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Title: Evolution
Author: Watson, James A. S.
Language: English
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*** Start of this LibraryBlog Digital Book "Evolution" ***
"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.
CHAPTER II
UNICELLULAR AND MULTICELLULAR ANIMALS
We must now turn to the main project of this book, which is to attempt
to trace out the lines along which animal Evolution has proceeded,
with special reference to that particular line which leads up to man.
Indeed, we shall have to stick somewhat closely to this one main
highway, and can but barely pause to glance along the numerous branch
roads, interesting though the travelling there might be.
It is perhaps necessary to say, at the outset, that the history of the
Evolution of man cannot be written as a plain, matter-of-fact tale.
Many portions of this history are tolerably well understood, but there
are other periods, in some of which notable steps of progress were
made, of which no record has ever been discovered. We must therefore
expect occasionally to be reduced to speculation, and here and there to
meet with controversy and with opposing theories.
It is not proposed here to enter into any full discussion as to the
origin of life. It may shortly be said that in the existing state
of knowledge, no very definite theory is possible. We know that
life is associated with a jelly-like or semi-fluid substance called
_protoplasm_, which consists of a very complex mixture of albuminoids.
These albuminoids are continually undergoing changes and interactions
of a complex kind, the sum total of which constitutes life. Many of
these reactions have been reproduced, or imitated, artificially, and
have been shown to be purely chemical or physical. The chemical nature
of the albuminoids is indeed so complex that some considerable time
must yet elapse before it can be completely investigated; and until
such time it is obvious that we cannot hope for any very definite
conceptions as to the nature of life. Broadly, however, the majority
of physiologists regard life as a highly intricate series of purely
physical and chemical processes, and if such a view be accepted,
there is no insuperable objection to a general theory of the origin
of living from non-living matter. By this it is not intended to imply
that the manufacture of living matter is an immediate possibility;
for even according to such a theory as we have indicated, it would
be supposed that living substance came into being by a very slow
process of Evolution, which it is hardly conceivable could ever be
repeated in the laboratory. Knowing, as we do, that there was a time
when no life existed upon the earth, and believing, as there is good
reason to believe, that there is no fundamental distinction between
living processes and ordinary chemical and physical reactions, we may
logically regard life itself as a product of a natural process of
Evolution.
[Illustration: FIG. 16.--A typical cell (greatly magnified).
(_k_) Nucleus; (_p_) cell protoplasm.]
[Illustration: FIG. 17.--The process of cell division.
_c_, The centrosome, the body which divides first, and which controls
the division of the nucleus.]
To begin at the beginning of our tale, we may ask ourselves what are
the lowest, simplest, living things that are known. The question does
not admit of any very definite answer. For as we look around among a
number of the most simple forms, we find ourselves handicapped in our
attempt to judge between them, by a lack of knowledge of their nature.
We come upon organisms so small that they appear, even under the most
powerful microscope, only as the tiniest specks; whose size is to be
measured in hundredths of thousandths of an inch. We even find good
evidence that living things exist which we are unable, in any manner
whatsoever, to see. Among the smallest known forms, and also among
some of the larger, we find organisms that we can only describe as
practically structureless, that appear as specks of almost homogeneous
protoplasm; but it seems reasonable to suppose that this appearance is
due rather to our imperfect observation than to an actual absence of
differentiation.
It is certain, however, that the lowest of the great groups is that of
the one-celled organisms. As all the higher types are built up of large
numbers of cells, essentially similar to those which constitute the
unicellular forms, it is important that we should know something of the
nature of this organic unit. A typical cell is illustrated in Fig. 16.
It consists of a mass of protoplasm, with a distinctly differentiated
portion called the nucleus. The function of the nucleus is that of
directing and controlling the activities of the cell; if it is removed,
the remaining portion of the cell soon dies; while, on the other hand,
a small portion of the cell, if it contains the nucleus, may frequently
live, and build up new protoplasm to replace what was lost. Cells are
formed only from previously existing cells, by a process of division,
which is usually simply one of halving. This process is begun in the
nucleus; it undergoes a complex rearrangement of its parts, the object
of which appears to be to insure an absolute equality in the halves,
and finally divides in two. The bulk of the protoplasm then separates
into two portions, a portion remaining round each of the nuclei. The
process of cell division is illustrated in Fig. 17.
Now it is a somewhat remarkable fact that we do not know whether or
not all the humbler forms of life possess a nucleus. It was formerly
believed that a considerable number of one-celled organisms were devoid
of the body in question, but in most of such it has been shown that
nuclear matter is present, though it may be distributed, in small
portions, throughout the cell. If organisms do exist which consist of a
cell without a nucleus, we must regard them as the simplest of living
things. In any case, the formation of a nucleus, a process by which a
kind of central government was formed, was probably one of the great
early steps of Evolution.
[Illustration:
FIG. 18.--Organism of sleeping sickness in blood. The round bodies are
red blood corpuscles.
Photo: F. Martin Duncan.]
The life-history of an ordinary one-celled organism may be briefly
summed up. It absorbs nourishment and energy, adds to its substance
until it reaches a certain fairly definite size, and then divides in
two, the halves separating, and going each its own way. In the world
of one-celled organisms there is no 'death from natural causes.' The
individual is potentially immortal, except in so far as we may regard
the individual life as ceasing when division takes place. Death occurs
only, as we say, accidentally--for example, from starvation or from the
attacks of enemies. A number of simple unicellular organisms are shown
in Figs. 18, 19, and 20.
The reader will have observed that we have referred to the group under
consideration in general terms, and without endeavouring to classify
its members as plants or animals. And indeed it is impossible to carry
this great distinction down to the lowest group of the organic world.
This stands below the first great forking of the tree of life; its
members remain in what has been described as a condition of 'chronic
indecision,' neither clearly vegetable nor definitely animal. But very
soon, in the march of progress, the forking of the roads was reached,
and whosoever was bent on journeying farther had perforce to make the
choice. We must here briefly consider what this choice was, and wherein
the fundamental distinction between a plant and an animal consists;
for, strange as the statement may seem, the basis of this distinction
is by no means generally appreciated.
[Illustration: FIG. 19.--The bacillus of bubonic plague (× 1000).
Photo: F. Martin Duncan.]
The typical plant lives by absorbing carbon dioxide gas, water, and
mineral salts from the surrounding media. These substances, by means of
energy which it gathers from the rays of the sun, the plant builds up
into organic substances, to be used in the maintenance of life, and for
growth and reproduction. This process of chemical construction occurs
only in the green, exposed parts of the plant, and indeed can occur
only in the presence of chlorophyll, the green colouring matter of the
leaves.
[Illustration: FIG. 20.--The bacillus of typhoid (× 2500 diameters).
Photo: F. Martin Duncan.]
The animal, on the other hand, lives by appropriating, either directly
or indirectly, what the plant has produced. All flesh is indeed grass,
in a different sense from that originally intended by the statement.
It is this essential difference which lies at the root of all the
plain and obvious distinctions between animals and plants. The plant
has neither the necessity to go forth in search of its food materials,
which nature brings to it, nor has it to spare of its painfully
collected energy for the labour of locomotion. Hence it remains
stationary. The animal must of necessity go to seek its more elaborate
fare, therefore it moves. Moreover, to be successful in its search,
the animal obviously requires a nervous system to direct and control
its movements, which system, except in the simplest and crudest forms,
is absent from the plant. In the main, then, the plant builds up and
saves, the animal breaks down and spends. The plant is the producer,
the animal the consumer.
[Illustration: FIG. 21.--Amœba.
_K_, Nucleus; _V_, contractile vacuole.]
Turning now to those of the lower organisms that are somewhat more
definitely animal in nature, we may describe the common Amœba.
Microscopic in size, this creature consists of a speck of semi-liquid
protoplasm, which is irregular and ever-changing in shape. It is
continually pushing out finger-like projections from various parts of
its surface, feeling, in a dim, vague way, for its food. It moves, if
but slowly, by withdrawing its substance in one direction and pouring
it forth in another. It indulges in such fare as bacteria or particles
of dead organic matter and feeds by the simple method of surrounding
the food particle with its protoplasm, and gradually digesting and
absorbing whatever it contains of nutriment. Undigested portions are
simply left behind as the creature moves on. The waste products are
drained into a simple cavity in the protoplasm called the contractile
vacuole, which empties itself periodically to the outside. The Amœba
reproduces by the ordinary process of simple fission, illustrated, with
the creature in its ordinary condition, in Figs. 21 and 22.
[Illustration: Fig. 22--Stages in division of Amœba.
K, nucleus.]
[Illustration: FIG. 23.--Paramœcium.
_EC_, Denser outer layer; _EN_, inner protoplasm; _N_, nucleus; _PV_,
contractile vacuole; _M_, mouth; _X_, cilia.
From Marshall and Hurst's _Practical Zoology_ (Smith, Elder & Co.).]
Somewhat higher than the Amœba, and apparently along the main line of
progress, stands the group which includes the slipper animalcule,
Paramœcium, shown in Fig. 23. This creature, barely visible to the
naked eye, is found in pools of water, or, for example, in drops of
rain or dew on plants, and it can generally be obtained in great
numbers by soaking a little hay in water for a day or two. It has,
as may be seen from the illustration, an elongated shape, with a
depression, the mouth, about the middle of one side. The progress made
good from the stage of the Amœba has been largely in the direction
of a more efficient method of locomotion. Instead of crawling, with
painful slowness, the Paramœcium swims freely and rapidly by means of
the numerous whip-like projections or _cilia_ which cover it, and with
which it lashes the water. An advance is also to be recognised in the
fact that the organism is surrounded by a dense outer wall; and that
its shape is consequently fixed. Hence also the Paramœcium cannot take
in food at any part of its surface, as the Amœba can, but only through
the special depression already mentioned. Excretion is carried on in
the same manner as in the Amœba. The Paramœcium is a water animal, yet
it can resist drying, and remain alive in the absence of water, for a
long period. This it accomplishes by becoming encysted, that is, by
contracting into a ball and surrounding itself with a resistant shell,
from which it can emerge when suitable conditions for active life
return. It is worth passing notice that there exist a number of forms
occupying a position intermediate between the two types which we have
described, and indicating that the second has, in all probability, been
derived from the first. One of these is shown on Fig. 24.
[Illustration:
FIG. 24.--Cercomonas, a form intermediate between the crawling Amœba
type and the free-swimming Paramœcium type.]
There is another interesting fact in connection with Paramœcium.
Under natural conditions, division and redivision continue in the
ordinary way for a large and indefinite number of generations. But
very occasionally, a process known as _conjugation_ occurs. Two
individuals lay themselves side by side, and partially unite; they
exchange portions of their nuclear substance, and finally separate
again, simple division afterwards proceeding as before. Conjugation,
although distinctly different from the ordinary process of sexual
reproduction, appears to serve the same purpose. Until quite lately its
meaning, and that of the process of sexual reproduction in general,
seemed to bid fair to remain a perpetual puzzle to biologists. But
at last we seem to be approaching the solution. The characters of a
species are determined, it is tolerably certain, by the constitution of
the cell nucleus, and accordingly as this varies from one individual
to another, so the characters of the individuals will vary. Now, if
simple division were to continue indefinitely, successive generations
would be produced on the same plan, and the racial characters would
in the main remain constant. But conditions of life vary from time to
time and from place to place, and the particular type which succeeds
best under one set of circumstances may be ill adapted for another. It
is therefore an advantage to a race to be capable of variation. And
the process of sexual reproduction, by continually bringing about a
mixture of the nuclear substance, ensures the regular production of a
variety of types. Of these various combinations of characters the few
that are suited to the prevailing conditions will, for the time being,
constitute the dominant types. When conditions change, fresh types will
be available to replace them. The process of conjugation is illustrated
in Fig. 25.
[Illustration: FIG. 25.--Stages in conjugation of Paramœcium.
_meg._, The meganucleus; _mic._, the micronucleus, which divides,
and half of which is exchanged; _p.b._, Polar bodies, which the
micronucleus throws off, and which disappear.
From Dendy's _Outlines of Evolutionary Biology_ (Constable).]
There are many groups of one-celled animals other than those typified
in the Amœba and the Paramœcium, but they do not appear to have any
significance so far as the descent of the higher animals is concerned,
and they therefore do not immediately concern us.
We have already mentioned that water is the life medium of the slipper
animalcule. It was destined to remain the natural element, both of
animals and of plants, throughout many subsequent stages of progress.
The reason of this is not far to seek. Active protoplasm consists to
the extent of about three-fourths of water, and a plentiful supply
of this is one of the essentials for the continuance of active life.
Therefore, before the conquest of the dry land could be accomplished,
devices had to be evolved both for maintaining and for conserving
the water supply--roots in the plant; in the animal, some method of
locomotion by land or air, so that water could be frequently reached;
protection against evaporation, in the form of a skin, in both; and
numerous other special devices. Add to this the fact that locomotion on
land presents much greater difficulties than that in water, and it will
hardly occasion surprise that vast ages were yet to be required before
the Evolution process could produce a land animal.
A striking analogy may be drawn between animal Evolution, from this
point onwards, and social Evolution. In the latter case we begin
with men, brought by a slow process of Evolution to a high state of
individual perfection, living in a state of savage individualism.
