"THROUGH THE EYE"
SERIES
EVOLUTION
"THROUGH THE EYE" SERIES
_THE FIRST TWO VOLUMES_
EVOLUTION. By J. A. S. WATSON, B.Sc.
THE CIVILIZATION OF THE ANCIENT EGYPTIANS.
By A. BOTHWELL GOSSE.
_Other Volumes in Preparation_
[Illustration: FIG. 1.--Tree illustrating the probable course of animal
evolution.]
EVOLUTION
BY J. A. S. WATSON, B.Sc.
[Illustration: THROUGH THE · EYE]
_Published by_ T. C. & E. C. JACK, LTD. 35 & 36 PATERNOSTER ROW,
LONDON, E.C. AND AT EDINBURGH
_Printed in Great Britain_
CONTENTS
CHAPTER I
PAGE
THE EVIDENCE FOR EVOLUTION 1
CHAPTER II
UNICELLULAR AND MULTICELLULAR ANIMALS 23
CHAPTER III
THE WORMS AND SOME OF THEIR POSTERITY 58
CHAPTER IV
THE EARLY VERTEBRATES AND THE FISHES 79
CHAPTER V
THE CONQUEST OF THE LAND 104
CHAPTER VI
THE MAMMALS AND MAN 123
For many of the illustrations the Publishers are indebted to the
Deutsche Verlags-Anstalt (from Günther's _Vom Urtier zum Menschen_)
and Herr Wilhelm Engelmann (from Haeckel's _Anthropogenie_).
Acknowledgment is also made to Messrs. Watts & Co., London, the
Publishers of the English Edition of Haeckel's _Anthropogenie_,
entitled _The Evolution of Man_.
EVOLUTION
CHAPTER I
THE EVIDENCE FOR EVOLUTION
The idea of Evolution is an old one. It is older than the Darwinian
hypothesis; it is older than Lamarck, who published his particular
theory in 1809, the year that Darwin was born; it is older than Buffon
or Kant. In a fairly definite form it is as old as Aristotle. The
Evolution idea has thus itself evolved, and is the product of many
centuries of thought. Yet it was only the last generation that began to
give the idea serious consideration, and it is perhaps only the present
that has granted it any general measure of acceptance; and it was
Darwin who wrought this change, who raised the conception of Evolution
from the status of a vague speculative idea to that of a well-grounded
theory, which appeals to the majority of educated minds as satisfactory
and reasonable.
We do not here propose to sketch the development of the idea, either
before or after Darwin; but only, in the first place, to state the
grounds on which the belief in Evolution is based, and, in the second,
to trace roughly the lines along which animal Evolution has proceeded.
In the first few pages of this book, then, we shall endeavour to bring
forward some of the evidence on which the modern Evolution theory rests.
+--------------+------------------+---------------+------------------+
| | First Appearance |Dominant Types.| |
| | of Types. | | |
+--------------+------------------+---------------+------------------+
| Modern | ... | MAN | } Post-tertiary, |
| Diluvium | Man | ... | } 1/2 per cent. |
+--------------+------------------+---------------+------------------+
| Pliocene | ... | } | } Tertiary or |
| Miocene | Monkeys | } MAMMALS | } Cænozoic, |
| Oligocene | ... | } | } 2-1/2 per cent.|
| Eocene | Lemurs | } | } |
+--------------+------------------+---------------+------------------+
| Cretaceous | _Higher mammals_ | } | } Secondary or |
| Jurassic | { _Birds_ | } REPTILES | } Mesozoic, |
| | { Marsupials | } | } 11 per cent. |
| Triassic | _Monotremes_ | } | } |
+--------------+------------------+---------------+------------------+
| Permian | _Reptiles_ | AMPHIBIANS | } Primary or |
| Carboniferous| _Amphibians_ | } FISHES | } Palæozoic, |
| Devonian | _Lung fishes_ | } | } 32 per cent. |
| Silurian | _Lower fishes_ | ... | } |
+--------------+------------------+---------------+------------------+
| Cambrian | ... | ... | } Archäen, 54 |
| Laurentian | ... | ... | } per cent. |
+--------------+------------------+---------------+------------------+
FIG. 2.--Table showing the chronological succession of the stratified
rocks, the subdivision of geological time, the approximate position of
the earliest fossils of each of the main types of vertebrates, and the
period of domination of each group.
