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

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

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The wing is made of a web of skin extended on the enormously elongated
outermost finger. The long tail served for balancing and steering. The
Pterodactyls varied from the size of sparrows to a wing-span of fifteen
feet--the largest flying creatures.]

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

PARIASAURUS: AN EXTINCT VEGETARIAN TRIASSIC REPTILE

Total length about 9 feet. (Remains found in Cape Colony, South
Africa.)]

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

TRICERATOPS: A HUGE EXTINCT REPTILE

(From remains found in Cretaceous strata of Wyoming, U.S.A.)

This Dinosaur, about the size of a large rhinoceros, had a huge
three-horned skull with a remarkable bony collar over the neck. But, as
in many other cases, its brain was so small that it could have passed
down the spinal canal in which the spinal cord lies. Perhaps this partly
accounts for the extinction of giant reptiles.]

[Illustration: _Photo: "Daily Mail."_

THE DUCKMOLE OR DUCK-BILLED PLATYPUS OF AUSTRALIA

The Duckmole or Duck-billed Platypus of Australia is a survivor of the
most primitive mammals. It harks back to reptiles, e.g. in being an
egg-layer, in having comparatively large eggs, and in being imperfectly
warm-blooded. It swims well and feeds on small water-animals. It can
also burrow.]

Evolution of the Voice

The first use of the voice was probably that indicated by our frogs and
toads--it serves as a sex-call. That is the meaning of the trumpeting
with which frogs herald the spring, and it is often only in the males
that the voice is well developed. But if we look forward, past
Amphibians altogether, we find the voice becoming a maternal call
helping to secure the safety of the young--a use very obvious when young
birds squat motionless at the sound of the parent's danger-note. Later
on, probably, the voice became an infantile call, as when the unhatched
crocodile pipes from within the deeply buried egg, signalling to the
mother that it is time to be unearthed. Higher still the voice expresses
emotion, as in the song of birds, often outside the limits of the
breeding time. Later still, particular sounds become words, signifying
particular things or feelings, such as "food," "danger," "home,"
"anger," and "joy." Finally words become a medium of social intercourse
and as symbols help to make it possible for man to reason.

Sec. 2

The Early Reptiles

In the _Permian_ period reptiles appeared, or perhaps one should say,
began to assert themselves. That is to say, there was an emergence of
backboned animals which were free from water and relinquished the method
of breathing by gills, which Amphibians retained in their young stages
at least. The unhatched or unborn reptile breathes by means of a
vascular hood spread underneath the egg-shell and absorbing dry air from
without. It is an interesting point that this vascular hood, called the
allantois, is represented in the Amphibians by an unimportant bladder
growing out from the hind end of the food-canal. A great step in
evolution was implied in the origin of this ante-natal hood or foetal
membrane and another one--of protective significance--called the amnion,
which forms a water-bag over the delicate embryo. The step meant total
emancipation from the water and from gill-breathing, and the two
foetal membranes, the amnion and the allantois, persist not only in
all reptiles but in birds and mammals as well. These higher Vertebrates
are therefore called Amniota in contrast to the Lower Vertebrates or
Anamnia (the Amphibians, Fishes, and primitive types).

It is a suggestive fact that the embryos of all reptiles, birds, and
mammals show gill-clefts--_a tell-tale evidence of their distant aquatic
ancestry_. But these embryonic gill-clefts are not used for respiration
and show no trace of gills except in a few embryonic reptiles and birds
where their dwindled vestiges have been recently discovered. As to the
gill-clefts, they are of no use in higher Vertebrates except that the
first becomes the Eustachian tube leading from the ear-passage to the
back of the mouth. The reason why they persist when only one is of any
use, and that in a transformed guise, would be difficult to interpret
except in terms of the Evolution theory. They illustrate the lingering
influence of a long pedigree, the living hand of the past, the tendency
that individual development has to recapitulate racial evolution. In a
condensed and telescoped manner, of course, for what took the race a
million years may be recapitulated by the individual in a week!

In the Permian period the warm moist climate of most of the
Carboniferous period was replaced by severe conditions, culminating in
an Ice Age which spread from the Southern Hemisphere throughout the
world. With this was associated a waning of the Carboniferous flora, and
the appearance of a new one, consisting of ferns, conifers, ginkgos, and
cycads, which persisted until near the end of the Mesozoic era. The
Permian Ice Age lasted for millions of years, and was most severe in the
Far South. Of course, it was a very different world then, for North
Europe was joined to North America, Africa to South America, and
Australia to Asia. It was probably during the Permian Ice Age that many
of the insects divided their life-history into two main chapters--the
feeding, growing, moulting, immature, larval stages, e.g. caterpillars,
and the more ascetic, non-growing, non-moulting, winged phase, adapted
for reproduction. Between these there intervened the quiescent,
well-protected pupa stage or chrysalis, probably adapted to begin with
as a means of surviving the severe winter. For it is easier for an
animal to survive when the vital processes are more or less in abeyance.

