult to see how modifications of her muscles or nerves can
affect them. Thus we cannot hope to learn anything, either way,
by cutting off the tails of cows, and experiments of that kind.
But it is acknowledged that certain diseases in the blood, which
nourishes the germs, may affect them, and recent experimenters
have found that they can reach and affect the germs in the body
by other agencies, and so produce inherited modifications in the
parent.* If this claim is sustained and enlarged, it may be
concluded that the greater changes of environment which we find
in the geological chronicle may have had a considerable influence
of this kind.
* See a paper read by Professor Bourne to the Zoological Section
of the British Association, 1910. It must be understood that when
I speak of Weismannism I do not refer to this whole theory of
heredity, which, he acknowledges, has few supporters. The
Lamarckian view is represented in Britain by Sir W. Turner and
Professor Darwin. In other countries it has a larger proportion
of distinguished supporters. On the whole subject see Professor
J. A. Thomson's "Heredity" (1909), Dewar and Finn's "Making of
Species" (1909--a Mendelian work), and, for essays by the leaders
of each school, "Darwinism and Modern Science" (1909).
The general issue, however, must remain open. The Lamarckian and
Weismannist theories are rival interpretations of past events,
and we shall not find it necessary to press either. When the fish
comes to live on land, for instance, it develops a bony limb out
of its fin. The Lamarckian says that the throwing of the weight
of the body on the main stem of the fin strengthens it, as
practice strengthens the boxer's arm, and the effect is inherited
and increased in each generation, until at last the useless
paddle of the fin dies away and the main stem has become a stout,
bony column. Weismann says that the individual modification, by
use in walking, is not inherited, but those young are favoured
which have at birth a variation in the strength of the stem of
the fin. As each of these interpretations is, and must remain,
purely theoretical, we will be content to tell the facts in such
cases. But these brief remarks will enable the reader to
understand in what precise sense the facts we record are open to
controversy.
Let us return to the chronicle of the earth. We had reached the
Devonian age, when large continents, with great inland seas,
existed in North America, north-west Europe, and north Asia,
probably connected by a continent across the North Atlantic and
the Arctic region. South America and South Africa were emerging,
and a continent was preparing to stretch from Brazil, through
South Africa and the Antarctic, to Australia and India. The
expanse of land was, with many oscillations, gaining on the
water, and there was much emigration to it from the
over-populated seas. When the fish went on land in the Devonian,
it must have found a diet (insects, etc.) there, and the insects
must have been preceded by a plant population. We have first,
therefore, to consider the evolution of the plant, and see how it
increases in form and number until it covers the earth with the
luxuriant forests of the Carboniferous period.
The plant world, we saw, starts, like the animal world, with a
great kingdom of one-celled microscopic representatives, and the
same principles of development, to a great extent, shape it into
a large variety of forms. Armour-plating has a widespread
influence among them. The graceful Diatom is a morsel of plasm
enclosed in a flinty box, often with a very pretty arrangement of
the pores and markings. The Desmid has a coat of cellulose, and a
less graceful coat of cellulose encloses the Peridinean. Many of
these minute plants develop locomotion and a degree of
sensitiveness (Diatoms, Peridinea, Euglena, etc.). Some
(Bacteria) adopt animal diet, and rise in power of movement and
sensitiveness until it is impossible to make any satisfactory
distinction between them and animals. Then the social principle
enters. First we have loose associations of one-celled plants in
a common bed, then closer clusters or many-celled bodies. In some
cases (Volvox) the cluster, or the compound plant, is round and
moves briskly in the water, closely resembling an animal. In most
cases, the cells are connected in chains, and we begin to see the
vague outline of the larger plant.
When we had reached this stage in the development of animal life,
we found great difficulty in imagining how the chief lines of the
higher Invertebrates took their rise from the Archaean chaos of
early many-celled forms. We have an even greater difficulty here,
as plant remains are not preserved at all until the Devonian
period. We can only conclude, from the later facts, that these
primitive many-celled plants branched out in several different
directions. One section (at a quite unknown date) adopted an
organic diet, and became the Fungi; and a later co-operation, or
life-partnership, between a Fungus and a one-celled Alga led to
the Lichens. Others remained at the Alga-level, and grew in great
thickets along the sea bottoms, no doubt rivalling or surpassing
the giant sea-weeds, sometimes 400 feet long, off the American
coast to-day. Other lines which start from the level of the
primitive many-celled Algae develop into the Mosses (Bryophyta),
Ferns (Pteridophyta), Horsetails (Equisetalia), and Club-mosses
(Lycopodiales). The mosses, the lowest group, are not preserved
in the rocks; from the other three classes will come the great
forests of the Carboniferous period.
