The Outline of Science, Vol. 1 (of 4)

J >> J. Arthur Thomson >> The Outline of Science, Vol. 1 (of 4)

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Getting on to dry land had a manifold significance.

It implied getting into a medium with a much larger supply of oxygen
than there is dissolved in the water. But the oxygen of the air is more
difficult to capture, especially when the skin becomes hard or well
protected, as it is almost bound to become in animals living on dry
ground. Thus this leads to the development of _internal surfaces_, such
as those of lungs, where the oxygen taken into the body may be absorbed
by the blood. In most animals the blood goes to the surface of
oxygen-capture; but in insects and their relatives there is a different
idea--of taking the air to the blood or in greater part to the area of
oxygen-combustion, the living tissues. A system of branching air-tubes
takes air into every hole and corner of the insect's body, and this
thorough aeration is doubtless in part the secret of the insect's
intense activity. The blood never becomes impure.

The conquest of the dry land also implied a predominance of that kind of
locomotion which may be compared to punting, when the body is pushed
along by pressing a lever against a hard substratum. And it also
followed that with few exceptions the body of the terrestrial animal
tended to be compact, readily lifted off the ground by the limbs or
adjusted in some other way so that there may not be too large a surface
trailing on the ground. An animal like a jellyfish, easily supported in
the water, would be impossible on land. Such apparent exceptions as
earthworms, centipedes, and snakes are not difficult to explain, for the
earthworm is a burrower which eats its way through the soil, the
centipede's long body is supported by numerous hard legs, and the snake
pushes itself along by means of the large ventral scales to which the
lower ends of very numerous ribs are attached.

Methods of Mastering the Difficulties of Terrestrial Life

A great restriction attendant on the invasion of the dry land is that
locomotion becomes limited to one plane, namely, the surface of the
earth. This is in great contrast to what is true in the water, where the
animal can move up or down, to right or to left, at any angle and in
three dimensions. It surely follows from this that the movements of land
animals must be rapid and precise, unless, indeed, safety is secured in
some other way. Hence it is easy to understand why most land animals
have very finely developed striped muscles, and why a beetle running on
the ground has far more numerous muscles than a lobster swimming in the
sea.

Land animals were also handicapped by the risks of drought and of frost,
but these were met by defences of the most diverse description, from the
hairs of woolly caterpillars to the fur of mammals, from the carapace of
tortoises to the armour of armadillos. In other cases, it is hardly
necessary to say, the difficulties may be met in other ways, as frogs
meet the winter by falling into a lethargic state in some secluded
retreat.

Another consequence of getting on to dry land is that the eggs or young
can no longer be set free anyhow, as is possible when the animal is
surrounded by water, which is in itself more or less of a cradle. If the
eggs were laid or the young liberated on dry ground, the chances are
many that they would be dried up or devoured. So there are numerous ways
in which land animals secure the safety of their young, e.g. by burying
them in the ground, or by hiding them in nests, or by carrying them
about for a prolonged period either before or after birth. This may mean
great safety for the young, this may make it possible to have only a
small family, and this may tend to the evolution of parental care and
the kindly emotions. Thus it may be understood that from the conquest of
the land many far-reaching consequences have followed.

[Illustration: _Photo: Rischgitz._

PROFESSOR THOMAS HENRY HUXLEY (1825-95)

One of the most distinguished of zoologists, with unsurpassed gifts as a
teacher and expositor. He did great service in gaining a place for
science in ordinary education and in popular estimation. No one
championed Evolutionism with more courage and skill.]

[Illustration: BARON CUVIER, 1769-1832

One of the founders of modern Comparative Anatomy. A man of gigantic
intellect, who came to Paris as a youth from the provinces, and became
the director of the higher education of France and a peer of the Empire.
He was opposed to Evolutionist ideas, but he had anatomical genius.]

[Illustration: AN ILLUSTRATION SHOWING VARIOUS METHODS OF FLYING AND
SWOOPING

Gull, with a feather-wing, a true flier. Fox-bat, with a skin-wing, a
true flier. Flying Squirrel, with a parachute of skin, able to swoop
from tree to tree, but not to fly. Flying Fish, with pectoral fins used
as volplanes in a great leap due to the tail. To some extent able to
sail in albatros fashion.]