Disappearance of many Ancient Types

We cannot leave the last period of the Palaeozoic era and its prolonged
ice age without noticing that it meant the entire cessation of a large
number of ancient types, especially among plants and backboneless
animals, which now disappear for ever. It is necessary to understand
that the animals of ancient days stand in three different relations to
those of to-day. (_a_) There are ancient types that have living
representatives, sometimes few and sometimes many, sometimes much
changed and sometimes but slightly changed. The lamp-shell,
_Lingulella_, of the Cambrian and Ordovician period has a very near
relative in the _Lingula_ of to-day. There are a few extremely
conservative animals. (_b_) There are ancient types which have no living
representatives, except in the guise of transformed descendants, as the
King-crab (_Limulus_) may be said to be a transformed descendant of the
otherwise quite extinct race to which Eurypterids or Sea-scorpions
belonged. (_c_) There are altogether extinct types--_lost races_--which
have left not a wrack behind. For there is not any representation to-day
of such races as Graptolites and Trilobites.

Looking backwards over the many millions of years comprised in the
Palaeozoic era, what may we emphasise as the most salient features? There
was in the _Cambrian_ the establishment of the chief classes of
backboneless animals; in the _Ordovician_ the first fishes and perhaps
the first terrestrial plants; in the _Silurian_ the emergence of
air-breathing Invertebrates and mud-fishes; in the _Devonian_ the
appearance of the first Amphibians, from which all higher land animals
are descended, and the establishment of a land flora; in the
_Carboniferous_ the great Club-moss forests and an exuberance of
air-breathing insects and their allies; in the _Permian_ the first
reptiles and a new flora.

THE GEOLOGICAL MIDDLE AGES

Sec. 1

The Mesozoic Era

In a broad way the Mesozoic era corresponds with the Golden Age of
reptiles, and with the climax of the Conifer and Cycad flora, which was
established in the Permian. But among the Conifers and Cycads our modern
flowering plants were beginning to show face tentatively, just like
birds and mammals among the great reptiles.

In the _Triassic_ period the exuberance of reptilian life which marked
the Permian was continued. Besides Turtles which still persist, there
were Ichthyosaurs, Plesiosaurs, Dinosaurs, and Pterosaurs, none of which
lasted beyond the Mesozoic era. Of great importance was the rise of the
Dinosaurs in the Triassic, for it is highly probable that within the
limits of this vigorous and plastic stock--some of them bipeds--we must
look for the ancestors of both birds and mammals. Both land and water
were dominated by reptiles, some of which attained to gigantic size. Had
there been any zoologist in those days, he would have been very
sagacious indeed if he had suspected that reptiles did not represent the
climax of creation.

The Flying Dragons

The _Jurassic_ period showed a continuance of the reptilian splendour.
They radiated in many directions, becoming adapted to many haunts. Thus
there were many Fish Lizards paddling in the seas, many types of
terrestrial dragons stalking about on land, many swiftly gliding
alligator-like forms, and the Flying Dragons which began in the Triassic
attained to remarkable success and variety. Their wing was formed by the
extension of a great fold of skin on the enormously elongated outermost
finger, and they varied from the size of a sparrow to a spread of over
five feet. A soldering of the dorsal vertebrae as in our Flying Birds was
an adaptation to striking the air with some force, but as there is not
more than a slight keel, if any, on the breast-bone, it is unlikely that
they could fly far. For we know from our modern birds that the power of
flight may be to some extent gauged from the degree of development of
the keel, which is simply a great ridge for the better insertion of the
muscles of flight. It is absent, of course, in the Running Birds, like
the ostrich, and it has degenerated in an interesting way in the
burrowing parrot (_Stringops_) and a few other birds that have "gone
back."