As our first witness, we may call the rocks which constitute the outer
portion of the earth, and ask them to tell us what they remember of the
history of life upon the planet. We cannot hope for the whole truth
from them, for their memory is imperfect; and yet they can tell us a
great number of important facts.
[Illustration: THE EVOLUTION OF THE HORSE
FIG. 3. From _The Guide to the American Museum of Natural History_.]
From the time when the world was sufficiently cooled for water to
condense on its surface, a continual process of unbuilding and
rebuilding of rocks has gone on. Wind and water, heat and cold have
laid their hands to the work, making sand and dust and gravel out of
solid stone, and these products of their labours have been carried
off to other places, laid down, and cemented together into new rocks.
We do not know the exact age of any particular rock that has been
made in this way, nor how long the process has been going on. At a
rough guess it may be three or four hundreds of millions of years.
The chronological succession of the different rock formations is,
however, known, and their relative ages may be judged with considerable
accuracy. Here and there, as time went on, the body of a plant or an
animal was deposited in the sand or mud or chalk, and has remained in
the resulting rocks, in the form of a fossil, through all the ages.
If, then, we study the occurrence of fossils in this succession of
deposits, we ought to get some indications as to the inhabitants of
the globe at various stages of its history. And if we do so, we meet
unmistakable evidence that the lower and simpler types, both of animals
and of plants, were in existence before the higher. Fig. 2 shows the
facts with regard to the vertebrates, the great upper class of the
animal kingdom. The first appearance of vertebrate fossils is in the
Upper Silurian rocks, that is to say, somewhere after the middle of
geological time. The fossils represent the lowest group of fishes. In
the next great formation, the Devonian, fossils of two higher groups
of fishes are to be found. The first land vertebrates, the amphibians,
are doubtfully represented in the upper or newer layers of the same
formation, and definitely so in the next, the Carboniferous. Towards
the end of the Carboniferous or early in the Permian epoch, the
first reptiles appear, and in the following period, or after about
three-fourths of geological time had passed, the earliest fossils of
mammals occur. The significance of this sequence will become plainer
when the differences and likenesses of these various groups are
explained. Each of these great groups in turn formed the dominant
animal population of the globe, and each in turn was superseded,
although not entirely, by the next. The mammal group itself appears
to be on the wane, overcome in the struggle for dominance by its own
latest and most remarkable member, man himself.
[Illustration: FIG. 4. From _The Feathered World_.]
THE VARIATION OF PIGEONS UNDER DOMESTICATION.
Centre--Rock Doves.
1. Carrier.
2. Pouter.
3. Almond Tumbler.
4. Trumpeter.
5. Barb.
6. Fantail.
7. Jacobin.
8. Capuchin.
9. Dragoon.
10. Modena.
11. Scandaroon.
12. Turbit.
13. English Owl.
14. Nun.
15. Mottle Tumbler.
16. Saddle Tumbler.
17. English Beard.
18. Baldhead.
19. Runt.
20. Magpie.
21. Show Homer.
22. Archangel.
23. Oriental Roller.
24. Norwich Cropper.
25. Cumulet.
26. Tippler.
27. African Owl.
28. Working Homer.
29. Mane.
30. Domino.
31. Oriental Turbit.
32. Blondinette.
33. Satinette.
34. Shortfaced Antwerp.
35. Priest.
36. Fairy.
37. Frillback.
38. Swallow.
39. Suabian.
40. Fire Spot.
The broad facts in the history of living things upon the earth are,
then, in accordance with the theory of Evolution. The chain of types
is indeed a broken one, the gaps being many, and some of them wide.
But this is readily to be understood from the comparative scarcity of
fossils, and the imperfection of the geological record.