Finally, it is worth dwelling on the risks of terrestrial life, because
they enable us better to understand why so many land animals have become
burrowers and others climbers of trees, why some have returned to the
water and others have taken to the air. It may be asked, perhaps, why
the land should have been colonised at all when the risks and
difficulties are so great. The answer must be that necessity and
curiosity are the mother and father of invention. Animals left the water
because the pools dried up, or because they were overcrowded, or because
of inveterate enemies, but also because of that curiosity and spirit of
adventure which, from first to last, has been one of the spurs of
progress.

Conquering the Air

6. The last great haunt of life is the air, a mastery of which must be
placed to the credit of insects, Pterodactyls, birds, and bats. These
have been the successes, but it should be noted that there have been
many brilliant failures, which have not attained to much more than
parachuting. These include the Flying Fishes, which take leaps from the
water and are carried for many yards and to considerable heights,
holding their enlarged pectoral fins taut or with little more than a
slight fluttering. There is a so-called Flying Frog (_Rhacophorus_) that
skims from branch to branch, and the much more effective Flying Dragon
(_Draco volans_) of the Far East, which has been mentioned already.
Among mammals there are Flying Phalangers, Flying Lemurs, and more
besides, all attaining to great skill as parachutists, and illustrating
the endeavour to master the air which man has realised in a way of his
own.

The power of flight brings obvious advantages. A bird feeding on the
ground is able to evade the stalking carnivore by suddenly rising into
the air; food and water can be followed rapidly and to great distances;
the eggs or the young can be placed in safe situations; and birds in
their migrations have made a brilliant conquest both of time and space.
Many of them know no winter in their year, and the migratory flight of
the Pacific Golden Plover from Hawaii to Alaska and back again does not
stand alone.

THE PROCESSION OF LIFE THROUGH THE AGES

Sec. 1

The Rock Record

How do we know when the various classes of animals and plants were
established on the earth? How do we know the order of their appearance
and the succession of their advances? The answer is: by reading the Rock
Record. In the course of time the crust of the earth has been elevated
into continents and depressed into ocean-troughs, and the surface of the
land has been buckled up into mountain ranges and folded in gentler
hills and valleys. The high places of the land have been weathered by
air and water in many forms, and the results of the weathering have been
borne away by rivers and seas, to be laid down again elsewhere as
deposits which eventually formed sandstones, mudstones, and similar
sedimentary rocks. Much of the material of the original crust has thus
been broken down and worked up again many times over, and if the total
thickness of the sedimentary rocks is added up it amounts, according to
some geologists, to a total of 67 miles. In most cases, however, only a
small part of this thickness is to be seen in one place, for the
deposits were usually formed in limited areas at any one time.

The Use of Fossils

When the sediments were accumulating age after age, it naturally came
about that remains of the plants and animals living at the time were
buried, and these formed the fossils by the aid of which it is possible
to read the story of the past. By careful piecing together of evidence
the geologist is able to determine the order in which the different
sedimentary rocks were laid down, and thus to say, for instance, that
the Devonian period was the time of the origin of Amphibians. In other
cases the geologist utilises the fossils in his attempt to work out the
order of the strata when these have been much disarranged. For the
simpler fossil forms of any type must be older than those that are more
complex. There is no vicious circle here, for the general succession of
strata is clear, and it is quite certain that there were fishes before
there were amphibians, and amphibians before there were reptiles, and
reptiles before there were birds and mammals. In certain cases, e.g. of
fossil horses and elephants, the actual historical succession has been
clearly worked out.

If the successive strata contained good samples of all the plants and
animals living at the time when the beds were formed, then it would be
easy to read the record of the rocks, but many animals were too soft to
become satisfactory fossils, many were eaten or dissolved away, many
were destroyed by heat and pressure, so that the rock record is like a
library very much damaged by fire and looting and decay.

Sec. 2

The Geological Time-table

The long history of the earth and its inhabitants is conveniently
divided into eras. Thus, just as we speak of the ancient, mediaeval, and
modern history of mankind, so we may speak of Palaeozoic, Mesozoic and
Cenozoic eras in the history of the earth as a whole.

