That the forms of life to-day found on the earth have come into
existence by the evolution of the more complex forms from the
simpler, and of these simpler forms from still simpler, through the
ever-operating law of Selection, is a necessary conclusion from the
following facts:
1. The existence in the animal world of all grades of structures,
from the humblest possible protozoan, whose body consists of a
single simple speck, to the most powerful and complex of mammals.
There are estimated to be something like a million species of
animals living on the earth to-day. There may be several times this
number. These species are linked together by millions of varieties,
and are so related to each other that they may be all gathered
together into various genera; these genera may be grouped into
families, the families into orders, and the orders into seven or
eight great primary phyla. By taking existing species and adding to
them the extinct species of the rocks, and placing them all
according to their structural affinities, it is possible to arrange
them in the form of a tree with the various phyla, orders, families,
genera, and species, branching and rebranching from the main trunk.
The existence of structures, so graduated as to render such an
arrangement possible, is in itself suggestive of a common
relationship and origin.
2. Evolution is suggested by the similarities and homologies of
structure found throughout the animal kingdom. Some of these
similarities and homologies have already been mentioned. They are
everywhere—remoter and more fundamental, some of them, others
closer and more detailed. To the untrained mind, which sees surfaces
only, and not even surfaces well, the animal world is an
interminable miscellany of forms. But to the biologist, who looks
deeper and with immense acumen over the whole field of animal life,
there are only seven or eight different types of structure in the
entire animal world. These seven or eight types correspond with the
primary classes, or phyla, into which animals are divided, viz.,
protozoa, sponges, celenterates, echinoderms, worms, mollusks,
arthropods, and vertebrates. However widely the members of each of
these great groups may differ among themselves in colour, size,
habits of life, and the like, the members of each group all resemble
each other fundamentally. Moles differ from monkeys, bats from men,
and birds from crocodiles and toads. They differ enormously. But
they are all vertebrates with red blood, double body cavities,
backbones, two pairs of limbs, and five fingers on each limb. When
they are looked at superficially, there is not much similarity
between a water-strider and a butterfly or between a stag-beetle and
a gnat. But they are all, in reality, built according to the same
plan. Like all other insects, they have six legs, a sheath-like
skeleton, and bodies characteristically divided into head, thorax,
and abdomen. It is the same with all other great classes of beings.
All worms resemble each other; and so do all mollusks, although they
may differ in particulars as widely as nautiluses and clams.
Echinoderms have a radiate structure, celenterates and sponges are
vase-like in shape, and protozoa are one-celled. The differences in
structure among the members of a group consist in different
modifications of a fundamental type. Among the vertebrates the
fore-limb may be an arm, a leg, a wing, a shovel, a flipper, or a
fin. But in all cases it is the same organ—that is, the same
implement modified to serve different ends. Take the mouth-parts of
insects. In the grasshopper and cricket these parts are fitted for
grinding; in the moths and butterflies they are fashioned into long
tubes for sucking the sweets of flowers; in the mosquito they form
an elaborate apparatus for drilling and drinking; and in the mayfly
the mouth-parts, though present, are not used at all. In all of
these animals these parts are essentially the same, although
differing so much in their forms and purposes that the unscientific
can scarcely be made to believe they are fundamentally alike. There
is no fact more familiar to the biologist or more frequently met
with in the fields of animal morphology than the fact that the same
general type may be hammered into dozens, or hundreds, or even
thousands, of different patterns by the incessant industry of its
surroundings, and that the same organic part may be moulded into
various implements serving totally different ends by the
environmental vicissitudes of time and space. On the hypothesis that
the members of each group of animals possessing common
characteristics, whether the group be large or small, have sprung
from a common ancestry, and that the differences in structure have
arisen as a result of differences in environment, the similarities
and homologies of structure existing among animals are perfectly
intelligible. But on any other supposition they are inexplicable.
3. Evolution is suggested by the remarkable series of phenomena
presented by embryology. There are at least four facts in the
developmental history of every creature which can hardly be
accounted for on any other supposition than that of organic
evolution.
_First_, the fact that every animal, above the lowest, individually
passes through an evolution between the beginning of its existence
and its maturity. Terrestrial beings are not born, like Minerva,
full-grown. They grow. They evolve. They commence close down to the
very atoms. And from this lowly genesis they rise, through a series
of marvellous changes, to that high state of perfection and
greatness from which they descend to dissolution.
If we knew by actual observation as little concerning the evolution
of individuals as we do of the evolution of species—if we had
always been used to seeing animals, including ourselves, in full
bloom—had never watched the tadpole, the pupa, and the babe pass
through their wonderful metamorphoses on their way to maturity, it
would probably be just as hard for many minds to believe that
animals evolve individually to be what they are as it is for them to
believe that species have grown to be what they are. In the case of
individuals, however, the evolution takes place right before our
eyes largely, while the evolution of species goes on so slowly and
stretches back so far into the past that it can only be inferred.