The First Known Bird

But the Jurassic is particularly memorable because its strata have
yielded two fine specimens of the first known bird, _Archaeopteryx_.
These were entombed in the deposits which formed the fine-grained
lithographic stones of Bavaria, and practically every bone in the body
is preserved except the breast-bone. Even the feathers have left their
marks with distinctness. This oldest known bird--too far advanced to be
the first bird--was about the size of a crow and was probably of
arboreal habits. Of great interest are its reptilian features, so
pronounced that one cannot evade the evolutionist suggestion. It had
teeth in both jaws, which no modern bird has; it had a long lizard-like
tail, which no modern bird has; it had claws on three fingers, and a
sort of half-made wing. That is to say, it does not show, what all
modern birds show, a fusion of half the wrist-bones with the whole of
the palm-bones, the well-known carpo-metacarpus bone which forms a basis
for the longest pinions. In many reptiles, such as Crocodiles, there are
peculiar bones running across the abdomen beneath the skin, the
so-called "abdominal ribs," and it seems an eloquent detail to find
these represented in _Archaeopteryx_, the earliest known bird. No modern
bird shows any trace of them. [Illustration: SKELETON OF AN EXTINCT
FLIGHTLESS TOOTHED BIRD, HESPERORNIS

(_After Marsh._)

The bird was five or six feet high, something like a swimming ostrich,
with a very powerful leg but only a vestige of a wing. There were sharp
teeth in a groove. The modern divers come nearest to this ancient
type.]

[Illustration: SIX STAGES IN THE EVOLUTION OF THE HORSE, SHOWING GRADUAL
INCREASE IN SIZE

(_After Lull and Matthew._)

1. Four-toed horse, Eohippus, about one foot high. Lower Eocene, N.
America.

2. Another four-toed horse, Orohippus, a little over a foot high. Middle
Eocene, N. America.

3. Three-toed horse, Mesohippus, about the size of a sheep. Middle
Oligocene, N. America.

4. Three-toed horse, Merychippus, Miocene, N. America. Only one toe
reaches the ground on each foot, but the remains of two others are
prominent.

5. The first one-toed horse, Pliohippus, about forty inches high at the
shoulder. Pliocene, N. America.

6. The modern horse, running on the third digit of each foot.]

There is no warrant for supposing that the flying reptiles or
Pterodactyls gave rise to birds, for the two groups are on different
lines, and the structure of the wings is entirely different. Thus the
long-fingered Pterodactyl wing was a parachute wing, while the secret of
the bird's wing has its centre in the feathers. It is highly probable
that birds evolved from certain Dinosaurs which had become bipeds, and
it is possible that they were for a time swift runners that took "flying
jumps" along the ground. Thereafter, perhaps, came a period of arboreal
apprenticeship during which there was much gliding from tree to tree
before true flight was achieved. It is an interesting fact that the
problem of flight has been solved four times among animals--by insects,
by Pterodactyls, by birds, and by bats; and that the four solutions are
on entirely different lines.

In the _Cretaceous_ period the outstanding events included the waning of
giant reptiles, the modernising of the flowering plants, and the
multiplication of small mammals. Some of the Permian reptiles, such as
the dog-toothed Cynodonts, were extraordinarily mammal-like, and it was
probably from among them that definite mammals emerged in the Triassic.
Comparatively little is known of the early Triassic mammals save that
their back-teeth were marked by numerous tubercles on the crown, but
they were gaining strength in the late Triassic when small arboreal
insectivores, not very distant from the modern tree-shrews (_Tupaia_),
began to branch out in many directions indicative of the great divisions
of modern mammals, such as the clawed mammals, hoofed mammals, and the
race of monkeys or Primates. In the Upper Cretaceous there was an
exuberant "radiation" of mammals, adaptive to the conquest of all sorts
of haunts, and this was vigorously continued in Tertiary times.

There is no difficulty in the fact that the earliest remains of definite
mammals in the Triassic precede the first-known bird in the Jurassic.
For although we usually rank mammals as higher than birds (being mammals
ourselves, how could we do otherwise?), there are many ways in which
birds are pre-eminent, e.g. in skeleton, musculature, integumentary
structures, and respiratory system. The fact is that birds and mammals
are on two quite different tacks of evolution, not related to one
another, save in having a common ancestry in extinct reptiles. Moreover,
there is no reason to believe that the Jurassic _Archaeopteryx_ was the
first bird in any sense except that it is the first of which we have any
record. In any case it is safe to say that birds came to their own
before mammals did.

Looking backwards, we may perhaps sum up what is most essential in the
Mesozoic era in Professor Schuchert's sentence: "The Mesozoic is the Age
of Reptiles, and yet the little mammals and the toothed birds are
storing up intelligence and strength to replace the reptiles when the
cycads and conifers shall give way to the higher flowering plants."