Each thinks and acts for himself, provides his own food, raiment, and
dwelling; constitutes his own standing army and police. From this
condition of affairs there has gradually been developed the modern
social arrangement, by which each individual helps to carry out some
distinctly special part of work for the community--be it wheat-growing,
cloth-weaving, bricklaying, or the arresting of burglars--and trusts
to the community for his requirements in all other directions. These
requirements themselves have so multiplied during the course of social
Evolution that innumerable forms of activity have sprung up between
those occupations which provide the original necessities of life. The
essence of the whole process has been co-operation and the division of
labour.
In the story of animal Evolution we have reached a point where a
highly perfected individual cell has been produced, which carries out
for itself, and for itself alone, all the activities of life. From
now onwards, co-operation and specialisation are the watchwords of
progress. There is a clubbing together, first of a few cells, then of
hundreds, and finally of millions upon millions, to form the bodies
corporate which we recognise as individual higher animals. Division
and distribution, subdivision and further distribution of the life
activities proceed at the same time, until we reach the condition
prevailing in the higher animals, where the degree of specialisation
almost passes conception. In such there is, to begin with, a vast
frontier army of skin cells, occupied in securing peace, as far as
possible, for the industries that go on within. There are directors
and controllers of these industries--the brain cells--with a myriad
of workers under their guidance, and a great and complex telegraph
system between. The workers themselves are of all descriptions--common
labourers like the cells of the muscles; transport workers like those
of the circulatory system; skilled factory hands like those of the
glands; even scavengers in the shape of the sweat gland and kidney
cells. Nay, there is even a numerous police force, of white blood
corpuscles, which patrol everywhere, arresting intruders and disposing
of them by the summary method of swallowing them whole.
[Illustration: FIG 26.--Spondylomorum, a small colony of flagellates.]
[Illustration: FIG. 27.--Magosphæra, a colonial flagellate.]
Our information regarding the early history of this co-operative
movement is fragmentary and incomplete, for only an odd species or
so seems to survive of the group which we regard as the earliest of
multicellular animals. In certain forms which are still essentially
unicellular, such as the Spondylomorum shown in Fig. 26, there is a
tendency to form smaller or larger cell colonies. When the individual
cell divides, the two daughter cells do not separate, but remain
somewhat loosely attached to each other, and the process of division
without separation continues until a considerable group is produced.
From this colony occasional individuals break away and proceed to form
new colonies. From such a type it is a comparatively easy step to
the Magosphæra described by Haeckel and illustrated in Fig. 27. This
consists of a simple ball of ciliated cells, which reproduces by the
occasional breaking away of an individual member, which divides and
redivides until a new sphere is produced. Unfortunately this animal
has only once been discovered, and many hold that it has not been
sufficiently investigated. No other of the same type is known.
If we turn to the plant kingdom, however, we find a comparatively
common organism of somewhat similar form. This is the Volvox, a plant
which consists of some thousands of cells, and reaches a size of about
a pin-head. It has the form of a hollow sphere, the wall of which
is one cell thick, and the cavity of which contains only water. The
cells bear whip-like cilia on their outer surface, by whose means
the organism is able to move, swimming by a rotary motion round a
definite axis. The individual cells are separated by layers of a
gelatinous substance, through which, however, pass connecting strands
of protoplasm. The cells, of course, contain the green colouring matter
common to plants in general. Distributed among the ordinary cells occur
a few that are distinguished by their larger size, and by the fact that
they lack cilia. These are the special reproductive members of the
colony. When the Volvox reaches maturity, these cells begin to divide,
and form new growths which take the form of hollow sacs, which project
into the cavity of the parent sphere. Later they separate from the
wall of the parent, and begin to move about, in the internal cavity,
by means of the cilia which they have developed. Finally the parent
breaks up and dies, and the progeny are set free to commence life for
themselves.
[Illustration: FIG. 28.--Volvox. A female, showing egg cells.]
[Illustration: FIG. 29.--Volvox. Male, showing packets of sperm cells.]
The fundamental importance of this type is that we have already
a division of the life activities. The majority of the cells are
concerned in the nutrition of the individual as a whole. These
ultimately perish. A minority, however, are fed and protected by them,
and these in return secure the perpetuation of the race. This division
into a mortal 'body' portion and an immortal reproductive portion is
the first and most important division of the life activities, whether
in the animal or in the plant kingdoms. The body cells, modified in
various directions for their special purposes, could not, and do not,
reproduce complete new individuals. Therefore a generalised type of
cell is maintained for the express purpose of the propagation of the
race. It is to be observed, now, that the process of reproduction
in Volvox is not always such as we have described. Sometimes the
reproductive cells are of two kinds. The one type divides into a great
number of small ciliated cells, which escape separately and directly
to the _outside_ of the sphere, and swim away. These free-swimming
individuals do not form new colonies, but seek out the reproductive
cells of the other type, which latter still form part of the organism
which has produced them. One of the free-swimming cells enters each
of those of the other kind, and the nuclei of the two merge into
one. The cell so produced, after a longer or shorter rest period,
commences to divide and redivide in the manner already described,
forming a new colony. The process that we have described is that of
sexual reproduction, and its essential features are the same as in
Volvox throughout the whole animal kingdom. The small free-swimming
cells are the male reproductive bodies or sperms, the others are the
female or egg cells. The union of the two produces the fertilised egg,
and the process of union is termed fertilisation. In Volvox, the male
and female elements are sometimes produced by the same individual, at
other times by different ones. Separation of the sexes is no necessary
accompaniment of the process of sexual reproduction, and indeed it is
only in the higher groups of animals that separate sexes are the rule.
The various conditions in Volvox are illustrated in Figs. 28, 29, and
30.
[Illustration: FIG. 30.--Volvox. Portion of a hermaphrodite individual,
showing egg cells (_O, O_{1}_), and sperms (_S_{1} S_{2} S_{3}_).]
The next great groups of animals are, on the one hand, that of the
sponges, and, on the other, that which includes the sea-anemones,
jelly-fishes, corals, etc. At first sight their structure seems vastly
different to that of the Volvox, from some form similar to which they
have probably been derived. The evidence obtained from the study of
their individual development, however, strongly suggests a process by
which we suppose that they evolved from Volvox-like ancestors. We shall
therefore briefly describe the earlier stages of the development of a
coral. The sexually produced individual starts life as a single cell,
the fertilised egg. This divides and redivides until a hollow ball of
cells is produced, which cells, like those of the Volvox, bear cilia.
Although simply spherical in shape, the creature moves by rotating
round a definite axis, like a planet. Moreover, nutriment is absorbed
not by any or every part of the surface, but only by a small area
round the lower pole. Now as development proceeds, the cells at this
pole divide more rapidly than the rest, with the natural result that
the ball begins to get out of shape. The distended portion, however,
develops to the inside, so that one part of the sphere is, as it were,
pushed into the other. When this process has been completed, the
original internal cavity is almost entirely eliminated, and a form is
produced which resembles a double-walled flask or vase. Such a form may
be taken as the fundamental architectural type of the groups that we
are now to consider. The meaning of this further step of Evolution is
again specialisation. The inner layer of cells takes on the functions
of digestion and absorption of food, there having been evolved, in
fact, the simplest possible form of mouth and stomach. Such other
functions as those of locomotion, protection, and support are exercised
by the outer layer. This process is illustrated in Fig. 31.
[Illustration: FIG. 31.--Process of gastrulation in a coral.
_A_, _B_, Blastula, or simple hollow ball; _C_, _D_, intermediate
condition; _E_, _F_, gastrula, or double-walled flask condition.]
But there is no known type of animal which, in its adult form, shows
quite the simple structure that we have described. Perhaps the
nearest approach is to be found in the lower sponges, in which two
modifications of the original plan have already been introduced.
In the first place, the creature is sedentary, being fixed, in an
inverted position, to some solid basis. It has, so to speak, ceased to
be a hunter, and is become a fisher. Secondly, its wall is pierced
in many places, so as to permit of a freer circulation, through the
digestive cavity, of the water which contains the food material. The
water passes in through these numerous perforations, and out through
the main central opening or 'mouth.' The sponges do not appear to
represent a stage in the main line of Evolution, but lead us almost
immediately into a cul-de-sac. We therefore cannot pause to describe
fully the many peculiar and interesting developments which occurred in
the group. An ordinary 'sponge,' by the way, bears the same relation to
the creature which produces it as does a 'coral' to the coral animal.
It represents, that is to say, the skeletons of a large colony of
individuals. The structure of a sponge is shown in Fig. 32.
[Illustration: FIG. 32.--Diagrammatic section of lower sponge.
_e_, inner cell layer.
_m_, middle jelly-like layer.
_z_, outer cell layer.
_a_, digestive cavity.
_i_, perforations in the wall.]
The other great group of primitive multicellular animals is that of the
Cœlenterata, and as an example of the most primitive of these we may
take the common freshwater Hydra. The Hydra reaches a length of nearly
half an inch, and is to be found attached to water-weed and the like
in streams. It consists of a hollow tube-shaped body which is fixed by
the so-called 'foot.' Two layers of cells form the wall of this tube,
these being separated by a thin membrane of gelatinous material. At
the upper end is the mouth, which leads immediately into the internal
cavity or stomach. The mouth is surrounded by a ring of from six to
eight tentacles, which are outgrowths of both cell layers. The cells
of the inner layer are large, and bear cilia that protrude into the
internal cavity. Their functions are those of digestion and absorption.
Part of the protoplasm of the outer cells is modified into a fibrous,
contractile substance, which represents the beginnings of muscle
tissue. The outer layer also forms a protective skin-like covering.
In the outer layer also occur a large number of stinging cells, each
of which has a complex mechanism for injecting a fluid poison into
any creature which should happen to come in contact with them. These
'nettle cells' occur in much greater numbers in the tentacles than
elsewhere, and here they are brought into play against the animals,
such as minute Crustaceans, which form the Hydra's prey. Coming in
contact with the tentacles, such creatures are caught, paralysed by
means of the stinging cells, and are gradually transferred into the
mouth by a slow contraction of the tentacles. The Hydra reproduces,
for the most part, by a simple process of budding. Small lateral
outgrowths are formed, which gradually develop mouth and tentacles
of their own. Ultimately these separate and are carried off by the
water, later to settle down and become attached to some fixed object.
Sometimes, however, sexual reproduction occurs. The reproductive
cells are produced, male and female on the same individual, among the
ordinary cells of the outer layer. These are set free, fertilisation
occurs in the water, and the egg develops in the same manner as that
of the coral. The Hydra is able, by means of the fibrous protoplasm of
its outer cells, to show well-marked movements. It can bend its body in
this direction or that, can contract its whole body into a small oval
mass, and is even able, by performing a number of slow somersaults, to
change its position. The structure and the methods of reproduction in
Hydra will be readily understood from the illustrations of the creature
on Figs. 33 and 34.
[Illustration: FIG. 33.--Specimens of Hydra on green water-weed.
_A_, Contracted; _B_, extended; _C_, specimen with vegetative buds;
_D_, specimen with sex cells; _sp_, sperm cells; _e_, egg.]
[Illustration: FIG. 34.--Diagrammatic section of Hydra.
_en_, Inner cell layer; _ec_, outer cell layer; _c_, nettle cell.]
If now we make a brief general survey of the group to which the Hydra
belongs, we find in it two somewhat strikingly different types. On
the one hand are sedentary forms that resemble, in a general way, the
Hydra; that consist of a tube-shaped body, with the mouth, surrounded
by a ring of tentacles, at the upper end. The sea-anemones and
corals are examples of this type, in which, however, the structure
shows various complexities as compared with that of the Hydra, which
complexities we cannot here pause to describe. On the other hand is the
well-known Medusa form, of which the common jelly-fish is a typical
example. This creature, as is well known, is mushroom shaped, with
tentacles round the edge. The mouth is in the middle of the lower
aspect, at the end of a short 'stalk.' This type is very different in
general appearance from the Hydra or sea-anemone, yet the one form
may be somewhat easily derived from the other; we have only to imagine
that a Hydra is turned upside down, that it is squashed, vertically,
until the internal cavity is greatly reduced, and the circumference,
especially in the region of the tentacles, greatly increased, and we
should have something resembling a Medusa. That the two types are
actually closely related is shown by the fact that there is in the
life-history of one group of Cœlenterates a regular alternation between
the one and the other.
[Illustration: FIG. 35.--Diagram of Medusa.
_rad_, Radial canals, with reproductive bodies, _o_; _r_, ring canal;
_t_, tentacle canal.]
If the above general conception of the structure of the Medusa be
borne in mind, its details will be easily understood. The internal
cavity, instead of being simple, has become complicated, through
the obliteration of certain parts of it, where the upper and lower
walls come in contact. What is left is a comparatively small cavity
immediately above the mouth, a number of symmetrically arranged canals
radiating out from this, and a ring canal connecting the ends of these
with each other. Another special characteristic is that there is a
great mass of gelatinous substance between the outer and inner cell
layers. The reproductive cells, as in the sea-anemones, are produced by
the inner cell layer, and escape by the mouth.
[Illustration: FIG. 36.--Diagrammatic section of Medusa.]