The early record of plant-life is so poor that it is useless to
speculate when the plant first left the water. We have somewhat
obscure and disputed traces of ferns in the Ordovician, and, as
they and the Horsetails and Club-mosses are well developed in the
Devonian, we may assume that some of the sea-weeds had become
adapted to life on land, and evolved into the early forms of the
ferns, at least in the Cambrian period. From that time they begin
to weave a mantle of sombre green over the exposed land, and to
play a most important part in the economy of nature.
We saw that at the beginning of the Devonian there was a
considerable rise of the land both in America and Europe, but
especially in Europe. A distant spectator at that time would have
observed the rise of a chain of mountains in Scotland and a
general emergence of land north-western Europe. A continent
stretched from Ireland to Scandinavia and North Russia, while
most of the rest of Europe, except large areas of Russia, France,
Germany, and Turkey, was under the sea. Where we now find our
Alps and Pyrenees towering up to the snow-line there were then
level stretches of ocean. Even the north-western continent was
scooped into great inland seas or lagoons, which stretched from
Ireland to Scandinavia, and, as we saw, fostered the development
of the fishes.
As the Devonian period progressed the sea gained on the land, and
must have restricted the growth of vegetation, but as the lake
deposits now preserve the remains of the plants which grow down
to their shores, or are washed into them, we are enabled to
restore the complexion of the landscape. Ferns, generally of a
primitive and generalised character, abound, and include the
ferns such as we find in warm countries to-day. Horsetails and
Club-mosses already grow into forest-trees. There are even
seed-bearing ferns, which give promise of the higher plants to
come, but as yet nothing approaching our flower and fruit-bearing
trees has appeared. There is as yet no certain indication of the
presence of Conifers. It is a sombre and monotonous vegetation,
unlike any to be found in any climate to-day.
We will look more closely into its nature presently. First let us
see how these primitive types of plants come to form the immense
forests which are recorded in our coal-beds. Dr. Russel Wallace
has lately represented these forests, which have, we shall see,
had a most important influence on the development of life, as
somewhat mysterious in their origin. If, however, we again
consult the geologist as to the changes which were taking place
in the distribution of land and water, we find a quite natural
explanation. Indeed, there are now distinguished geologists (e.g.
Professor Chamberlin) who doubt if the Coal-forests were so
exceptionally luxuriant as is generally believed. They think that
the vegetation may not have been more dense than in some other
ages, but that there may have been exceptionally good conditions
for preserving the dead trees. We shall see that there were; but,
on the whole, it seems probable that during some hundreds of
thousands of years remarkably dense forests covered enormous
stretches of the earth's surface, from the Arctic to the
Antarctic.
The Devonian period had opened with a rise of the land, but the
sea eat steadily into it once more, and, with some inconsiderable
oscillations of the land, regained its territory. The latter part
of the Devonian and earlier part of the Carboniferous were
remarkable for their great expanses of shallow water and
low-lying land. Except the recent chain of hills in Scotland we
know of no mountains. Professor Chamberlin calculates that
20,000,000, or 30,000,000 square miles of the present continental
surface of Europe and America were covered with a shallow sea. In
the deeper and clearer of these waters the earliest Carboniferous
rocks, of limestone, were deposited. The "millstone grit," which
succeeds the "limestone," indicates shallower water, which is
being rapidly filled up with the debris of the land. In a word,
all the indications suggest the early and middle Carboniferous as
an age of vast swamps, of enormous stretches of land just above
or below the sea-level, and changing repeatedly from one to the
other. Further, the climate was at the time--we will consider the
general question of climate later--moist and warm all over the
earth, on account of the great proportion of sea-surface and the
absence of high land (not to speak of more disputable causes).