In certain particular instances, however, very complete series of
fossil forms have been discovered, connecting, by small gradations,
modern animals with greatly different extinct types. One of the most
complete of such series has been discovered for the horse. The changes
that have occurred in the evolution of this animal have been mainly in
three directions--increase in size, reduction in the number of toes
from the original five to the final one, and deepening of the crowns
of the teeth, so as to render them capable of longer wear. From the
Eohippus of early tertiary times, an animal of about the size of a fox
terrier, with five toes behind, and four with the vestige of the fifth
in front, there is a complete connecting series reaching up to the
modern horse, with its single remaining toe and the vestiges of two
others. A few of the main links in this chain are illustrated in Fig.
3. It is impossible to regard such a series without having the idea of
Evolution strongly suggested to the mind.
In the second place, there is evidence for Evolution in the fact that
marked changes can and do occur in the characters of living races of
organisms. There is ample evidence, for example, that all our modern
breeds of pigeons are descended from the wild rock-dove. How markedly
some of these differ from their wild ancestor, and among themselves,
may be seen from Fig. 4. The size of some is twice as great as that
of others. The bill in some is greatly increased in length, is almost
ludicrously reduced in others. Colour, feathering, build, even
the instincts and the voice, vary enormously as between different
varieties. In short, there is hardly any obvious character that has
not, in one or other of the breeds, undergone great modification. As
Darwin remarked, any naturalist coming upon such a group of forms in
nature would have no hesitation in placing them in different species or
genera, or even perhaps in different families. Even granting that the
conditions of domestication are peculiar, we must admit that if such
large changes can occur in a few centuries, it is possible that man
has evolved from the lowest of living organisms during a period some
hundreds of thousands of times as long.
[Illustration: O. LATA O. LAMARCKIANA O. NANELLA.
FIG. 5.
Mutation in _Oenothera lamarckiana_. The parent species (in the middle)
with two of the 'sports' from it.
From De Vries, _The Evolution Theory_. By permission of The Open Court
Publishing Co.]
But marked changes of type occur not only under conditions of
domestication; nor is it necessary to infer the occurrence of any such
changes without actual direct evidence. The formation of new types
occurs in nature, and has taken place under the very eyes of scientific
observers. Perhaps the most striking case that can be quoted is that
of Lamarck's Evening Primrose, which, under the observation of Prof.
De Vries in Amsterdam, produced some half-dozen of 'sports' which seem
well entitled to rank as new species. Fig. 5 shows the parent plant
and two of the new types that were produced by it. One is a dwarf in
habit, the other is characterised by the greatly increased breadth of
its foliage. Others showed different peculiarities. One might quote
many other instances of violent changes of type--of the appearance of
six-fingered children, whose peculiarity was afterwards inherited;
of web-footed pigeons, and of new varieties of fruits, flowers, and
vegetables. The causes of such 'sports' or mutations are unknown,
but their moderately frequent occurrence is abundantly demonstrated.
Such facts show, at all events, that the old conception of species as
permanently fixed, unchanging types, can no longer reasonably be held.
[Illustration: FIG. 6.--Horse's Foot, with well-developed Side Digit.
From Bateson's _Materials for the Study of Variation_ (Macmillan)].
[Illustration: FIG. 7.--Persistent Coccyx in Man.]
[Illustration: FIG. 8.--Persistent Gill Slits in Man.]
Not all of the abnormalities which thus suddenly appear, we know not
how or wherefore, are new. Many recall characters in lower or older
groups, and may reasonably be interpreted as 'reversions.' Thus the
horse's leg shown in Fig. 6 bears a well-developed side toe, in place
of the small vestige that is normally present. Horses with this
peculiarity have occurred with some frequency, probably before, and
certainly since, the most famous of their kind, which Julius Cæsar
rode. It seems reasonable to regard this peculiarity as a return to
the old ancestral condition illustrated before, in which the side toes
were well developed. The same applies to the instance of a persistent
tail and persistent gill slits in man (Figs. 7 and 8), and to many
other instances that might be quoted. One must indeed deal carefully
with such cases, for it is always difficult to say what changes are new
departures, and what are returns to ancestral types. There is danger of
arguing in a circle--of supposing the ancestry from the abnormality,
and of terming the latter a reversion because it suggests the supposed
ancestry. Nevertheless, when variations occur, suggesting characters
which are believed, on other grounds, to be ancestral, they must tend
to strengthen the other evidence as to the evolution of the type in
question.