[Illustration: FIG. 37.--Group of Cœlenterates--Medusæ, Sea-anemones,
and Corals.]
Something remains to be said regarding the specialisation of tissues
in this group. We have already mentioned the stinging cells, and
the beginnings of muscular tissue, in Hydra. The former are a
constant feature of the Cœlenterates, while the latter reaches a very
considerable development in the higher forms, as may be judged from
the surprising rapidity with which the Medusa can swim, or from the
strength with which the sea-anemone can retract its tentacles and draw
itself together. Important, further, is the nerve tissue. This consists
of cells whose business it is to receive and transmit stimuli. They
have long fibrous projections connecting them with each other, so that
there is a network of communication throughout the whole animal. In the
Medusa, where co-ordinated movements of various portions is necessary,
there is a concentration of nerve cells into a double ring near the
edge. Here also there are special organs, probably of sight and of the
sense of balance; but as these cannot be regarded as the forerunners of
the analogous organs in higher animals, we need not pause to describe
them. The anatomy of the Cœlenterates will be better understood if the
reader will study the diagrams in Figs. 35 and 36, while some idea of
the beauty and variety met with in the group may be obtained from Fig.
37.
[Illustration: FIG. 38.--Diagram of Ctenophore.
_f_, Tentacle; _fs_, tentacle sac; _t_, central cavity; _tg_,
upper canal; _rud_, plate bearing cilia; _g_, radial canal; _r_,
longitudinal canal; _si_, sense organ.]
There is another group of jellyfish-like marine animals which have been
given the name of Ctenophora. By some they are regarded as a divergent
sub-class of the Cœlenterates, by others as a distinct main group;
in any case they appear to be important from our point of view. The
structure of a typical member is shown in Fig. 38, and a few other
forms are illustrated in Fig. 39. Our typical example is pear-shaped,
with the mouth at the lower pole. The internal cavity is complex, but
is on a different plan from that of the Medusa. There is a central
cavity communicating with the outside not only by the mouth but also by
two canals opening near the upper pole. There are two radial canals,
each of which divides into four, the branches of which lead at right
angles into other canals, running from pole to pole and blind at both
ends. There are two tentacles, as shown, which can be withdrawn into
special sacs. At the opposite end from the mouth are sense organs,
seemingly of smell and balance respectively. On the outer surface,
above each of the longitudinal canals, is a row of small plates bearing
cilia. It is by the movement of these cilia, like a multitude of minute
oars, that the animal swims--a method of locomotion which does not
occur in the true Cœlenterates. An additional feature is the formation,
at an early stage of development, of a definite third layer of cells
between the outer and the inner. This layer ultimately forms the
greater part of the jelly-like mass of the body.
[Illustration: FIG. 39.--Group of Ctenophora.]
Regarding the interrelationships of the various types that we have
described, and their respective importance with reference to the
descent of man, opinions are somewhat divided. Some believe the
Ctenophora to have been derived from the Medusa form, but the more
probable view seems to be that they have evolved separately from
some earlier and more primitive type than any existing Cœlenterate,
and that their ancestors have all been free-swimming and ciliated.
Now the Ctenophora are considered, on good grounds, to be somewhat
nearly akin to the lowest worms, and thus to stand fairly close to
the main line of Evolution. If this view be correct, the whole group
of existing Cœlenterates forms a side branch of the Evolution tree.
This fact, however, does not take away the importance of the group in
relation to the theory of the descent of the higher animals, for the
Cœlenterates have certainly retained many of the characters which were
possessed by the direct ancestors of man, such, for instance, as the
simple digestive cavity, the primitive type of body, consisting of
two cell layers, the diffuse and elementary nervous system, and the
radial arrangement of parts. Moreover, the course of Evolution in the
group, leading from the Hydra to the sea-anemone and the Medusa, has
probably been in many respects parallel to that which started from some
primitive extinct form, and led up to the Ctenophora. Therefore the
study of the group has thrown much light on the earlier history of the
animal world. Regarding the age of the group, it may be mentioned that
fossil corals, etc., are found, along with Crustaceans and Molluscs, in
the earliest known fossil-bearing beds, belonging to the Cambrian age.
CHAPTER III
THE WORMS AND SOME OF THEIR POSTERITY
The somewhat miscellaneous collection of animals that have been thrown
together and termed _worms_ is of the greatest importance for our
theory of descent. Indeed, it seems probable that all of the four great
groups which we have yet to mention have descended directly from worm
ancestors. This, at all events, is the view of Haeckel, although it
must be admitted that many other theories have been proposed. Nor can
it be taken as a matter for surprise that agreement concerning this
part of our history should be hard to reach, for the difficulties which
are met in it are many and perplexing.
The worms comprise many greatly divergent groups, and the difference
between the lowest and the highest of these has been produced by many
important steps in Evolution. Of these groups but few immediately
concern us; the first and lowest of those which do, is that of
the Turbellarians, a section of the Platodes or flat-worms. The
Turbellarians are small or microscopic tongue-shaped organisms, of
which the majority of species live on the sea-floor, others however
being found in fresh water. The surface of the body is covered
uniformly with cilia, which serve, in the smaller forms, as organs
of propulsion, while in the larger they appear to have the function
of maintaining a flow of fresh water over the surface, and thus of
assisting respiration. In some respects there has been little advance
from the condition of the Cœlenterate. The digestive cavity is a
simple or more or less divided sac, communicating with the exterior
only by means of the mouth. Unlike the condition of affairs in the
Cœlenterates and Ctenophora, however, the sex glands do not discharge
the reproductive bodies into the digestive cavity, but directly to
the exterior by means of a special opening. Each individual has a pair
of male and a pair of female reproductive glands, but the eggs are
not self-fertilised; nor is the fertilisation of the eggs trusted to
chance and the sea-water, as in the lower groups. Instead there is
a definite exchange of sperms between two individuals, and the eggs
are fertilised before they leave the body. They are also frequently
supplied with a store of nutritive material by a pair of special yolk
glands. A distinct step of progress can thus be recognised in the
arrangements for reproduction. Between the outer skin and the inner
digestive layer is developed a considerable mass of cells, forming
muscular and connective tissue, etc. It will readily be understood that
the development of such thick tissue masses occasions two distinct
new difficulties in the animal economy; for where cells are in direct
contact neither with the digestive layer nor with the exterior, their
nutrition and the removal of their waste products can no longer be
efficiently carried on without special devices. Hence on the one
hand a circulatory system, for the transport of food materials,
and on the other an excretory system, become necessary. The first
of these new departures was not destined to be made until the next
stage of progress; the Turbellarians seem to have temporarily got
over the difficulty, like the Ctenophora, by developing a complex
and ramifying digestive cavity. An excretory system, however, makes
its appearance here. Indeed, the beginnings of such a system can be
seen in the Ctenophora, in which there are small excretory organs
opening into the digestive cavity. The corresponding organs in the
worms, as in all subsequent types, open directly to the outside. In
the Turbellarians these organs, which are termed nephridia, are two in
number, and consist of long tubes which branch and ramify throughout
the body, the small branches terminating in special excreting cells,
and the whole constituting a complete and thorough drainage system.
The nervous system consists of one or two small masses of nerve cells
termed ganglia in the front region, with a somewhat complex network
of nerves connecting them with various parts of the body. There are
frequently two pairs of sense organs, probably rudimentary eyes and
ears respectively. The main features of the digestive, reproductive,
excretory, and nervous system are shown in the figures in Fig. 40.
[Illustration: FIG. 40.--A simple Turbellarian--Rhabdocœlum
(diagrammatic).
_m_, Mouth; _d_, digestive cavity; _nc_, nephridia; _au_, eyes; _na_,
sense organs; _g_, brain; _n_, nerves; _h_, male, and _e_, female,
reproductive glands.]
The Turbellarians probably arose from Ctenophora or from some nearly
related form, a view that receives support from the occurrence of
several apparently intermediate types. The differences may, in fact,
be partly accounted for as adaptations to meet the change of habitat
from that of the upper waters to that of the sea floor. A spherical, or
pear, or bell shape is suitable enough for a swimming animal, but would
be impossible for one that was to crawl. The first change, then, we may
imagine, was a flattening, which produced a disc-shaped animal, with
the mouth in the centre of the lower aspect and the sense organs in the
middle of the upper. Secondly, a definite mode of progression, by which
one part of the body continually went first, would be an advantage, as
permitting of a better co-ordination of movements, and an elongation of
the body in the line of movement would have the effect of diminishing
resistance and of making progression easier. Finally the sense organs,
like the scouts of an army, would be best in front, and would migrate
thither, and the mouth, in order to get the full benefit of the food
which the sense organs sought out, would gradually shift to a position
beside them. These adaptions, it is obvious, have produced a complete
change in the architecture of the animal. Our sea-anemone, or Medusa,
or Ctenophore is _radially symmetrical_. That is to say, its parts are
arranged like the spokes of a wheel, and it may be divided into two
equal halves by each of several planes passing through the main axis.
It has an upper and a lower surface, but no head and tail ends. The
lowest of the worms now can be divided into two halves only in one
direction, that which separates the right and left sides. They are, in
scientific language, _bilaterally symmetrical_. The change to this type
of architecture was a very important step of Evolution, particularly
in relation to locomotion. Bilateral symmetry was destined to remain a
constant feature of three of the four great groups that evolved from
the worms. The star-fishes reverted to the earlier condition.
[Illustration: FIG. 41.--A primitive flat-worm--Aphanostomum (× 50).
_a_, Mouth; _g_, sense organ; _i_, internal digestive tissue; _s_,
male, and _o_, female, reproductive glands; with _m_ and _f_, external
openings.]
The next class of worms with which we have to deal is that of the
Rotifera. In their general structure, and in their excretory and
sensory-nervous systems, the Rotifers do not differ essentially from
the Turbellarians. They do differ, however, in that the digestive
cavity has a second opening to the exterior, at the end opposite to
the mouth. The advantage of this arrangement, which was retained
in the subsequent stages of Evolution, is obvious, for it renders
possible a much more regular and thorough digestive process. Instead
of the food passing in, and the undigested remains passing out, by
the same opening, and instead of the contents of the digestive cavity
being a general mixture of food material in all stages of digestion,
there is now a regular stream of food passing through the cavity in
one direction, and being digested as it goes. A near relative of the
Rotifers is shown in Fig. 42.
[Illustration: FIG. 42.--Chænonotus, a lower worm.
_m_, Mouth; _e_, eye; _ss_, sensory hairs; _œ_, œsophagus; _sk_, skin;
_d_, digestive canal; _n_, nephridia; _ex_, excretory opening; _c_,
cilia; _a_, anus; _b_, brain; _mc_, muscle cells; _n_, nerves; _o_,
ovary.]
Thirdly, we must briefly allude to the Nemertines. These are a group of
flattened thread-like worms of very variable size, found both in fresh
and salt water. The most notable advance in this group is to be seen
in the occurrence of a special circulatory system. It has already been
indicated that the gastric cavity of the lower forms has the double
function of digestion and of the transport of nutritive substances to
the various parts of the body. In the Nemertines the second of these
functions is carried out by the blood system, which consists of two or
three vessels that run parallel throughout the length of the body and
anastomose at either end. There is no indication of any enlarged or
specially contractile portion of any of these, no indication, that is
to say, of a heart. The blood conveys not only nutritive substances,
but also, as in the higher animals, oxygen. Some Nemertines have indeed
red blood, containing true hæmoglobin, which is well known as the
oxygen-carrying material in the vertebrates. A typical Nemertine is
shown in Fig. 44, and a diagram showing some features of the anatomy
in Fig. 45. It will be seen that the nervous system is of the same
type as in the worms already described. There are two pairs of sense
organs, one pair being eyes, and the other probably having the function
of gauging the chemical nature of the water. The Nemertines possess
a peculiar organ in a snout or proboscis, which they can protrude or
withdraw into a special sac. The snout is armed with a sharp sting, and
forms an effective weapon whether against the creature's enemies or its
prey.
[Illustration: FIG. 43.--Nephridium of a Turbellarian.]
About this stage of Evolution, the exact point being somewhat difficult
to fix, there appears the body cavity. This, which is altogether
distinct from the digestive cavity, is a familiar feature of the
anatomy of the higher animals. In it are suspended the heart and lungs
and the whole of the digestive organs and glands. The question of the
origin of the body cavity and the blood system is a very difficult one,
and a thorough theoretical discussion would take us too far.
Before proceeding to the question of the origin of the vertebrates,
we may pause briefly to consider the other groups to which the worms
appear to have given rise. First of these we may take the Echinoderms,
which include the well-known star-fishes and sea-urchins, and the
very beautiful feather stars. As already indicated, it is believed
that the radial symmetry, which is so characteristic of this group,
is not a primitive feature, but that, in fact, the Echinoderms are
descended from bilaterally symmetrical ancestors. One reason for
this view is that the larval or immature form is always markedly
bilaterally symmetrical. In an ordinary star-fish, which we may take as
typical of the group, the mouth is in the middle of the lower aspect,
and the excretory opening of the digestive cavity in the upper side
just opposite. There is no blood system, or excretory organs, and no
concentration of nerve cells into any form of brain. Eyes, however,
are present, and sensitiveness to light may be easily demonstrated.