These were ideal conditions for the primitive vegetation, and it
spread over the swamps with great vigour. To say that the
Coal-forests were masses of Ferns, Horsetails, and Club-mosses is
a lifeless and misleading expression. The Club-mosses, or
Lycopodiales, were massive trees, rising sometimes to a height of
120 feet, and probably averaging about fifty feet in height and
one or two feet in diameter. The largest and most abundant of
them, the Sigillaria, sent up a scarred and fluted trunk to a
height of seventy or a hundred feet, without a branch, and was
crowned with a bunch of its long, tapering leaves. The
Lepidodendron, its fellow monarch of the forest, branched at the
summit, and terminated in clusters of its stiff, needle-like
leaves, six' or seven inches long, like enormous exaggerations of
the little cones at the ends of our Club-mosses to-day. The
Horsetails, which linger in their dwarfed descendants by our
streams to-day, and at their exceptional best (in a part of South
America) form slender stems about thirty feet high, were then
forest-trees, four to six feet in circumference and sometimes
ninety feet in height. These Calamites probably rose in dense
thickets from the borders of the lakes, their stumpy leaves
spreading in whorls at every joint in their hollow stems. Another
extinct tree, the Cordaites, rivalled the Horsetails and
Club-mosses in height, and its showers of long and extraordinary
leaves, six feet long and six inches in width, pointed to the
higher plant world that was to come. Between these gaunt towering
trunks the graceful tree-ferns spread their canopies at heights
of twenty, forty, and even sixty feet from the ground, and at the
base was a dense undergrowth of ferns and fern-like seed-plants.
Mosses may have carpeted the moist ground, but nothing in the
nature of grass or flowers had yet appeared.
Imagine this dense assemblage of dull, flowerless trees pervaded
by a hot, dank atmosphere, with no change of seasons, with no
movement but the flying of large and primitive insects among the
trees and the stirring of the ferns below by some passing giant
salamander, with no song of bird and no single streak of white or
red or blue drawn across the changeless sombre green, and you
have some idea of the character of the forests that are
compressed into our seams of coal. Imagine these forests spread
from Spitzbergen to Australia and even, according to the south
polar expeditions, to the Antarctic, and from the United States
to Europe, to Siberia, and to China, and prolonged during some
hundreds of thousands of years, and you begin to realise that the
Carboniferous period prepared the land for the coming dynasties
of animals. Let some vast and terrible devastation fall upon this
luxuriant world, entombing the great multitude of its imperfect
forms and selecting the higher types for freer life, and the
earth will pass into a new age.
But before we describe the animal inhabitants of these forests,
the part that the forests play in the story of life, and the
great cataclysm which selects the higher types from the myriads
of forms which the warm womb of the earth has poured out, we must
at least glance at the evolutionary position of the Carboniferous
plants themselves. Do they point downward to lower forms, and
upward to higher forms, as the theory of evolution requires? A
close inquiry into this would lead us deep into the problems of
the modern botanist, but we may borrow from him a few of the
results of the great labour he has expended on the subject within
the last decade.
Just as the animal world is primarily divided into Invertebrates
and Vertebrates, the plant world is primarily divided into a
lower kingdom of spore-bearing plants (the Cryptogams) and an
upper kingdom of seed-bearing plants (the Phanerogams). Again,
just as the first half of the earth's story is the age of
Invertebrate animals, so it is the age of Cryptogamous plants. So
far evolution was always justified in the plant record. But there
is a third parallel, of much greater interest. We saw that at one
time the evolutionist was puzzled by the clean division of
animals into Invertebrate and Vertebrate, and the sudden
appearance of the backbone in the chronicle: he was just as much
puzzled by the sharp division of our plants into Cryptogams and
Phanerogams, and the sudden appearance of the latter on the earth
during the Coal-forest period. And the issue has been a fresh and
recent triumph for evolution.
Plants are so well preserved in the coal that many years of
microscopic study of the remains, and patient putting-together of
the crushed and scattered fragments, have shown the Carboniferous
plants in quite a new light. Instead of the Coal-forest being a
vast assemblage of Cryptogams, upon which the higher type of the
Phanerogam is going suddenly to descend from the clouds, it is,
to a very great extent, a world of plants that are struggling
upward, along many paths, to the higher level. The characters of
the Cryptogam and Phanerogam are so mixed up in it that, although
the special lines of development are difficult to trace, it is
one massive testimony to the evolution of the higher from the
lower. The reproductive bodies of the great Lepidodendra are
sometimes more like seeds than spores, while both the wood and
the leaves of the Sigillaria have features which properly belong
to the Phanerogam. In another group (called the Sphenophyllales)
the characters of these giant Club-mosses are blended with the
characters of the giant Horsetails, and there is ground to think
that the three groups have descended from an earlier common
ancestor.