[Illustration: FIG. 9.
(a) The blind-gut of a kangaroo (_bl_), and (b) the corresponding
reduced structure, the vermiform appendix in man (_bl_).]
[Illustration:
FIG. 10.--Skeleton of Cassowary, showing reduced wing-bones (a piece
of black paper is placed under them).
From Dendy's _Outlines of Evolutionary Biology_ (Constable).]
Another and a very strong evidence of Evolution is to be found in what
are termed vestigeal structures, two of which are illustrated in Figs.
9 and 10. They are, for the most part, obviously useless, and their
occurrence has never been satisfactorily explained except by supposing
them to be remnants of organs that were functional in the past history
of their possessors' race. The appendix of man, for instance, is not
only useless, but is frequently a source of danger. But its presence
is readily explained by supposing that it represents the blind-gut,
which is large and functional in many of the lower animals. Again, how
should we account for the presence of small functionless wing-bones in
the cassowary, unless by supposing that its ancestors were accustomed
to fly like ordinary birds? How should we explain the bones which
represent the hind limbs of the whale, unless by regarding the whale
as descended from an animal which had functional hind limbs, or the
representatives of eyes in animals that live in the dark, unless
by supposing that these are descended from ancestors which saw? It
has been well said that the bodies of many animals are veritable
antiquarian museums, filled with relics of their own ancestors.
The next argument for Evolution to which we would refer is based on the
similar structure and origin of organs or members that have entirely
different uses. In Fig. 11 are figured the bones of the fore limbs of
four different mammals, a whale, a bat, a dog, and man. The first is
used for swimming, the second for flight, the third for locomotion on
land, and the fourth as a grasping and holding organ. If these organs
had been specially designed, each for its specific purpose, we should
expect to find fundamental differences in structure. Actually the
general arrangement of bones is the same in each case. A fact like this
points strongly to a common origin of the four types mentioned, and to
a general primitive arrangement of the bones of the limb. This primary
type, it seems natural to suppose, has been modified for various
special purposes in many different directions, the general features
remaining recognisable. Many other cases of homology, or similarity of
structure and origin, in organs whose function is dissimilar, might
be quoted. Thus the poison gland of the poison snakes is not an organ
which has been specially developed, but is a modified portion of one
of the salivary glands. The hoof of the horse and the finger nail
of man can evidently be satisfactorily explained as modifications
of a general type of terminal claw, and the scales of the scaly
ant-eater and the quills of the porcupine are only modified hairs. The
significance of facts like these, when carefully considered, is very
great.
[Illustration: FIG. 11.--The bones of the fore limbs of (_a_) whale,
(_b_) bat, (_c_) dog, and (_d_) man, showing essential similarity in
arrangement.]
[Illustration: FIG. 12.--Distribution of Marsupials or pouch-bearing
animals.
Australia, New Guinea, etc. 36 Genera. 144 Species.
America 3 Genera. 28 Species.]
The study of the geographical distribution of animals has brought forth
a great mass of facts which, considered by themselves, seem chaotic
and meaningless, but which, in the light of Evolution, are full of
significance. Observe, for example, the distribution of the Marsupials
or pouch-bearing animals, shown on the accompanying map (Fig. 12).
Australia is full of them, while they are relatively meagrely
represented in a few other parts of the world. At the same time the
greater and higher group of mammals was represented in Australia,
at the time of its discovery, only by the bushman and his dog and a
few species of mice. It is not as if the Australian environment were
specially well adapted for marsupials, or specially ill-adapted for
higher mammals; for the sheep has proved itself splendidly adapted for
the conditions, and the rabbit most inconveniently so. Why, then, this
curious state of affairs? It is an undoubted fact that the marsupials
are both lower in their position in the animal kingdom, and older,
than the main group to which all our European mammals belong. Now it
is believed that Australia was once connected by land with the Asiatic
Continent, and that it was finally separated from it before the higher
mammals were in existence. The great step of further progress occurred
elsewhere than in Australia, and the mammals of the latter continent
were left in their obsolete condition, preserved through lack of
competition of that higher type which elsewhere became dominant.