The most remarkable feature of the group is the water-vascular system,
consisting of a series of radial canals, one in each ray, which join a
circular one situated in the central portion of the body. The system of
canals communicates with the exterior by means of a sieve-like plate
on the upper surface, and it is kept full of water by the continual
pumping action of cilia on the walls of the tube which leads down from
the sieve plate.
[Illustration: FIG. 44.--A Nemertine--Tetrastemma.
Actual length about 1-1/2 inch.]
[Illustration: FIG. 45.--Diagram of Nemertine--Nemertopsis.
_cg_ and _sg_, Sense organ; _a_, eyes; _gh_, brain; _bl_, blood
vessels; _n_, nerve cord; _d_, alimentary canal; _ex_, nephridia; _r_,
snout, withdrawn.]
The ordinary star-fish is carnivorous, and lives largely on ordinary
mussels, which it bridges over with its arms, and opens by a steady
and long-continued pulling, the soft parts being then sucked up by the
partially protruded stomach. A few types of Echinoderms are shown in
Figs. 46, 47, 48.
[Illustration: FIG. 46.--Star-fishes.]
[Illustration: FIG. 47.--Feather Star.
Photo: Harold Bastin.]
The group of the Mollusca includes such common forms as cuttle-fishes,
whelks, slugs, snails, mussels, and oysters. These, it will be
observed, comprise marine, freshwater, and land forms. The molluscs,
like the next two groups with which we have to deal, have made a
conquest of the land, though in the present instance it cannot be
regarded as very complete. The anatomy of the group shows much
variation, and only a few of the leading features can be alluded to.
The digestive system is highly developed. The mouth is provided with
a jaw or jaws, and with a tongue-like ribbon, which is covered with
rows of teeth, like a file, and by whose action the food is torn and
disintegrated. A gullet leads from the mouth to a stomach, which is
followed by an intestine. Salivary glands and a large hepatic gland
or liver are present. Respiration occurs partly through the skin, but
special organs also exist for this function, gills in the water forms,
and a lung cavity in those which breathe air. There is a well-developed
blood system, and generally a heart; the blood is pumped direct from
the heart to the general body tissues, and returns to it by way of
the kidneys or nephridia, which purify it of waste materials, and the
respiratory organs, where it is freed of carbon dioxide and supplied
with oxygen. The nervous system varies greatly, but a pair of cerebral
ganglia--a brain--is usually present. There is a particularly keen
sense of smell, and taste and hearing may also easily be shown to
exist. Some forms are blind, from which condition there is a regular
series of stages of development of the eye, up to forms in which it
becomes a highly perfected organ, with cornea, iris, lens, and retina.
The close similarity between this and the ordinary vertebrate eye,
which must have evolved quite separately, is one of the strangest
coincidences of Evolution. Thus in many ways the molluscs are to be
regarded as highly specialised types. But in two important directions,
in intelligence and in their arrangements for locomotion, they stand as
a group on a low plane of development. Figs. 49, 50, and 51 illustrate
some of the forms met with in the group. The origin of the molluscs,
as well as that of the Echinoderms, is wrapped in obscurity. That
each group is derived from some form of worm is probable, yet some
zoologists hold even such a general statement as this to be lacking in
support.
[Illustration: FIG. 48.--Sea Urchin.
1, With spines broken off; 2, with spines on.]
[Illustration: FIG. 49.--Molluscs--Univalves.]
[Illustration: FIG. 50.--Molluscs--Bivalves.]
[Illustration: FIG. 51.--Molluscs--Cuttle-fish, with eggs.]
Our third great group is that of the Arthropods (literally 'jointed
footed'), including the Crustaceans (crabs, lobsters, shrimps, etc.),
spiders and mites, centipedes and insects. The Arthropods are sometimes
classed together with their ancestors, the ringed worms (such as the
common earth-worm), as Articulata, a name which refers to a very
obvious feature, the repetition of similar segments in a regular series
from front to rear. This is perhaps most apparent in the ringed worms
and centipedes, but it is to be seen in all members of the group. This
same tendency to reduplication of parts in a regular series may be
observed in the vertebrates, as we shall see. Slight indications of it
are also to be found in the Nemertines. Numerous theories have been
proposed which derive the vertebrates from some of the Articulata--from
the ringed worms or the Crustaceans, and even from the air-breathing
members; and at first sight such theories seem attractive, for in some
of their more obvious characters there is a certain resemblance between
the two groups. But there are also many and fundamental differences,
and few zoologists have accepted any hypothesis of this type. We may
briefly allude to some of these differences.
[Illustration: FIG. 52.
1, Marine swimming ringed worm; 2, giant centipede; 3, Peripatus.
Photo: Martin Duncan, Berridge, and Bastin.]
In the Arthropods, where the body consists of hard and soft parts, the
'skeleton' is an external one, and encloses the soft parts. Respiration
occurs by means of the skin or of gills, or, in air-breathing forms,
by 'trachea,' which are small branching tubes opening on the sides
of the body. But in no case has the mouth or the digestive tract any
connection with the respiratory system, a condition of affairs very
different from that obtaining in the vertebrates. The nervous system
consists of a brain, situated above the gullet, a nerve ring round
the latter, and a double nerve cord running along the body, _below_
the digestive canal. This is obviously the opposite position to that
occupied by the main nerve cord in the vertebrates, an important point
of difference.
The Arthropods are an extraordinarily successful group. A multitude
of forms of Crustaceans populate the waters, and they are excelled in
numbers and variety only by the insects upon land. While the individual
size appears to be somewhat strictly limited, probably by the nature
of the respiratory and blood systems, many types show exceedingly high
development in various directions--in intelligence, in social and
parental instincts, etc. The insects are of course to be regarded as
the highest Articulata, and have, like the highest vertebrates, the
mammals and birds, almost completely forsaken the water for the dry
land and the air. An interesting member of the Articulata, from the
standpoint of the Evolution theory, is the Peripatus, shown with a
ringed worm on Fig. 52 (3). It gains its interest for us from the fact
that, while classed as an Arthropod, it stands very nearly half-way
between the ringed worms (Fig. 52 (1)) and the true Arthropoda, and
thus forms a solitary link between the two types. In Fig. 52 (2) and
Figs. 53 to 58 are shown a number of types of Arthropods.
[Illustration: FIG. 53.--Arthropods--The Lobster.]
[Illustration: FIG. 54.--Arthropods--Scorpion.
Photo: Leonard Bastin.]
[Illustration: FIG. 55.--Arthropods (insects)--Stag-horn beetle.
Photo: Harold Bastin.]
[Illustration: FIG. 56.--Arthropods (insects)--Dragon-fly.
Photo: Harold Bastin.]
[Illustration: FIG. 57.--Arthropods (insects)--Mantis.
Photo: Harold Bastin.]
[Illustration: FIG. 58.--Arthropods (insects)--Swallow-tail butterfly
and larva.
Photos: Harold Bastin.]
[Illustration: FIG. 59.--Balanoglossus.]
We must now go back and take up the main thread of our story. The next
stage that falls to be described is that of a highly interesting group
of worms known as Enteropneusta, a name signifying 'gut breathers.'
This group contains a very small number of worm forms, which are to
be found burrowing in the sand of the sea floor. A typical example is
the Balanoglossus, a worm of some four inches in length, whose general
appearance is illustrated in Fig. 59. The creature has, as will be
observed, a large muscular snout or proboscis, behind which follows a
small portion called the collar, and behind this again the long body.
The most noteworthy feature of the group, as the name implies, is the
respiratory system. The mouth, which is situated in the region of the
'collar,' leads into a gullet, which is partially divided into an upper
and a lower canal by means of two inwardly projecting longitudinal
folds, one on either side. Only the lower of these canals is used as a
food passage; the upper communicates with the outside by means of
a large number of transverse slits on its sides. Water is continually
being taken in by the mouth and passed along this upper canal, to reach
the outside by way of the gill slits, and in doing so it passes over
the gills, where the blood is circulating in fine capillary vessels.
Here the blood is supplied with oxygen from the water, and is at the
same time relieved of carbonic oxide. This, it will be observed, is
the same method of respiration as that of the fishes. Behind the last
gill slit the digestive canal becomes a simple tube, with two digestive
glands or liver sacs. There are two main blood vessels, the larger
running along above the digestive canal, and the smaller below it, the
two being connected by means of numerous branches. There is a swelling
of the dorsal vessel--a heart--at its forward extremity, in the base
of the proboscis. The nervous system is peculiar; it consists of two
nerve cords, the smaller below the gut and the larger above it--the
latter therefore occupying a position similar to that of the spinal
cord in the vertebrates. Thus in two respects, in its respiratory
and nervous systems, Balanoglossus must be regarded as a highly
extraordinary member of the worm group, and in both its peculiarities
it shows an approach to the vertebrate. There can be little doubt as
to the position of this group as an important connecting link between
the ordinary worms and the vertebrates. Fig. 60 illustrates the main
features of the anatomy.
[Illustration: FIG. 60.--Section of front end of Balanoglossus.
_e_, Snout; _m_, mouth; _h_, heart; _cö_, body cavity; _d_, alimentary
canal; _n_, _n_, nerve cords; _vg_, ventral blood vessel; _dg_, dorsal
blood vessel; _f_, fold dividing the alimentary canal; _vd_, food
canal; _k_, gill slits.]
As to the origin of the Enteropneusta, opinions are somewhat divided.
Their blood system and their development would seem to suggest a
descent from the ringed worms. On the other hand, their possession of
a snout, and their very slight indication of division into segments,
would seem to separate them from the group mentioned and to connect
them rather with the Nemertines. The latter view is perhaps the more
probable.
CHAPTER IV
THE EARLY VERTEBRATES AND THE FISHES
The lowest of the vertebrates--if indeed it can be called a vertebrate
at all, seeing that it has no vertebræ--is the lancelet, Amphioxus.
The common species of this animal (there are some eight in all) occurs
in the sea off our own coasts, and is usually to be found half buried
in the sand or mud of the sea floor. It is some two inches in length
and has the shape of a laterally flattened cigar, and one of its very
obvious features is the arrangement of the muscles in regular layers
from front to back, in the same manner as those of a fish.
To describe some of its features in detail, the alimentary canal bears
a somewhat striking similarity to that of Balanoglossus. There is a
round, simple mouth, unprovided with jaws, and surrounded by a number
of projecting bristles. This leads into a large pharynx, through the
walls of which, on either side, pass a large number of gill slits. The
pharynx is not divided into an upper and a lower canal, but there is a
shallow groove along the bottom which serves the same purpose as the
food canal in Balanoglossus. The remaining, digestive, part of the gut
is practically a simple tube, with a blind sac attached, representing
the liver. The gill slits do not open directly to the exterior, but
into the so-called peribranchial chamber, formed by the junction below
the body of two flap-like outgrowths, one from the upper part of either
side. This chamber opens to the outside by a single pore.
Above the gut lies a straight, cylindrical rod of cartilage, pointed
at either end. This is the highly important structure known as the
_Notochord_, which is present in all the vertebrates, although in the
higher forms it is replaced during development by the vertebræ, the
bony segments of the backbone. Above the notochord again lies the
main nerve cord, a position which it retains throughout the whole
vertebrate group. The nerve cord is simple in structure, with only a
very slight swelling at the front end, representing the brain. There
are two main blood vessels, an upper and a lower, which expand and
contract alternately throughout their whole length, and thus maintain
the circulation. The blood passes forward in the ventral vein, is
pumped through the fine vessels of the gills, and collected into the
upper artery. From this it is distributed throughout the body by branch
vessels, to be re-collected into the ventral vein. If the reader will
refer to the illustrations in Figs. 61, 62, and 63, the relationships
of these parts will be more easily understood. There is a single small
eye-spot, a single organ of smell, but no hearing organs. It seems
probable that this extremely ill-developed condition of the sensory
system is due in some measure to degeneration, and is not a primitive
characteristic. There are numerous pairs of simple nephridia, which
open into the peribranchial chamber, and bear a close resemblance to
those of the worms.
The lancelet forms a most important link between the lower and the
higher animals. It is in all probability derived from some form similar
to Balanoglossus, and it certainly leads up to the round-mouths, which
form the next step in the ladder.
[Illustration: FIG. 61.--The Lancelet--Amphioxus.]
[Illustration: FIG. 62.--Diagrammatic cross-section of Amphioxus.
_n_, Nerve cord; _ch_, notochord; _mus_, muscle tissue; _ec_, skin;
_bl_, blood vessel; _cöl_, body cavity; _kd_, pharynx; _ld_, liver
sac; _g_, reproductive gland; _p_, peribranchial chamber.]
[Illustration: FIG. 63.--Diagrams of Tunicate (on the left), Amphioxus
(centre), and young Lamprey (right).
_o_, Mouth; _au_, eye; _c_, peribranchial chamber; _ch_, notochord;
_d_, alimentary canal; _g_, ear; _hz_, heart; _k_, gills; _lb_, liver;
_m_, nerve cord; _m_, brain; _mg_, stomach; _mt_, mantle; _z_, tunicate
embryos.]