Geologists cannot tell us except in an approximate way how long the
process of evolution has taken. One of the methods is to estimate how
long has been required for the accumulation of the salts of the sea,
for all these have been dissolved out of the rocks since rain began to
fall on the earth. Dividing the total amount of saline matter by what is
contributed every year in modern times, we get about a hundred million
years as the age of the sea. But as the present rate of
salt-accumulation is probably much greater than it was during many of
the geological periods, the prodigious age just mentioned is in all
likelihood far below the mark. Another method is to calculate how long
it would take to form the sedimentary rocks, like sandstones and
mudstones, which have a _total_ thickness of over fifty miles, though
the _local_ thickness is rarely over a mile. As most of the materials
have come from the weathering of the earth's crust, and as the annual
amount of weathering now going on can be estimated, the time required
for the formation of the sedimentary rocks of the world can be
approximately calculated. There are some other ways of trying to tell
the earth's age and the length of the successive periods, but no
certainty has been reached.

The eras marked on the table (page 92) as _before the Cambrian_
correspond to about thirty-two miles of thickness of strata; and all the
subsequent eras with fossil-bearing rocks to a thickness of about
twenty-one miles--in itself an astounding fact. Perhaps thirty million
years must be allotted to the Pre-Cambrian eras, eighteen to the
Palaeozoic, nine to the Mesozoic, three to the Cenozoic, making a grand
total of sixty millions.

The Establishment of Invertebrate Stocks

It is an astounding fact that at least half of geological time (the
Archaeozoic and Proterozoic eras) passed before there were living
creatures with parts sufficiently hard to form fossils. In the latter
part of the Proterozoic era there are traces of one-celled marine
animals (Radiolarians) with shells of flint, and of worms that wallowed
in the primal mud. It is plain that as regards the most primitive
creatures the rock record tells us little.

[Illustration: _From Knipe's "Nebula to Man."_

ANIMALS OF THE CAMBRIAN PERIOD e.g. Sponges, Jellyfish, Starfish,
Sea-lilies, Water-fleas, and Trilobites]

[Illustration: _Photo: J. J. Ward, F.E.S._

A TRILOBITE

Trilobites were ancient seashore animals, abundant from the Upper
Cambrian to the Carboniferous eras. They have no direct descendants
to-day. They were jointed-footed animals, allied to Crustaceans and
perhaps also to King-crabs. They were able to roll themselves up in
their ring-armour.]

[Illustration: _Photo: British Museum (Natural History)._

THE GAMBIAN MUD-FISH, PROTOPTERUS

It can breathe oxygen dissolved in water by its gills; it can also
breathe dry air by means of its swim-bladder, which has become a lung.
It is a _double-breather_, showing evolution in process. For seven
months of the year, the dry season, it can remain inert in the mud,
getting air through an open pipe to the surface. When water fills the
pools it can use its gills again. Mud-nests or mud encasements with the
lung-fish inside have often been brought to Britain and the fish when
liberated were quite lively.]

[Illustration: THE ARCHAEOPTERYX

(_After William Leche of Stockholm._)

A good restoration of the oldest known bird, Archaeopteryx (Jurassic
Era). It was about the size of a crow; it had teeth on both jaws; it had
claws on the thumb and two fingers; and it had a long lizard-like tail.
But it had feathers, proving itself a true bird.]

[Illustration: WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS

The longest feathers or primaries (PR) are borne by the two fingers (2
and 3), and their palm-bones (CMC); the second longest or secondaries
are borne by the ulna bone (U) of the fore-arm; there is a separate tuft
(AS) on the thumb (TH).]

The rarity of direct traces of life in the oldest rocks is partly due to
the fact that the primitive animals would be of delicate build, but it
must also be remembered that the ancient rocks have been profoundly and
repeatedly changed by pressure and heat, so that the traces which did
exist would be very liable to obliteration. And if it be asked what
right we have to suppose the presence of living creatures in the absence
or extreme rarity of fossils, we must point to great accumulations of
limestone which indicate the existence of calcareous algae, and to
deposits of iron which probably indicate the activity of iron-forming
Bacteria. Ancient beds of graphite similarly suggest that green plants
flourished in these ancient days.

Sec. 3

The Era of Ancient Life (Palaeozoic)

The _Cambrian_ period was the time of the establishment of the chief
stocks of backboneless animals such as sponges, jellyfishes, worms,
sea-cucumbers, lamp-shells, trilobites, crustaceans, and molluscs. There
is something very eloquent in the broad fact that the peopling of the
seas had definitely begun some thirty million years ago, for Professor
H. F. Osborn points out that in the Cambrian period there was already a
colonisation of the shore of the sea, the open sea, and the deep waters.