[Illustration: FIG. 13.--Distribution of Lemurs.
Madagascar 12 Genera. 36 Species.
Africa, India, Malay 5 Genera. 12 Species.]
Madagascar offers a similar case. It abounds with forest vegetation and
seems to offer a highly suitable environment for the monkey tribe. Yet
there are no apes on the island. Their place is occupied by the Lemur
tribe, which, there is every reason to believe, is the older group
of the two, and that from which the apes have sprung. It is supposed,
then, that Madagascar was separated from Africa before the ape had
evolved. The lemurs thenceforward were free from the competition of
their more highly developed relatives, and have branched out into a
great variety of types, while still remaining on a relatively low plane
of intelligence and specialisation. The distribution of the Lemurs is
shown in Fig. 13.
In Dr. Alfred Russel Wallace's book on _Island Life_ there are set
forth a great number of interesting facts on the subject of the animal
population of islands, and many striking interpretations of these
facts in the light of the Evolution theory. Coral islands, and those
caused by volcanic eruptions, are peopled with inhabitants which have
accidentally come thither by flight, or have been brought, for example,
on floating timber by ocean currents. On the other hand, islands which
represent separated fragments of continents have usually a fauna of the
same general type as that of the continent of which they have formed
a part. But the actual species are frequently different, and if the
separation is of more ancient date, the differences are still more
marked. The fact of this divergence of an isolated animal population
from that from which it has originated is sufficiently striking, and
would remain an inexplicable problem, were we without an Evolution
theory. According to the Evolution hypothesis, however, the restricted
and somewhat special environment favours a modification of the
original types with which the island was provided, and a satisfactory
explanation is offered.
Finally, we may mention the evidence that has been gathered from the
study of embryology and development. It has been stated, in a metaphor
which is perhaps more clever than it is exact, that every animal
climbs up its own ancestral tree; and while it would be absurd to
say, for instance, that a mammalian embryo resembles successively a
fish, an amphibian, and a reptile, still many of the broad facts in
the evolution of a race seem to be repeated, in a more or less blurred
and indistinct fashion, in the development of the individual. Thus,
for example, gill-slits and a tail are possessed in common by the
embryos of all higher animals, only afterwards to disappear in those
types in which the adult animal is without these structures. The heart
of the mammal or bird is at first simple, then two chambered like that
of a fish, then three chambered like an amphibian's, and finally four
chambered. Some of the main phases in the development of the rabbit and
of man are shown in Figs. 14 and 15 respectively.
[Illustration: FIG. 14.--Stages in development of embryo of rabbit.
_a_, 10 days; _b_, 11 days; _c_, 15 days; _d_, 17 days old.]
[Illustration: FIG. 15.--Stages in development of human embryo.
_a_, 18-21 days; _b_, 27-30 days; _c_, 35 days; _d_, 52-54 days old.]
The young flat-fish is like an ordinary member of the fish tribe, with
an eye on either side of its head, and its body built on the ordinary
symmetrical lines. It is only later, when it begins habitually to be
upon one side on the sea bottom, that the eye from the under side
wanders round to the opposite aspect beside its fellow, and the upper
side becomes pigmented, while the lower remains white.
In similar fashion a primitive form of kidney is, as it were, sketched
in, in the development of the higher animals, only to be erased at
a later stage and replaced by a better form. The human child has a
complete body covering of hair, which disappears soon after birth. In
these and many more instances, one cannot avoid the impression that
the organism has not been specially designed for what it finally comes
to be. It cannot forget, and must needs repeat, or so it seems, some
considerable part of the history of its race.
Manifestly, then, all this evidence, gleaned from many different
sources, points to a common origin of living things, and to the gradual
evolution of the higher from the lower types. It may also be said that
there is no scientific evidence against such a view.