Further, it is now believed that a large part of what were
believed to be Conifers, suddenly entering from the unknown, are
not Conifers at all, but Cordaites. The Cordaites is a very
remarkable combination of features that are otherwise scattered
among the Cryptogams, Cycads, and Conifers. On the other hand, a
very large part of what the geologist had hitherto called "Ferns"
have turned out to be seed-bearing plants, half Cycad and half
Fern. Numbers of specimens of this interesting group--the
Cycadofilices (cycad-ferns) or Pteridosperms (seed-ferns)--have
been beautifully restored by our botanists.* They have afforded a
new and very plausible ancestor for the higher trees which come
on the scene toward the close of the Coal-forests, while their
fern-like characters dispose botanists to think that they and the
Ferns may be traced to a common ancestor. This earlier stage is
lost in those primitive ages from which not a single leaf has
survived in the rocks. We can only say that it is probable that
the Mosses, Ferns, Lycopods, etc., arose independently from the
primitive level. But the higher and more important development is
now much clearer. The Coal-forest is not simply a kingdom of
Cryptogams. It is a world of aspiring and mingled types. Let it
be subjected to some searching test, some tremendous spell of
adversity, and we shall understand the emergence of the higher
types out of the luxuriant profusion and confusion of forms.
* See, especially, D. H. Scott, "Studies of Fossil Botany" (2nd
ed., 1908), and "The Evolution of Plants" (1910--small popular
manual).
CHAPTER IX. THE ANIMALS OF THE COAL-FOREST
We have next to see that when this period of searching adversity
comes--as it will in the next chapter --the animal world also
offers a luxuriant variety of forms from which the higher types
may be selected. This, it need hardly be said, is just what we
find in the geological record. The fruitful, steaming, rich-laden
earth now offered tens of millions of square miles of pasture to
vegetal feeders; the waters, on the other hand, teemed with
gigantic sharks, huge Cephalopods, large scorpion-like and
lobster-like animals, and shoals of armour-plated, hard-toothed
fishes. Successive swarms of vegetarians--Worms, Molluscs, etc.--
followed the plant on to the land; and swarms of carnivores
followed the vegetarians, and assumed strange, new forms in
adaptation to land-life. The migration had probably proceeded
throughout the Devonian period, especially from the calmer shores
of the inland seas. By the middle of the Coal-forest period there
was a very large and varied animal population on the land. Like
the plants, moreover, these animals were of an intermediate and
advancing nature. No bird or butterfly yet flits from tree to
tree; no mammal rears its young in the shelter of the ferns. But
among the swarming population are many types that show a
beginning of higher organisation, and there is a rich and varied
material provided for the coming selection.
The monarch of the Carboniferous forest is the Amphibian. In that
age of spreading swamps and "dim, watery woodlands," the stupid
and sluggish Amphibian finds his golden age, and, except perhaps
the scorpion, there is no other land animal competent to dispute
his rule. Even the scorpion, moreover, would not find the
Carboniferous Amphibian very vulnerable. We must not think of the
smooth-skinned frogs and toads and innocent newts which to-day
represent the fallen race of the Amphibia. They were then heavily
armoured, powerfully armed, and sometimes as large as alligators
or young crocodiles. It is a characteristic of advancing life
that a new type of organism has its period of triumph, grows to
enormous proportions, and spreads into many different types,
until the next higher stage of life is reached, and it is
dethroned by the new-comers.
The first indication--apart from certain disputed impressions in
the Devonian--of the land-vertebrate is the footprint of an
Amphibian on an early Carboniferous mud-flat. Hardened by the
sun, and then covered with a fresh deposit when it sank beneath
the waters, it remains to-day to witness the arrival of the
five-toed quadruped who was to rule the earth. As the period
proceeds, remains are found in great abundance, and we see that
there must have been a vast and varied population of the Amphibia
on the shores of the Carboniferous lagoons and swamps. There were
at least twenty genera of them living in what is now the island
of Britain, and was then part of the British-Scandinavian
continent. Some of them were short and stumpy creatures, a few
inches long, with weak limbs and short tails, and broad,
crescent-shaped heads, their bodies clothed in the fine scaly
armour of their fish-ancestor (the Branchiosaurs). Some (the
Aistopods) were long, snake-like creatures, with shrunken limbs
and bodies drawn out until, in some cases, the backbone had 150
Back to Full Books
|