The _Ordovician_ period was marked by abundant representation of the
once very successful class of Trilobites--jointed-footed,
antenna-bearing, segmented marine animals, with numerous appendages and
a covering of chitin. They died away entirely with the end of the
Palaeozoic era. Also very notable was the abundance of predatory
cuttlefishes, the bullies of the ancient seas. But it was in this period
that the first backboned animals made their appearance--an epoch-making
step in evolution. In other words, true fishes were evolved--destined in
the course of ages to replace the cuttlefishes (which are mere molluscs)
in dominating the seas.

_______________________________________________________________________

_RECENT TIMES_ Human civilisation.
_______________________________________________________________________

{PLEISTOCENE OR GLACIAL TIME Last great Ice Age.
_CENOZOIC ERA_ {MIOCENE AND PLIOCENE TIMES Emergence of Man.
{EOCENE AND OLIGOCENE TIMES Rise of higher mammals.
_______________________________________________________________________

{CRETACEOUS PERIOD Rise of primitive mammals,
{ flowering plants,
{ and higher insects.
_MESOZOIC ERA_ {JURASSIC PERIOD Rise of birds and flying
{ reptiles.
{TRIASSIC PERIOD Rise of dinosaur reptiles.
_______________________________________________________________________

{PERMIAN PERIOD Rise of reptiles.
{CARBONIFEROUS PERIOD Rise of insects.
{DEVONIAN PERIOD First amphibians.
_PALAEOZOIC ERA_ {SILURIAN PERIOD Land animals began.
{ORDOVICIAN PERIOD First fishes.
{CAMBRIAN PERIOD Peopling of the sea.
_______________________________________________________________________

_PROTEROZOIC AGES_ Many of the Backboneless stocks began.
_ARCHAEOZOIC AGES_ Living creatures began to be upon the earth.
_______________________________________________________________________

{Making of continents and ocean-basins.
{Beginnings of atmosphere and hydrosphere.
_FORMATIVE TIMES_ {Cooling of the earth.
{Establishment of the solar system.
_______________________________________________________________________

In the _Silurian_ period in which the peopling of the seas went on
apace, there was the first known attempt at colonising the dry land. For
in Silurian rocks there are fossil scorpions, and that implies ability
to breathe dry air--by means of internal surfaces, in this case known as
lungbooks. It was also towards the end of the Silurian, when a period of
great aridity set in, that fishes appeared related to our mud-fishes or
double-breathers (Dipnoi), which have lungs as well as gills. This,
again, meant utilising dry air, just as the present-day mud-fishes do
when the water disappears from the pools in hot weather. The lung-fishes
or mud-fishes of to-day are but three in number, one in Queensland, one
in South America, and one in Africa, but they are extremely
interesting "living fossils," binding the class of fishes to that of
amphibians. It is highly probable that the first invasion of the dry
land should be put to the credit of some adventurous worms, but the
second great invasion was certainly due to air-breathing Arthropods,
like the pioneer scorpion we mentioned.

[Illustration: PICTORIAL REPRESENTATION OF THE SUCCESSIVE STRATA OF THE
EARTH'S CRUST, WITH SUGGESTIONS OF CHARACTERISTIC FOSSILS

E.g. Fish and Trilobite in the Devonian (red), a large Amphibian in the
Carboniferous (blue), Reptiles in Permian (light red), the first Mammal
in the Triassic (blue), the first Bird in the Jurassic (yellow), Giant
Reptiles in the Cretaceous (white), then follow the Tertiary strata with
progressive mammals, and Quaternary at the top with man and mammoth.]

The _Devonian_ period, including that of the Old Red Sandstone, was one
of the most significant periods in the earth's history. For it was the
time of the establishment of flowering plants upon the earth and of
terrestrial backboned animals. One would like to have been the
discoverer of the Devonian foot-print of _Thinopus_, the first known
Amphibian foot-print--an eloquent vestige of the third great invasion of
the dry land. It was probably from a stock of Devonian lung-fishes that
the first Amphibians sprang, but it was not till the next period that
they came to their own. While they were still feeling their way, there
was a remarkable exuberance of shark-like and heavily armoured fishes in
the Devonian seas.