Before describing these latter, however, we must briefly allude to the
highly remarkable group of the tunicates or sea squirts, one of which
is shown in Fig. 64. They are sedentary creatures found attached to
rocks or weeds on the sea floor, and in appearance they remind one
rather of misshapen potatoes than of higher animals. They are in fact
regarded by the fishermen who bring them to the surface as plants, and
they were for long looked upon by zoologists as akin to the molluscs.
The only definite external features of the tunicates are two apertures
at the upper end, one in the centre and one somewhat on one side. The
absence of any other definite external characters is due to the fact
that the creature is enclosed in a mantle of cellulose. The central
opening is the mouth, which leads into a large pharynx, the walls
of which are perforated by numerous gill slits. This is surrounded by
the mantle cavity, which connects with the outer water by means of the
second pore. The gut is continued into a simple stomach and intestine,
the latter bending back upon itself and opening into the bottom of the
mantle cavity, as shown in the diagram in Fig. 63. In the adult animal
there is no trace of the notochord, and only a remnant of the nerve
cord; and there are either no special sense organs or only traces of
these. On the other hand, the tunicates possess a centralised heart.
They are hermaphrodite, and, very curiously, a number of forms multiply
like corals, by a simple process of budding.
[Illustration: FIG. 64.--A Tunicate.
_in.ap._, Opening leading to mouth; _ex.ap._, opening of peribranchial
chamber.
From Dendy's _Outlines of Evolutionary Biology_ (Constable).]
[Illustration: FIG. 65.--Larva of Tunicate.
_n_, Nerve cord; _s_, sense organs; _m_, mouth; _kd_, pharynx; _vd_,
alimentary canal; _ch_, notochord.]
Now the remarkable fact has been made out that the young tunicate (see
Fig. 65) bears a most striking resemblance to the immature lancelet. It
is a free-swimming, tadpole-like creature, and possesses a notochord
and nerve cord and in general the same characters that we described
for Amphioxus. It is only later that the creature settles down and
assumes its final degenerate sedentary form. There can be no doubt that
the tunicates have been derived from some lancelet-like form, but the
course of their evolution has been unique. The type is the lost brother
of the vertebrate family, who has chosen a distinctly downward path in
life; yet who has come to no miserable end, but lives on, more or less
successfully, in his lower social sphere.
The round-mouths, including the lampreys and the hag-fish, stand midway
between the lancelet and the fishes, and therefore constitute for us an
important group.
The lamprey is a fairly generally known eel-like creature, of which
there is a smaller fresh-water, and a larger salt-water species, the
latter reaching a length of about a yard. It is found attached to, and
feeding on, the dead bodies of fish, and less frequently on living
specimens. The hag is much more definitely parasitic in its habits,
and often occurs in the body cavity of living fish. These forms were
for long regarded as fishes, and are sometimes even yet included
in that group, but all their characteristics point to a very much
lower position in the animal world than such a classification would
indicate. One of the most striking external differences is that the
round-mouths have nothing to represent the two pairs of fins which
occur uniformly in the fishes, and which are, in a true sense, the
forebears of our own arms and legs. Even more important than this is
the absence of jaws. The mouth in this group is a simple round opening,
whose edges are armed with pointed teeth, the latter, however, bearing
no real resemblance to the teeth of the higher animals. By means of
these teeth, and a pointed, tongue-like organ, and by suction, the
round-mouths are able to bore into the tissues of the animals on which
they prey. The absence of jaws and of extremities is, of course, a
feature which they share with the lancelet. Turning to the internal
structure, we may first observe that there is still no vertebral
column, but only a simple, rod-like notochord, similar in its shape,
and in its relations to other parts, to that of Amphioxus. There
is, however, an additional development of cartilage in the region
of the head, forming, in particular, a sheath-like covering for the
brain and also a kind of basket-work support for the pharynx and
gills. The original tube-like form of the dorsal nerve cord is easily
recognisable, but it is distinctly distended at its front end into a
brain, which shows a division into a series of three distinct portions,
called respectively the fore, mid, and hind brain. This division, it
is interesting to observe, is the first process in the development of
the brain in all the higher animals. There is a pair of well-developed
hearing organs, and in the lamprey a pair of similarly well-developed
eyes. In the hag fish the latter are greatly reduced, a condition which
is explained by the creature's mode of life. The nostril is unpaired,
a condition which is probably primitive. Respiration is carried out by
means of gills, which are situated in a series of six to eight pouches,
each of which opens into the gullet and again directly to the outside,
the external openings being an obvious feature of the animal. There
is a very distinct, simple heart, which pumps the blood to the gills,
from whence it is collected and distributed throughout the body. The
digestive canal is a simple tube, provided, however, with a liver and
a pancreas, the two most important digestive glands in the higher
animals. The sexes are separate, but traces of a previous hermaphrodite
condition seem to persist. Henceforward in the vertebrate group the
sexes are always separate. A character of the sex organs which is to be
regarded as primitive is that they are unconnected with the excretory
system, whereas in the higher vertebrates the two systems are always
strangely interconnected. As in all the higher types, there is but one
pair of male or female reproductive glands. Finally, the round-mouths
differ markedly from the lancelet in the structure of the skin. In the
latter animal the skin is composed of a single layer of cells. In the
former it consists of an epidermis, some three or four layers thick,
and an underlying cuticle or 'true skin'; in other words, the skin has
the same general structure as that of the higher types. The lamprey
and the hag are illustrated in Figs. 66 and 67, and some of the main
anatomical features of the group are shown in Figs. 63, 68, and 69.
[Illustration: FIG. 66.--The Lamprey--Petromyzon.]
[Illustration: FIG. 67.--The Hag-fish--Myxine.]
There remain, even after the most thorough investigation of
Balanoglossus, the lancelet, and the round-mouths, some questions with
regard to the origin of the vertebrates that are still unanswered. It
must, however, be regarded as an extremely fortunate circumstance that
representatives have come down to us of the three ancient groups to
which these three types respectively belong. This is the more fortunate
in that the groups in question are known only from their few living
members, a circumstance which is of course easily accounted for by the
absence of any hard parts capable of being preserved as fossils. From
this point onwards there is a skeleton, and we are consequently enabled
to draw valuable information from fossils. Partly in consequence of
this, the Evolution chain from this point onwards is much more complete
than the portion that we have dealt with thus far.
[Illustration: FIG. 68.--Diagrammatic cross-section of lamprey larva.
_n_, Nerve cord; _ch_, notochord; _ar_, artery; _v_, vein; _g_,
reproductive body; _d_, alimentary canal.]
[Illustration: FIG. 69.--Mouth of lamprey.]
We have already observed that the true fishes, to which we must
now direct our attention, differ from the round-mouths in several
important characters. They possess two pairs of extremities, the
pectoral and pelvic fins; they have a pair of nostrils; there is also
a well-developed skull, which includes a series of cartilaginous or
bony arches situated in the wall of the gut and between the successive
gill clefts. These branchial arches bear a certain resemblance to
the basket-work arrangement of cartilage in the round-mouths, but
for various reasons are not regarded as having been derived from the
latter. It is from the first pair of these arches that the jaws are
formed, organs which make their first appearance in the lower fishes.
The skeleton shows great development in other directions. The notochord
is present in its primitive condition during the earlier stages of
development, but it becomes surrounded, and in many cases largely
suppressed, by the portions of the vertebræ. Each vertebra consists
of an upper and a lower portion, the upper forming an arch round the
nerve cord and the lower bearing lateral processes or ribs. In the
higher forms the two portions become united round the notochord, and
the resulting vertebra may encroach inwards until it becomes solid, the
notochord then remaining only as a series of small pieces of cartilage
between the successive units of the vertebral column. There is also,
of course, a skeleton in connection with the limbs, but this does not
yet correspond in detail to that of the other classes of vertebrates.
The brain is much more highly developed than in the round-mouths; in
many forms, particularly, there is a considerable development of the
cerebral hemispheres, a portion of the fore brain, and the seat of the
higher intelligence. The eyes and ears show the same main features as
in the higher groups. The ear has three semicircular canals, the same
number as in man, as against two in the lamprey and one in the hag.
Fishes are possessed of a peculiar 'sixth sense,' the organs for which
are situated in two lines running along the sides of the body, the
latter forming a familiar feature of a cod or whiting. The nature of
this sense is not definitely known, but it appears to be of the nature
of a very refined appreciation of wave motions in the water. It is
probably by means of these 'lateral line' sense organs, for instance,
that fishes are so easily able to avoid obstacles when swimming in the
dark.
[Illustration: FIG. 70.--Diagrammatic dissection of dog-fish (Scyllium).
_sk_, Skull; _gh_, brain; _n_, nerve cord; _ch_, notochord; _ho_,
reproductive gland; _un_, kidney; _wk_, vertebræ; _k_, jaw; _z_,
tongue; _ks_, gill slits; _ar_, arteries; _ph_, pharynx; _h_, heart;
_lb_, liver; _m_, stomach; _mil_, spleen; _pan_, pancreas; _vd_,
intestine, with spiral fold; _cöl_, body cavity; _r_, rectum.]
The heart has one auricle and one ventricle, except in a single group
which we shall afterwards mention. The heart is situated immediately
behind the gills, to which the blood is pumped directly by the
ventricle. From the gills, the blood is collected and distributed
throughout the body, is re-collected and returned to the auricle. The
circulatory system is provided with a set of blood-glands, essentially
similar to those in man himself. There is a spleen, a thymus and a
thyroid gland, and a pair of suprarenal bodies. The several functions
of these glands form an extremely difficult chapter of physiology,
but, broadly speaking, they are concerned in the formation of the
white blood corpuscles, the removal of worn-out red corpuscles, and in
certain obscure but important chemical changes in the composition of
the blood. The blood itself consists of a fluid plasma in which float
white and red blood corpuscles, the latter being flat and oval, and
containing the same oxygen-carrying substance, hæmoglobin, as is found
in mammals.
The alimentary canal is simple. The mouth cavity is succeeded by the
pharynx, the walls of which are perforated by the gill clefts. Next
follow the gullet, the stomach, and the intestine, the division into
the three portions being apparent often only after close examination.
There are generally gastric glands, of simple form, a large liver, and
almost always a pancreas. The kidneys and the reproductive organs open
to the exterior by a common duct. A further characteristic feature of
the fishes is their external covering of scales. True teeth, comparable
to those of the higher vertebrates, appear first in this group. Some of
the main features that we have mentioned are illustrated in Fig. 70.
Careful study of the fishes makes it evident that they have very much
in common with the higher groups of vertebrates. It is not too much to
say, with Haeckel, that there is far more difference between Amphioxus
and the fishes than between the fishes and man.
There are four main divisions of the fish group. The first, that of the
Elasmobranchs, comprises the sharks and dog-fishes, the skates and the
rays. The second group, the Ganoids, includes the sturgeon and a few
less well-known forms. The third, the so-called bony or food fishes,
includes the vast majority of ordinary species, such as the salmon
and trout, the cod, herring, eel, and all our ordinary freshwater
species. The fourth, the 'lung fishes,' consists of three very
remarkable species, which we shall later describe in detail. The mutual
relationships of these groups is well understood, and it is possible to
make fairly definite statements regarding their evolution.
The Elasmobranchs are at once the most primitive and, so far as is
known, the oldest of the four. From these evolved the lower Ganoids,
which then divided into two main branches, the first of which led up to
the higher Ganoids and through them, at a comparatively late date, to
the bony fishes. The second led to the lung fishes and, either through
them or along a somewhat parallel line, to the amphibians and the land
vertebrates generally. It is with the second line, therefore, that we
shall be mainly concerned.
The Elasmobranchs are characterised by the fact that the gill slits
open individually to the exterior, there being no gill cover, such
as is found in the other groups. Their scales are simple, tooth-like
projections, and in fact there is no essential difference between them
and the teeth. The skull is more primitive than in the other groups,
but a discussion of its details would necessarily be very involved. The
living members of the group show a fairly high stage of development
of the vertebræ--considerably higher, in fact, than that found in the
lung fishes--but some extinct members showed a very primitive condition
with regard to this point. In the fossil skeleton shown in Fig. 71, for
instance, it is apparent that the notochord was present as a simple
continuous rod. The skeleton in question is from the Permian and
belongs to what is regarded as the most primitive type of fish known.
Two specimens of Elasmobranchs are shown in Figs. 72 and 73, and the
teeth of a shark in Fig. 74.
[Illustration:
FIG. 71.--Fossil skeleton of _Pleuracanthus Decheri_, a primitive
Elasmobranch.]
[Illustration:
Photo: Thiele.
FIG. 72.--Spotted dog-fish.]
[Illustration:
Photo: Thiele.
FIG. 73.--Blue Shark.]
[Illustration: FIG. 74.--Teeth of Shark.]
In the Ganoids and bony fishes there is a gill cover, and in all
but a few Ganoids there is some formation of true bone, whereas in
the Elasmobranchs the skeleton is wholly cartilaginous. One of the
most striking anatomical features of these groups, and one which
distinguishes them from the Elasmobranchs, is the presence of a swim
bladder, a large sac-like outgrowth from the upper part of the gut. The
function of the swim bladder is that of regulating the specific gravity
of the fish, which becomes greater or less according as air is expelled
or taken in. The Ganoids and bony fishes are illustrated in Figs. 75 to
79.