Sec. 2

The Cenozoic or Tertiary Era

In the _Eocene_ period there was a replacement of the small-brained
archaic mammals by big-brained modernised types, and with this must be
associated the covering of the earth with a garment of grass and dry
pasture. Marshes were replaced by meadows and browsing by grazing
mammals. In the spreading meadows an opportunity was also offered for a
richer evolution of insects and birds.

During the _Oligocene_ the elevation of the land continued, the climate
became much less moist, and the grazing herds extended their range.

The _Miocene_ was the mammalian Golden Age and there were crowning
examples of what Osborn calls "adaptive radiation." That is to say,
mammals, like the reptiles before them, conquer every haunt of life.
There are flying bats, volplaning parachutists, climbers in trees like
sloths and squirrels, quickly moving hoofed mammals, burrowers like the
moles, freshwater mammals, like duckmole and beaver, shore-frequenting
seals and manatees, and open-sea cetaceans, some of which dive far more
than full fathoms five. It is important to realise the perennial
tendency of animals to conquer every corner and to fill every niche of
opportunity, and to notice that this has been done by successive sets of
animals in succeeding ages. _Most notably the mammals repeat all the
experiments of reptiles on a higher turn of the spiral._ Thus arises
what is called convergence, the superficial resemblance of unrelated
types, like whales and fishes, the resemblance being due to the fact
that the different types are similarly adapted to similar conditions of
life. Professor H. F. Osborn points out that mammals may seek any one of
the twelve different habitat-zones, and that in each of these there may
be six quite different kinds of food. Living creatures penetrate
everywhere like the overflowing waters of a great river in flood.

Sec. 3

The _Pliocene_ period was a more strenuous time, with less genial
climatic conditions, and with more intense competition. Old land bridges
were broken and new ones made, and the geographical distribution
underwent great changes. Professor R. S. Lull describes the _Pliocene_
as "a period of great unrest." "Many migrations occurred the world over,
new competitions arose, and the weaker stocks began to show the effects
of the strenuous life. One momentous event seems to have occurred in the
Pliocene, and that was the transformation of the precursor of humanity
into man--the culmination of the highest line of evolution."

The _Pleistocene_ period was a time of sifting. There was a continued
elevation of the continental masses, and Ice Ages set in, relieved by
less severe interglacial times when the ice-sheets retreated northwards
for a time. Many types, like the mammoth, the woolly rhinoceros, the
sabre-toothed tiger, the cave-lion, and the cave-bear, became extinct.
Others which formerly had a wide range became restricted to the Far
North or were left isolated here and there on the high mountains, like
the Snow Mouse, which now occurs on isolated Alpine heights above the
snow-line. Perhaps it was during this period that many birds of the
Northern Hemisphere learned to evade the winter by the sublime device of
migration.

Looking backwards we may quote Professor Schuchert again:

"The lands in the Cenozoic began to bloom with more and more
flowering plants and grand hardwood forests, the atmosphere is
scented with sweet odours, a vast crowd of new kinds of insects
appear, and the places of the once dominant reptiles of the lands
and seas are taken by the mammals. Out of these struggles there
rises a greater intelligence, seen in nearly all of the mammal
stocks, but particularly in one, the monkey-ape-man. Brute man
appears on the scene with the introduction of the last glacial
climate, a most trying time for all things endowed with life, and
finally there results the dominance of reasoning man over all his
brute associates."

In man and human society the story of evolution has its climax.

The Ascent of Man

Man stands apart from animals in his power of building up general ideas
and of using these in the guidance of his behaviour and the control of
his conduct. This is essentially wrapped up with his development of
language as an instrument of thought. Some animals have words, but man
has language (Logos). Some animals show evidence of _perceptual_
inference, but man often gets beyond this to _conceptual_ inference
(Reason). Many animals are affectionate and brave, self-forgetful and
industrious, but man "thinks the ought," definitely guiding his conduct
in the light of ideals, which in turn are wrapped up with the fact that
he is "a social person."

Besides his big brain, which may be three times as heavy as that of a
gorilla, man has various physical peculiarities. He walks erect, he
plants the sole of his foot flat on the ground, he has a chin and a good
heel, a big forehead and a non-protrusive face, a relatively uniform set
of teeth without conspicuous canines, and a relatively naked body.