EVOLUTION OF LAND ANIMALS

Sec. 1

Giant Amphibians and Coal-measures

The _Carboniferous_ period was marked by a mild moist climate and a
luxuriant vegetation in the swampy low grounds. It was a much less
strenuous time than the Devonian period; it was like a very long summer.
There were no trees of the type we see now, but there were forests of
club-mosses and horsetails which grew to a gigantic size compared with
their pigmy representatives of to-day. In these forests the
jointed-footed invaders of the dry land ran riot in the form of
centipedes, spiders, scorpions, and insects, and on these the primeval
Amphibians fed. The appearance of insects made possible a new linkage of
far-reaching importance, namely, the cross-fertilisation of flowering
plants by their insect visitors, and from this time onwards it may be
said that flowers and their visitors have evolved hand in hand.
Cross-fertilisation is much surer by insects than by the wind, and
cross-fertilisation is more advantageous than self-fertilisation because
it promotes both fertility and plasticity. It was probably in this
period that _coloured_ flowers--attractive to insect-visitors--began to
justify themselves as beauty became useful, and began to relieve the
monotonous green of the horsetail and club-moss forests, which covered
great tracts of the earth for millions of years. In the Carboniferous
forests there were also land-snails, representing one of the minor
invasions of the dry land, tending on the whole to check vegetation.
They, too, were probably preyed upon by the Amphibians, some of which
attained a large size. Each age has had its giants, and those of the
Carboniferous were Amphibians called Labyrinthodonts, some of which were
almost as big as donkeys. It need hardly be said that it was in this
period that most of the Coal-measures were laid down by the immense
accumulation of the spores and debris of the club-moss forests. Ages
afterwards, it was given to man to tap this great source of
energy--traceable back to the sunshine of millions of years ago. Even
then it was true that no plant or animal lives or dies to itself!

The Acquisitions of Amphibians.

As Amphibians had their Golden Age in the Carboniferous period we may
fitly use this opportunity of indicating the advances in evolution which
the emergence of Amphibians implied. (1) In the first place the passage
from water to dry land was the beginning of a higher and more promiseful
life, taxed no doubt by increased difficulties. The natural question
rises why animals should have migrated from water to dry land at all
when great difficulties were involved in the transition. The answers
must be: (_a_) that local drying up of water-basins or elevations of the
land surface often made the old haunts untenable; (_b_) that there may
have been great congestion and competition in the old quarters; and
(_c_) that there has been an undeniable endeavour after well-being
throughout the history of animal life. In the same way with mankind,
migrations were prompted by the setting in of prolonged drought, by
over-population, and by the spirit of adventure. (2) In Amphibians for
the first time the non-digitate paired fins of fishes were replaced by
limbs with fingers and toes. This implied an advantageous power of
grasping, of holding firm, of putting food into the mouth, of feeling
things in three dimensions. (3) We cannot be positive in regard to the
soft parts of the ancient Amphibians known only as fossils, but if they
were in a general way like the frogs and toads, newts and salamanders of
the present day, we may say that they made among other acquisitions the
following: true ventral lungs, a three-chambered heart, a movable
tongue, a drum to the ear, and lids to the eyes. It is very interesting
to find that though the tongue of the tadpole has some muscle-fibres in
it, they are not strong enough to effect movement, recalling the tongue
of fishes, which has not any muscles at all. Gradually, as the tadpole
becomes a frog, the muscle-fibres grow in strength, and make it possible
for the full-grown creature to shoot out its tongue upon insects. This
is probably a recapitulation of what was accomplished in the course of
millennia in the history of the Amphibian race. (4) Another acquisition
made by Amphibians was a voice, due, as in ourselves, to the rapid
passage of air over taut membranes (vocal cords) stretched in the
larynx. It is an interesting fact that for millions of years there was
upon the earth no sound of life at all, only the noise of wind and wave,
thunder and avalanche. Apart from the instrumental music of some
insects, perhaps beginning in the Carboniferous, the first vital sounds
were due to Amphibians, and theirs certainly was the first voice--surely
one of the great steps in organic evolution.

[Illustration: _Photo: British Museum (Natural History)._

FOSSIL OF A PTERODACTYL OR EXTINCT FLYING DRAGON

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