[Illustration:
Photo: Underwood.
FIG. 75.--Ganoid fishes--The Sturgeon.]
The lung fishes or Dipnoi present a curious mixture of primitive and
of highly advanced characters. In their persistent notochord and their
inconsiderable formation of bone, they are much more primitive than
the food fishes. On the other hand, an extremely important departure
is seen in the adaption of the swim bladder as a respiratory organ. In
one of the three existing species this organ is single, in the others
it is double. The wall of the swim bladder is thick, and contains
considerable muscle tissue. Its inner surface is covered with a complex
system of pits and blind sacs, the walls of which contain numerous
capillary vessels.
[Illustration: FIG. 76.--Ganoid fishes--_Polypterus bihir_.]
[Illustration: FIG. 77.--Ganoid fishes--American bony pike,
Lepidosteus.]
[Illustration:
Photo: Thiele.
FIG. 78.--Bony- or food-fishes--The Salmon.]
[Illustration: FIG. 79.--Bony- or food-fishes--The Common Eel.
Photo: Underwood.]
[Illustration: FIG. 80.--Lung fishes--Australian lung fish, Ceratodus.]
[Illustration: FIG. 81.--Lung fishes--Protopterus (Africa).]
[Illustration: FIG. 82.--Lung fishes--Lepidosiren (South America).]
[Illustration:
FIG. 83.--Swim bladder (lung) of Protopterus.]
There are three living species of lung fishes, one of which is found in
Australia, another in Tropical Africa, and a third in the tributaries
of the Amazon. All live under conditions which make ordinary
respiration by gills difficult. The Australian species inhabits
rivers which become reduced, in the dry season, to stagnant pools of
foul water, in which ordinary fish frequently fail to survive. Under
such circumstances the creature comes periodically to the surface to
breathe. The other two species live in rivers which actually dry up
in summer, and the fishes bury themselves in the mud, and remain
in a torpid condition, breathing air by their lungs, until the rainy
season comes round, perhaps four or even six months later. Correlated
with the special method of respiration is a special type of blood
system, whereby part of the blood is pumped direct to the lungs, and
returns direct to the heart. There are two auricles, to receive the
blood from the lungs and the general circulation respectively, but only
one ventricle, in which the two streams become mixed. Figs. 80 to 82
illustrate the three existing Dipnoi, and the structure of the lung is
shown in Fig. 83.
It is obvious, from the distribution of the lung fishes, and also
from geological evidence, that the group was once very plentifully
represented, and has only been preserved from total extinction by
peculiar circumstances.
Regarding the position of the group, some zoologists regard them as the
direct ancestors of the Amphibians. Others believe that the group had a
common origin with the bony fishes and the Amphibia in some early form
of Ganoid. In any case, the Dipnoi possess an extraordinary interest as
showing the beginnings of adaption to a life out of the water.
CHAPTER V
THE CONQUEST OF THE LAND
The Amphibia are the oldest and the lowest group of vertebrates that
are able to lead an active existence on land, and the characters
which distinguish them most definitely from the fishes are all to be
interpreted as adaptions to the new mode of life. One of the most
obvious external differences between the two groups is in the structure
of the extremities, the fish having fins, while the amphibian has limbs
constructed on the same general lines as our own arms and legs. The
fish's fin is to be regarded as an extremity with a very great number
of fingers or toes. It has the function of a paddle, and is obviously
useless whether for supporting or propelling the body on land. The
first obvious necessity for a land existence is some mechanism by
which the limb can be alternately pushed forward and, being fixed to
some solid object, drawn upon, so as to pull the body after it. A
different arrangement of bones and muscles, so as to give a much more
complex lever system than that of a fin, and some kind of clawing
arrangement at the end, were thus necessary. The similarity in the
limbs of all the land vertebrates is very striking, as is indicated by
the comparison of human and a frog's limbs on Fig. 84. In each case
there is a single bone in the upper arm or thigh, which is attached
to a bony girdle in the trunk. There are two elements in the forearm
and in the lower leg respectively, below which, in either case, is
a group of small bones constituting a complex joint at the wrist or
ankle. Then follows the set of five bones in the foot or hand, to each
of which is attached a jointed finger or toe. We have no reason to
believe that this particular arrangement, and the particular number
of digits, was arrived at except by accident. Once arrived at,
however, the arrangement was adhered to with considerable strictness.
For although the number of digits is in some groups--in the birds
especially--reduced, the primary design is almost always readily
recognisable. The second function of the limb, that of supporting
the body, was developed very slowly. In the amphibians and reptiles,
and even in the lower mammals, the legs are comparatively weak and
sprawling, and the creature crawls on the belly.
[Illustration: Left hind leg of frog.
Left leg of man.
Left arm of man.
Left arm of frog.
FIG. 84.]
[Illustration: FIG. 85.--Development of the Frog.]
The second great change which required to be made was of course in
the method of breathing. An ordinary fish, when taken out of the
water, dies of suffocation, because its gills become inefficient for
respiration as soon as they become dry. An entirely new type of organ
had therefore to be evolved, and this occurred on the same lines as
in the Dipnoi, by the development of a pair of sacs from the upper
part of the digestive canal, in which the blood is made to circulate,
and which are kept filled with air taken direct from the atmosphere.
It is of course very well known that an ordinary amphibian is not
a lung breather throughout its whole life. The metamorphosis of a
gill-breathing tadpole into a lung-breathing frog, illustrated in
Fig. 85, is a phenomenon with which everyone is familiar. And this
condition, in which a change in the mode of life is made by each
individual in the course of its development, is the typical one. But
the modern amphibians include types ranging from completely water to
perfect land forms. Some, like the Austrian Olm (Fig. 86), are gill
breathers throughout their whole life. One which is normally of this
type, the Axolotyl, illustrated in Fig. 87, can be made to acquire
lungs and assume a land mode of life. Others, which normally make
the metamorphosis, can be prevented from doing so by being confined
to the water, and complete their life-histories in the condition of
gill breathers. In still other forms (_e.g._ the Cœcilians, Fig. 89)
the change is made before the young creature leaves the egg, and the
independent life is commenced in the condition of a land animal.
Correlated with the development of the lungs is a change in the
structure of the nostrils, from the condition of blind sacs, as they
occur in the fishes, to that of air passages, communicating with the
upper part of the alimentary canal, and thence with the lungs.
[Illustration: FIG. 86.--The Olm--_Proteus anguincus_.]
[Illustration: FIG. 87.--Axolotyl. The ordinary gill-breathing form,
and the artificially developed land form.]
[Illustration: FIG. 88.--Amphibians--The Fire Salamander.]
The living forms of the Amphibia differ considerably from the types
which constituted the group in those long-distant ages when it was
in the heyday of its prosperity. The latter forms were characterised
especially by a system of armour-plating over the head, which
frequently extended under the breast and even covered the greater part
of the lower surface, and which appears to have formed a protection
against the multitude of sharks which populated the waters in which the
amphibians partly lived. The armour-plated type has long ago become
extinct, but it is in it, rather than in the modern forms of Amphibia,
that we must look for the direct ancestor of man. A fossil of this
earlier group is shown on Fig. 90.
[Illustration: FIG. 89.--The Ceylon Cœcilian, _Ichthyophis glutinosa_,
with eggs.]
[Illustration: FIG. 90.--Fossil amphibian, _Brachiosaurus_, from the
Permian.]
Our next group in the order of Evolution is that of the reptiles, the
main differences between which and the Amphibia are of the nature of
more complete adaptions to a life on land. The reptiles have in fact
completed the conquest of the land which was undertaken by the previous
group, and many of them, living as they do in dry and hot deserts, are
as independent of the water as any form of animal life. Thus whereas
the amphibian has a thin skin, which is kept moist by the secretions of
numerous skin glands, the reptile has a body-covering of scales, which
form an effective protection against a too rapid loss of moisture.
Evidently with the same object, the reptile egg is enclosed in a hard
and resistant shell. Correlated with the change in the skin is a
much more perfect development of the lungs, for while the amphibian
breathes to a considerable extent through its thin moist skin, this
method of assisting respiration is not available to the reptile.
Again connected with the improvement of the respiratory process,
there is a partial development of a septum dividing the ventricle or
pumping chamber of the heart. The value of this division of course
lies in the fact that the purified and oxygenated blood from the lungs
is prevented from mixing with the venous blood from the body. The
course of the blood is from the body to the right auricle, thence to
the right ventricle, and thence to the lungs. The pure blood from the
lungs returns to the left auricle, passes thence to the ventricle on
the same side to be pumped to the general circulation. The disadvantage
of a single ventricle, such as occurs in the Amphibia, and the
advantage of the regular double circulation, such as that in man, are
sufficiently obvious. The division of the ventricle into two chambers
is less complete in the lizards and snakes, very nearly perfect in the
crocodiles.
The reptiles, like the Amphibia, are 'cold blooded,' by which is meant,
not that their blood is necessarily cold, but that its temperature
varies with that of the surroundings, while that of the blood of the
mammals and birds is practically constant.
A very important feature of the reptiles, which they possess in common
with the mammals and the birds, is that the embryo produces two
membranous outgrowths called respectively the amnion and the allantois,
which completely envelop it, and which have important functions in
connection with nutrition, respiration, and excretion during the period
when the young creature is enclosed in the egg. It is, of course, not
until we reach the higher mammals that these membranes assume their
greatest importance.
[Illustration: FIG. 91.--Imaginative European landscape in the
Cretaceous period, with reconstructions of typical reptiles.
Plesiosaurus (swimming, up to 50 ft. long).
Three Ichthyosaurians.
Pterodactyls (flying).
Iguanodon.
Megalosaurus (40 ft. long).
Rhamphorhynchus.]
For a considerable time in the world's history the reptiles were the
dominant vertebrate class, and in the chalk period especially they were
represented by a great variety of forms, and by a number of species of
colossal stature, one at least of which was over a hundred feet long.
In those times the reptiles were by no means all condemned to crawl on
their bellies, for they included a large number of marine forms,
comparable to the porpoises and whales among the mammals, and flying
forms whose aspect must have resembled, and been equally terrifying
with, that of our mythical dragons. A few reconstructions of these are
shown in Fig. 91.
[Illustration:
Photo: Berridge.
FIG. 92.--Reptiles--A Chameleon.]
[Illustration:
Photo: Berridge.
FIG. 93.--Reptiles--Indian Python.]
[Illustration: FIG. 94.--Reptiles--Turtle and Tortoises.]
[Illustration: FIG. 95.--Reptiles--Alligator.]
All living reptiles, with a single exception, belong to the four
comparatively modern types of the lizards, snakes, tortoises, and
crocodiles, of which examples are illustrated in Figs. 92 to 95. None
of these are closely related to the mammals or birds. For the common
ancestor of all these types we must go back to some primitive reptile
form. Fortunately such a type is represented at the present day in a
single peculiar species found in New Zealand, which bears the Maori
name of the Tuatara. It was formerly found commonly on the mainland,
but is now confined to a few small islands in the Bay of Plenty,
North Island, where it enjoys Government protection. It is, as the
illustration in Fig. 96 shows, a lizard-like creature, and reaches
a length of about two feet. It lives in burrows near the shore, and
feeds on small animals that are left behind by the tide. The Sphenodon,
as zoologists have named it, has apparently been preserved owing
to the absence of competition by the mammals, and by adopting the
rather curious mode of life just described. In all its features, but
especially in the primitive condition of its vertebræ, it is very much
lower than any other living reptile, and it connects the higher groups
with the Amphibia. Many closely related fossil species are known, one
of which is shown in Fig. 97.
[Illustration: FIG. 96.--The Tuatara, _Sphenodon punctatus_.]
[Illustration: FIG. 97.--_Homeosaurus pulchellus._ A fossil early
reptile from the Jurassic.]
To the lay mind the distinctions between the Amphibia and the reptiles
are not very obvious, and indeed in the older classifications the
former group was not separated from the latter. The differences between
a reptile and a bird, on the other hand, are very striking. It might
therefore be regarded as a matter for surprise that zoologists now make
the greater distinction between the Amphibia and the reptiles, grouping
the former in one great class with the fishes, the latter in a second
great section with the birds. But in fact there are many fundamental
points of agreement between reptiles and birds, and it is impossible
to doubt that the latter have sprung from a reptilian stock. Indeed, a
most interesting connecting link is known, in the fossil Archiopteryx
shown in Fig. 98, of which only two specimens have been found, and
which is the only creature of its type of which we have any record. In
all its skeletal features, the Archiopteryx is reptilian, and it would
undoubtedly have been classed as a new type of reptile but for the
obvious and unmistakable traces of feathers. From what particular class
of reptiles the birds have sprung is not known.
The birds have assumed the position of almost unquestioned masters
of the air, but like other great groups they show possibilities of
evolution in other directions wherever opportunity offers, and types
like the kiwi and the penguin shown in Figs. 100 and 101 have forsaken
their native element--the one for the land, the other for the water.