[Illustration: DIAGRAM SHOWING SEVEN STAGES IN THE EVOLUTION OF THE
FORE-LIMBS AND HIND-LIMBS OF THE ANCESTORS OF THE MODERN HORSE,
BEGINNING WITH THE EARLIEST KNOWN PREDECESSORS OF THE HORSE AND
CULMINATING WITH THE HORSE OF TO-DAY

(_After Marsh and Lull._)

1 and 1A, fore-limb and hind-limb of Eohippus; 2 and 2A, Orohippus; 3
and 3A, Mesohippus; 4 and 4A, Hypohippus; 5 and 5A, Merychippus; 6 and
6A, Hipparion; 7 and 7A, the modern horse. Note how the toes shorten and
disappear.]

[Illustration: A. Fore-limb of Monkey B. Fore-limb of Whale

WHAT IS MEANT BY HOMOLOGY? ESSENTIAL SIMILARITY OF ARCHITECTURE, THOUGH
THE APPEARANCES MAY BE VERY DIFFERENT

This is seen in comparing these two fore-limbs, A, of Monkey, B, of
Whale. They are as different as possible, yet they show the same bones,
e.g. SC, the scapula or shoulder-blade; H, the humerus or upper arm; R
and U, the radius and ulna of the fore-arm; CA, the wrist; MC, the palm;
and then the fingers.]

But in spite of man's undeniable apartness, there is no doubt as to his
solidarity with the rest of creation. There is an "all-pervading
similitude of structure," between man and the Anthropoid Apes, though it
is certain that it is not from any living form that he took his origin.
None of the anatomical distinctions, except the heavy brain, could be
called momentous. Man's body is a veritable museum of relics (vestigial
structures) inherited from pre-human ancestors. In his everyday bodily
life and in some of its disturbances, man's pedigree is often revealed.
Even his facial expression, as Darwin showed, is not always human. Some
fossil remains bring modern man nearer the anthropoid type.

It is difficult not to admit the ring of truth in the closing words of
Darwin's _Descent of Man_:

"We must, however, acknowledge, as it seems to me, that man, with
all his noble qualities, with sympathy which feels for the most
debased, with benevolence which extends not only to other men but to
the humblest living creature, with his God-like intellect which has
penetrated into the movements and constitution of the solar
system--with all these exalted powers--man still bears in his bodily
frame the indelible stamp of his lowly origin."

THE EVOLVING SYSTEM OF NATURE

There is another side of evolution so obvious that it is often
overlooked, the tendency to link lives together in vital
inter-relations. Thus flowers and their insect visitors are often
vitally interlinked in mutual dependence. Many birds feed on berries and
distribute the seeds. The tiny freshwater snail is the host of the
juvenile stages of the liver-fluke of the sheep. The mosquito is the
vehicle of malaria from man to man, and the tse-tse fly spreads sleeping
sickness. The freshwater mussel cannot continue its race without the
unconscious co-operation of the minnow, and the freshwater fish called
the bitterling cannot continue its race without the unconscious
co-operation of the mussel. There are numerous mutually beneficial
partnerships between different kinds of creatures, and other
inter-relations where the benefit is one-sided, as in the case of
insects that make galls on plants. There are also among kindred animals
many forms of colonies, communities, and societies. Nutritive chains
bind long series of animals together, the cod feeding on the whelk, the
whelk on the worm, the worm on the organic dust of the sea. There is a
system of successive incarnations and matter is continually passing from
one embodiment to another. These instances must suffice to illustrate
the central biological idea of the web of life, the interlinked System
of Animate Nature. Linnaeus spoke of the Systema Naturae, meaning the
orderly hierarchy of classes, orders, families, genera, and species; but
we owe to Darwin in particular some knowledge of a more dynamic Systema
Naturae, the network of vital inter-relations. This has become more and
more complex as evolution has continued, and man's web is most complex
of all. It means making Animate Nature more of a unity; it means an
external method of registering steps of progress; it means an evolving
set of sieves by which new variations are sifted, and living creatures
are kept from slipping down the steep ladder of evolution.

Parasitism

It sometimes happens that the inter-relation established between one
living creature and another works in a retrograde direction. This is the
case with many thoroughgoing internal parasites which have sunk into an
easygoing kind of life, utterly dependent on their host for food,
requiring no exertions, running no risks, and receiving no spur to
effort. Thus we see that evolution is not necessarily progressive;
everything depends on the conditions in reference to which the living
creatures have been evolved. When the conditions are too easygoing, the
animal may be thoroughly well adapted to them--as a tapeworm certainly
is--but it slips down the rungs of the ladder of evolution.

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