The birds agree with the mammals in the development of a four-chambered
heart, in their warm blood, in their external covering for the skin,
and in the development of arrangements and instincts for the parental
care of the young. Their line of Evolution has thus been to some
extent parallel to that of the mammals. On the other hand, they differ
obviously in the structure and function of their fore limbs, in the
absence of a diaphragm, and in their special methods for the care of
the young, and there can be no doubt that the two groups have had quite
different origins in the reptile class.
[Illustration: FIG. 98.--Archiopteryx, a fossil lizard-like bird.
Photo: Underwood.]
[Illustration: FIG. 99.--Eagle, with prey.]
[Illustration: FIG. 100.--The Kiwi, Apteryx. A practically wingless
running bird.
Photo: W. P. Dando.]
[Illustration: FIG. 101.--The King Penguin. A highly specialised diving
bird.
Photo: W. P. Dando.]
CHAPTER VI
THE MAMMALS AND MAN
The subject of our discussion is now narrowed down to the group of the
mammals. The mammals are characterised by two very obvious features:
a body-covering of hair, and a set of special glands in the female
which secrete milk for the nourishment of the young. These are constant
characters, and neither is ever found in any other group. As to how
the hair originated, nothing definite is known; but (while there are
certain difficulties in regard to the theory) it is on the whole
reasonable to suppose that the mammalian hair arose, as the bird's
feather undoubtedly did, as a modification of the reptile's scale. The
mammary gland appears to represent a modification of other skin glands,
either of sweat glands or more probably of the oil glands which exist
in connection with the hairs.
Another important character, already mentioned at the end of the last
chapter, is the diaphragm, a muscular partition separating the body
cavity into a thoracic and an abdominal portion. The diaphragm has
important functions in connection with the mechanism of breathing. By
means of it the thoracic cavity can be increased or diminished in size,
and air thus drawn into or expelled from the lungs. It is interesting
to observe that a similar partition, with the same function, occurs
in the Crocodiles, but this has different relations to the abdominal
organs, and has evidently evolved quite independently.
Very characteristic of the mammals are, further, their teeth, for
whereas the teeth of the reptiles are indefinite in number, and
generally very numerous, those of the mammal are relatively few, and
each species has a definite normal number. Moreover, the teeth of
the reptile are all of a kind, and they may be replaced almost any
number of times during the animal's life, whereas those of the mammal
show differentiation according to their respective functions, and are
only once changed, that is, when the milk teeth are replaced by the
permanent set. The teeth of the mammal are of four kinds: incisors, or
chisel-like cutting teeth, canines, which are especially well developed
in carnivorous species, and which are used in the tearing up of flesh,
etc., and two groups of grinders--premolars, which are replaced during
the animal's life, and molars, which occur only as permanent teeth.
[Illustration: FIG. 102.--_Pareiosaurus Baini._ Fossil skeleton from
the Permian of South Africa.]
[Illustration: FIG. 103.--Skull of Galesaurus, from the Permian of
South Africa.
_A_, From side; _B_, from below; _C_, from above; _D_, back tooth.]
[Illustration: FIG. 104.--Skull of Tritylodon from below.]
The mammals in all probability arose from a particular group of
reptiles which flourished in the Permian and Triassic periods, and
which disappeared very shortly afterwards. These pass under the name of
the Theromorpha, and a typical specimen, Pareiosaurus, is illustrated
in Fig. 102. The skull of another, illustrated in Fig. 103, shows
distinct indications of a mammal-like differentiation of the teeth. The
Tritylodon, whose skull is shown in Fig. 104, is an intermediate type
between this group and the mammals, some zoologists regarding it, as it
were, as the last reptile, others as the first of the mammals.
As to how the special mammalian features arose, or what special
conditions called them into existence, we are of course without
definite knowledge, for neither hair nor mammary glands are
recognisable in fossils. But it seems likely that the warm blood and
the hairy covering evolved in correlation with one another, and as
adaptions to meet a gradual cooling of the climate. It is certain at
all events that the present-day mammals (and also the birds) are far
better adapted for a cold environment than the reptiles, which very
easily get frozen to death; and it is also known that ice periods
occurred in South Africa, where many fossil Theromorpha and the
Tritylodon are found, at the time when these creatures existed; both of
which facts support the theory indicated.
At the present time the mammals are the highest and on the whole the
most successful of the vertebrate groups. They include the largest
and strongest, the swiftest-footed and the most intelligent of the
animal kind. They show refinements of the senses of sight, hearing, and
smell such as are met with nowhere else. They range from the Equator to
the coldest regions of the earth in which any food is to be found; they
people alike the forest and the plain, and have their representatives
both in the air and in the sea.
[Illustration: FIG. 105.--The Australian Duck-mole or Duck-billed
Platypus.]
The most lowly of the mammals are the Monotremes, which include the
well-known Australian duck-mole or duck-billed platypus, and two
species of spiny ant-eaters, one of which is found in Australia, the
other in New Guinea. The two types are shown in Figs. 105 to 107.
The best-known and most striking fact concerning these is that, like
the birds and reptiles, and unlike all other mammals, they lay eggs.
Beyond this feature they show many affinities with the reptiles, in
their skeleton for example, and particularly in their reproductive
organs. Another interesting fact is that the blood temperature, of
the ant-eater at least, is low, and varies considerably. It has been
found to range between 80 and 93 degrees Fahrenheit, whereas all other
mammals, so far as is known, have a blood temperature between 97 and
105, and moreover one that is remarkably constant for each species
during normal health. The brain development and the intelligence are
also much lower than in other mammals, though distinctly superior to
those met with among the reptiles. The method of rearing the young in
the case of the ant-eater (that of the duck-mole not being yet fully
known) is that the single egg is placed, as soon as it is laid, in the
pouch under the belly of the female. Here it hatches in a very short
time, and here the young animal remains for the first two or three
months of its life, being nourished by milk produced by the mammary
glands, which open into the pouch. It is certainly owing to the absence
of the competition of the higher mammals in the regions where they are
found, that these two creatures, so interesting from the standpoint of
Evolution, have been preserved to us.
[Illustration: FIG. 106.--New Guinea Spiny Ant-eater.]
[Illustration: FIG. 107.--Australian Spiny Ant-eater.]
The next group, the Marsupials, is the lowest in which we get the true
mammalian characteristic of the bearing of living young. For while
occasional members of other groups, of the reptiles especially, produce
live young, the actual state of affairs is fundamentally different. In
these latter the egg is merely retained in the genital duct until the
young creature emerges. It is merely hatched inside the body of the
mother instead of outside. But in the Marsupials the developing young
receive nourishment from the mother, during prenatal life, other than
what is contained in the yolk. This nourishment is obtained in the form
of a secretion from the wall of the uterus, there being as yet (with a
single partial exception) no real connection between mother and young.
The peculiar method of nourishing and protecting the young Marsupial
after birth is of course well known. The young are born in a very
immature and helpless condition, and are placed by the mother in her
pouch. The mouth becomes permanently attached to the nipple of the dam,
and the young creature remains thus for a considerable time, the milk
being pumped into it by the mammary gland rather than sucked in by the
efforts of the creature itself.
The Marsupials are further characterised by the possession of an extra
pair of bones in the pelvis, which function as supports for the pouch;
by the peculiar and primitive arrangement of the reproductive organs;
by their still poorly developed brain; and by their generally large
number of teeth, which reach a total of fifty or fifty-two in some
species, whereas forty-four is the ordinary maximum number in the main
mammal group. This last is to be regarded as a character derived from
the reptiles.
[Illustration: FIG. 108.--Kangaroo, with young.]
[Illustration: FIG. 109.--Newborn young of Kangaroo.]
It is very interesting to observe how, in Australia, where the
Marsupials have been free from the competition of other mammals,
they have evolved along many of the same general lines as the higher
group. We have indeed no marsupial whales, bats, or seals, but there
is a mole, very similar in its appearance and habits to our European
species, and a carnivorous type which closely resembles a wolf or
jackal; again we have bandicoots, occupying the same place in nature
as our rabbits and other rodents; tree-dwelling, squirrel-like forms;
and kangaroos, which compare in their mode of life, if not in their
appearance, with the cattle, deer, and antelopes of other countries.
The Marsupials are illustrated in Figs. 108 to 113. This group, so far
as can be judged from fossils, is considerably older than that of
the higher mammals, and everything points to some marsupial type as
a connecting link between the egg-laying Monotremes and the placental
mammals.
[Illustration: FIG. 110.--The Marsupial Mole.]
[Illustration: FIG. 111.--Tasmanian Wolf, a carnivor-like marsupial.]
[Illustration: FIG. 112.--Marsupials--Long-nosed Bandicoot.]
[Illustration: FIG. 113.--Squirrel-like marsupial (_Phascologale
penicillata_).]
As has previously been indicated, the most important characteristic
of the third great group is a modification of the membranes of the
embryo to form a connection between it and the wall of the uterus.
The allantois develops as a highly vascular membrane, the small blood
vessels of which are brought into very close contact with those in the
wall of the uterus. So that, while the blood of the mother does not
actually mix with that of the child, the two fluids are separated only
by thin membranes, through which nutritive substances easily pass. The
broad advantage of this is, of course, that the young animal passes
the earlier stages of its life inside the mother's body, where it is
exposed to a minimum of risk, is efficiently nourished, and from which
it is not sent forth into the world until it is tolerably well able to
look after itself.
[Illustration: FIG. 114.--Oeba Armadillo.
Photo: Berridge.]
[Illustration: FIG. 115.--Sea-cows.]
We shall now ask the reader to conceive a primitive placental mammal
type, and to consider briefly the relationships of the various groups
that have sprung from it. The group whose evolution has been perhaps
least progressive is that of the Edentates, including the sloths,
armadillos, and American ant-eaters (Fig. 114). Then, on the one hand,
we have the herbivorous types, including the remarkable sea-cows (Fig.
115), the rodents (Fig. 116), and the hoofed animals, leading up from
such comparatively primitive forms as the coneys (Fig. 117) to such
highly specialised types as the giraffe (Fig. 118). The other series of
groups are believed to be more nearly related to each other than to the
aforementioned, and the nearest approach to their common ancestor,
among modern mammals, is probably to be found in the Insectivora
(Figs. 119 and 120). Differing sharply from the Insectivora in their
possession of wings, but otherwise closely similar, are the bats (Fig.
121). From this same insectivor-like type have probably evolved the
whales (Fig. 122), the seals, etc. (Fig. 123), the carnivora (Fig.
124), and finally the primates, leading up through the lemurs to the
monkeys and man. Thus merely indicating the many and devious roads of
mammalian Evolution, we turn again to the one particular line that we
set out to follow.
[Illustration: FIG. 116.--Porcupine.]
[Illustration: FIG. 117.--Coney (Hyrax).]
[Illustration: FIG. 118.--Giraffe.
Photo by _Sport General_.]
[Illustration: FIG. 119.--_Centetes ecaudatus_, a primitive insectivor.]
[Illustration: FIG. 120.--Hedgehog.
Photo: Berridge.]
The Insectivora are by many of their features recognisable as low
types of placental mammals. They have a primitive type of skull, and
frequently show a rather marked similarity between the several classes
of teeth. The brain is relatively ill-developed, reminding one rather
strongly of the Marsupials, and the hemispheres show little if any
tendency to develop those wrinkles and fissures which always accompany
the higher type of intelligence. Probably the most primitive member of
the whole group is the Centetes from Madagascar, shown in Fig. 119. The
hedgehog and the shrews are its best-known examples. An interesting
point regarding the hedgehog is that, like few other mammals, it has
persisted almost unchanged from early tertiary times. It is thus to be
regarded as a very antique type, not only in its main features, but in
its details.
[Illustration: FIG. 121.--Vampires.]
[Illustration: FIG. 122.--Porpoises.]
[Illustration: FIG. 123.--Sea Lions.]
[Illustration: FIG. 124.--Indian Leopard.
Photo: Underwood.]
The Lemurs are an interesting group, standing, as they do, midway
between the primitive placental forms and the monkeys. Their special
home, as already mentioned, is Madagascar, to which some thirty-six of
the fifty known species are confined, but they occur also in Africa
and in South-Eastern Asia. They are arboreal and mostly nocturnal
in habits, and their food consists partly of fruit, etc., partly of
insects. They were formerly much more widely distributed, and many
fossils have been unearthed, for example, in North America. They
show certain characters of a distinctly primitive kind, such, for
instance, as their habit of hibernation. Their typical number of teeth
is thirty-six, the same as in the lower monkeys, but fossil forms are
known which possessed the full number of forty-four.
In their general build they show marked adaptation to their arboreal
life, and approach, some more and some less, the appearance of the
monkeys. The fore-limbs are considerably modified from the condition
in which they occur in ordinary mammals, in which they are placed
vertically under the body. They are placed in a more lateral position,
so that they can be moved through greater angles, and extended over
the head. In common language, they are ceasing to be legs, and are
becoming arms. As in the monkeys, the thumb and the great toe are
opposed to the other digits so as to render the hand and foot more
efficient as grasping organs. Hence the Lemurs may be included with
the apes as 'Quadrumana,' or four-handed animals. The fingers and toes
either bear claws, as in the lower animals, or flattened nails like
those of the higher apes and man, many species possessing the two
types of structures on different digits. The face is fox-like, and
lacks the human expression that is seen in the monkeys. The brain shows
considerable variation, being in some species far more primitive, in
others rather more highly developed, than in the lower monkeys.
[Illustration: FIG. 125.--Ring-tailed Lemur.]
[Illustration: FIG. 126.--The Slow Lemur or Loris.]
[Illustration: FIG. 127.--The Tarsier (Lemur).]
The history of the group has been very completely made out from
fossils, and it is possible to work back to forms which, apart from
their known subsequent evolution, could not be definitely separated
from the ancestors of other mammal groups. The Lemurs are illustrated
in Figs. 125 to 127.
The monkeys, to which we now turn, are divided into two groups,
the American or Western and the Eastern or Old-World types, each
of which is definitely confined to the regions indicated by the
names. The Western apes (Figs. 128 and 129) are by much the lower
group of the two, and in fact they lead back to fossil forms which
cannot be definitely distinguished from the Lemurs. They are a side
branch of the monkey stem, but are in all probability the nearest
living representatives of the first of our ape ancestors. They are
characterised by a wide septum between the nostrils, which makes the
latter open in an outward direction, a feature which enables them to
be distinguished at a glance from the other group. They have further
a prehensile tail, which is of much use in climbing, and generally
thirty-six teeth, or four more than the Old-World group. The lowest
of them are the well-known Marmosets, which have claws on all the
digits except the great toe, which last has a nail. These are further
characterised by the presence of four nipples, all other monkeys having
only two. Correlated with this feature, the Marmosets normally bear two
or three young at a birth, whereas in all other apes one is the usual
number.
[Illustration: FIG. 128.--A Marmoset (_Hapale jacchus_).]
The Eastern monkeys have thirty-two teeth, the same number as in man;
the septum between the nostrils is narrow, so that these open downwards
and forwards; the tail is never prehensile and is frequently absent.
They include such well-known forms as the baboons, the Gibraltar ape,
the sacred Hanuman of India, the Diana monkey (Fig. 130), and the
comical-looking Nasalis (Fig. 131). These represent a further step
towards the final climax of the primate group.
[Illustration: FIG. 129.--Squirrel Monkey (_Chrysothrix sciurca_).]
[Illustration: FIG. 130.--The Diana Monkey (_Cercopithecus Diana_).]
[Illustration: FIG. 131.--The Proboscis Monkey (_Nasalis larvatus_).]
[Illustration: FIG. 132.--Gibbon (_Hylobates leuciscus_).]
[Illustration: FIG. 133.--Orang-Utan.]
[Illustration: FIG. 134.--Young Chimpanzee.]
[Illustration: FIG. 135.--The Gorilla.]
A special class has to be made for the four genera of Old-World apes
illustrated in Figs. 132 to 136. These are the Gorilla and Chimpanzee,
which are African in distribution, and the Gibbon and Orang, which are
Asiatic. They are termed the Anthropoid or man-like apes, and there
can be no question that they are the nearest living relatives of the
human species. This is seen in their general build, which is man-like
in a high degree; by many similarities, even in the minutest details,
in their skeleton and muscular system; and by the fact that their
brain, while still greatly inferior to that of man, is by as much
superior to that of any other animal. The voice is frequently well
modulated, and the expression of the emotions, whether by it or by the
countenance, is very man-like. The position of the group was summed
up by Huxley, after a most thorough investigation of their anatomy,
in these words: "Thus, whatever system of organs be studied, the
comparison of their modification in the ape series leads to one and the
same result--that the structural differences which separate man from
the Gorilla and Chimpanzee are not so great as those which separate
the Gorilla from the lower apes." An interesting physiological proof
of the close relationship between the anthropoids and man has more
lately been discovered. It is found that the blood serum of any animal
destroys the blood corpuscles of any other when these are mixed with
it, except those of closely related species. Now the human blood serum
is destructive of the corpuscles of all the lower animals, so far as is
known, except those of the anthropoids.
[Illustration: FIG. 136.--Skeletons (left to right) of Gibbon,
Orang-Utan, Chimpanzee, Gorilla, and Man.
From Huxley's _Man's Place in Nature_.]
As regards the inter-relationships of the four species, it is
certain that the Gibbon is the lowest, and the nearest to the common
ancestor of the other three and of man. It has indeed the man-like
characteristic of walking in the erect position; but it has thirteen or
fourteen pairs of ribs, as against the normal twelve in man; it has
long arms, like the lower Old-World apes, and large canine teeth. It
is further the smallest of the group, and in habits the most like the
lower monkeys.
The Gorilla and Chimpanzee are closely related. The Gorilla is not
only the largest ape of the four, but in shape and build the most
man-like, which is accounted for by the fact that it is less strictly
arboreal than the others, and confines itself largely to the ground.
Its skull is superficially much less human in appearance than that of
the Chimpanzee, due to the strongly developed crests, which serve as
attachments for the powerful muscles of the lower jaw. The teeth of the
Chimpanzee are more uniform in size, and the skull smoother. There is a
marked difference in temperament between the two species, the Gorilla
being fierce and gloomy and quite untamable, while the Chimpanzee is
of a pleasant and lively disposition, and can, as is well known, be
trained to wear clothes, eat with a fork and knife, etc.
The Orang-Utan is found inhabiting forest ground in Borneo and Sumatra,
and living largely in trees, in which it builds nests as temporary
sleeping-places. It is a clumsy-looking animal, supporting itself on
the knuckles of its hands when travelling along the ground, and moving
but slowly. It has twelve pairs of ribs, the same number as in man, and
one fewer than in the Gorilla and Chimpanzee.
Practically all the important anatomical distinctions between man
and the anthropoids are reducible to two causes: the change from the
arboreal existence to a life on the ground and in the open country, and
the great development of the intelligence. When the change in the mode
of life occurred, it is obvious that an erect carriage would possess
an advantage over the stooping gait of the ape, in which it is neither
definitely a biped nor a quadruped. The erect position necessitated
a better-developed heel, stronger calf and hip muscles, and a more
parallel position and stronger development of the great toe. It also
brought about a shortening of the fore-arms and a widening of the
pelvis. Much has been made of the difference in the foot, between the
condition of a grasping organ, with an opposable great toe, and that
seen in man. But the change is just what we should devise in order to
make the foot stronger for propelling the body on the ground. Moreover,
the difference is more obvious than fundamental, and it is well known
that in armless persons, who develop the possibilities of their feet,
the latter members can still be turned into wonderfully efficient
'hands.'
[Illustration: FIG. 137.--Brains of (_F_) Cercopithecus (Eastern
Monkey); (_H_) Australian Bushman; (_L_) Chimpanzee; (_M_) European.]
In brain development the anthropoids are, as already mentioned,
greatly inferior to man; in the Gorilla, the largest-brained type,
the cranial capacity never, so far as is known, exceeds 600 cubic
centimetres, whereas the average in man is about 1500, and the smallest
known about 930. Apart from the difference in size, however, there is
a surprisingly close similarity between the anthropoid and the human
brains, which may be followed even in the particular arrangement of
the fissures. That the gap between the two types in respect of the
character in question is by no means extraordinarily wide may be seen
from Fig. 137, in which a series of brains are depicted. The size of
the hemispheres and the wrinkling of their surface are the characters
from which we judge the brain development, and it is obvious that in
these respects the step from the lower monkeys to the Chimpanzee is
greater than that between the Chimpanzee and the lowest human type. In
the teeth also, as is shown on Fig. 138, there is a gradual change from
the lemur to the human type.
[Illustration: FIG. 138.
_A_, Lower jaw of Pelycodus, a primitive extinct lemur, with eleven
teeth on either side; _B_, Lower jaw of red howling monkey (a Western
ape), with nine teeth on either side; _C_, Lower jaw of chimpanzee,
with eight teeth on either side; _D_, Lower jaw of man.]
[Illustration: FIG. 139.--The Pithecanthropus skull from the side and
from above.]
[Illustration:
FIG. 140.--The Pithecanthropus femur, from behind. The bone shows an
exostosis, evidently caused by an injury.]
Thus there can be no reasonable doubt that man has evolved from an
ancestor which, if it existed to-day, we should without hesitation
class as an anthropoid ape. Could any doubt have remained, it would
have been set aside by the discovery, in 1891, of a being occupying a
position about midway between the highest apes and the most primitive
known man. This is the Pithecanthropus, whose remains were discovered
in Java in a volcanic deposit of somewhat doubtful age, but probably
belonging to an era when a primitive type of man was already in
existence. The remains were indeed somewhat scanty, consisting of the
roof of the skull, a thigh bone, and a fragment of the lower jaw, the
former two of which are illustrated in Figs. 139 and 140. From these
fragments of the skeleton numerous deductions have been drawn, of
greater or less probability. It may be said with practical certainty,
however, that this ape-man was of the size of a smallish man, and that
he was accustomed to walk and stand in the characteristically human
erect attitude. The cranial capacity has been calculated at from 850
to 1000 cubic centimetres, or considerably greater than the highest
existing apes, and about equal to that of the lowest known human
specimens. His statue, as his discoverer conceived him, is illustrated
in Fig. 141.
[Illustration:
FIG. 141.--Imaginative statue of _Pithecanthropus erectus_.]
A restoration of the skull is shown in Fig. 142, and it is apparent
that in respect of the shape of the roof, at least, the Pithecanthropus
stands just about midway between the Chimpanzee and the most primitive
living man. The gradual approach to the human type, as we move upwards
in the primate scale, is very striking. The Pithecanthropus is by
some regarded as the result of an abortive attempt at 'man making,'
by others as a true transition form. We cannot in any case be very
far from the truth if we hang up his picture among the portraits of
our ancestors, for the transitional form would necessarily be closely
similar to him in its main features.
The next ancestor of whom we have any knowledge is definitely a human
being. This is the primitive man who inhabited Europe in earlier
Diluvial times, particularly in the interval between the first and the
second great ice ages. To him, from the place of his first discovery,
the name of the Neandertal man has been applied, and he is classed
by scientists as belonging to a different species from modern man,
the latter being named _Homo sapiens_, while he is given the less
flattering name of _Homo primigenius_. He was characterised, as may
readily be seen from the skull illustrated in Fig. 143, by a very
low and receding forehead, with heavy ridges of bone over the eyes.
In shape of head he stands midway between the Javan ape-man and the
Australian native, the lowest type existing at the present day. Further
ape-like peculiarities are the prominence of the lower part of the
face, the very large and massive lower jaw, and the receding chin,
shown in Fig. 144. From the skulls which have been found it is possible
to form a good idea of the man's appearance, which idea has been
expressed by a German sculptor in the bust illustrated in Fig. 145.
One may be permitted to hazard a guess at the cause of the process
"running to brain," which is the main feature of the last phases of
man's Evolution. The most probable theory seems to be that man came
into existence owing to the disappearance of forest over an area
inhabited by some high anthropoid ape. Ill-adapted as this ape would
certainly be for a life on the plains, it was saved from extinction
only by its high intelligence. And as cunning and reason would now,
in the new environment, be the most important assets, the process
of natural selection made for progress chiefly in respect of these
characters.
The faculty of articulate speech, which we must regard as an accidental
result of the great brain development, has given the human species that
great advantage which it possesses over all other animals of being able
to accumulate knowledge and experience from generation to generation.
It is this mass of experience, which is not inherent in man's nature,
but has to be impressed afresh on each successive generation, which
accounts for man's unique position in the animal world. But it is no
part of the scheme of this book to deal with the evolution of language
or invention or culture, and we must conclude.
Mankind are divisible into many types and races, some of which, like
the Australian aborigines and the Veddas of Ceylon, are relatively
primitive, others like the Germanic races being undoubtedly high in
the series. None of the differences are sufficient, however, to make
it necessary to regard mankind except as members of a single species.
The lower races have from time to time disappeared before the higher,
and the process continues at the present time. However much we
may regret it, this process has undoubtedly been a great factor in the
progress of the species as a whole.
[Illustration: FIG. 142.
Skulls of
Lemur.
Chimpanzee.
Australian Bushman.
Skulls of
Howling monkey (Mycetes).
Pithecanthropus (restored).
European man.]
[Illustration: FIG. 143.--Skull of Primitive Man, from Le Moustier.]
[Illustration: FIG. 144.--The Heidelberg lower jaw (_Homo
primigenius_).]
[Illustration: FIG. 145.--Bust of _Homo primigenius_, by Hyatt Meyer.]
[Illustration: FIG. 146.--Australian Bushman.]
Within the cultivated races, however, man has practically ceased to
evolve, at least in so far as concerns the main lines on which his
Evolution has thus far proceeded. For in creating the artificial
conditions of civilisation, he refuses any longer to be governed by
the stern law of nature, which decides that the fit shall live and
multiply, and the unfit surely perish. There is in fact evidence that
conditions of civilisation are making for retrogression rather than
for progress, a state of affairs that is worthy of the most serious
consideration. The only rational and scientific remedy that has been
offered for this state of affairs is the institution of some moderate
system of artificially guiding man's further Evolution.
PRINTED IN GREAT BRITAIN.
* * * * *
Transcriber's Notes:
Italic text is denoted by _underscores_.
Minor punctuation and printer errors repaired.
Every effort has been made to replicate this text as faithfully
as possible, including obsolete and variant spellings and other
inconsistencies.
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