_Second_, the fact that animals, no matter how much they may differ
from each other at maturity, all begin existence at the same place.
Every animal commences its organic existence as an egg—as a
one-celled animal—as an organism identical in structure with the
simplest protozoan. The ova of whales ‘are no larger than fern
seeds.’ The eggs of the coral, the crab, the ape, and the man are
so precisely alike that the highest powers of the microscope cannot
distinguish between them.
_Third_, the fact that the members of the same great group of
animals in their individual development pass through similar stages
of evolution. The ‘worm’ stage in the development of most
insects and the ‘fish’ stage of frogs are well known.
There are no more remarkable instances of individual evolution in
the whole range of animal life. The fish, the reptile, the bird, the
dog, and the human being—all vertebrates, in short—cannot for
some time after their embryonic commencement be distinguished from
each other. ‘The feet of lizards and mammals, the wings and feet
of birds, and the hands and feet of men,’ says the illustrious Von
Baer, as quoted by Darwin, ‘all arise from the same fundamental
form’.[1]
‘It is quite in the later stages of development,’ says Huxley,
‘that the human being presents marked differences from the ape,
while the latter departs as much from the dog in its development as
the man does’.[2]
Not only frogs, but reptiles, birds, and mammals, including man, all
have gills at a certain stage in their embryonic development. Nearly
all the lower invertebrate animals are hermaphroditic—that is, in
the body of each animal is found the two kinds of sex organs which
in the higher animals exist in distinct animals. And frogs, birds,
and other higher animals, which as adults are unisexual, have, as an
inheritance from these primitive forms, hermaphroditic embryos.[3]
_Fourth_, the fact that the structural stages through which animals
in embryo pass correspond in a wonderful manner with the permanent
structures of those lower forms which extend serially back to the
beginnings of life. It is the proudest boast of the embryologist
that he is able to know the route through which any species has come
to be what it is by a simple study of the individual evolution of
its members. Each animal repeats in its individual evolution the
evolution of its species. This recapitulation is not always
complete—is, in fact, frequently vague, sometimes circuitous, and
often broken or abbreviated. Processes requiring originally
centuries or thousands of years to accomplish are here telescoped
into a few months, or even days. It is not strange that the process
is imperfect. But so firmly is the belief in the correspondence of
ontogeny and phylogeny fixed in the minds of modern biologists that,
in determining the classification and affinities of any particular
animal, more reliance is placed on the facts of embryology than on
those of adult structure.
The first thing that an animal becomes after it is an egg—unless
it is a one-celled animal, in which case it remains always an
egg—is two cells; these two cells become four; these four become
eight; and so on, until the embryo becomes a many-celled ball,
consisting of a single layer of cells surrounding a fluid interior.
A dimple forms in the cell layer on one side of this ball, and, by
deepening to a hollow, changes the ball into a double-walled sac.
This is the gastrula—the permanent structure of the sponges and
celenterates, and an (almost) invariable stage in the larval
development of all animals above the sponges and celenterates. The
gastrula becomes a worm (or an insect or a fish through the worm) by
elongation and enlargement, and by the development of the endoderm,
which is the inner layer of the cell wall, into organs of nutrition
and reproduction, and by the development of the ectoderm, which is
the outer cell layer, into organs of motion and sensation.
The embryonic development of a human being is not different in kind
from the embryonic development of any other animal. Every human
being at the beginning of his organic existence is a protozoan,
about 1/125 inch in diameter; at another stage of development he is
a tiny sac-shaped mass of cells without blood or nerves, the
gastrula; at another stage he is a worm, with a pulsating tube
instead of a heart, and without head, neck, spinal column, or limbs;
at another stage he has, as a backbone, a rod of cartilage extending
along the back, and a faint nerve cord, as in amphioxus, the lowest
of the vertebrates; at another stage he is a fish with a
two-chambered heart, mesonephric kidneys, and gill-slits with gill
arteries leading to them, just as in fishes; at another stage he is
a reptile with a three-chambered heart, and voiding his excreta
through a cloaca like other reptiles; and finally, when he enters
upon post-natal sins and actualities, he is a sprawling, squalling,
unreasoning quadruped. The human larva from the fifth to the seventh
month of development is covered with a thick growth of hair and has
a true caudal appendage, like the monkey. At this stage the embryo
has in all thirty-eight vertebrae, nine of which are caudal, and the
great toe extends at right angles to the other toes, and is not
longer than the other toes, but shorter, as in the ape.
These facts are unmistakable. There is a reason for everything, and
there is a reason for these transformations through which each
generation of living beings journeys. The individual passes through
them because the species to which he belongs has passed through
them. They represent ancestral wanderings. As if to emphasise the
kinship of all of life’s forms and to render incontrovertible the
fact of universal evolution, Nature compels every individual to
commence existence at the same place, and to recapitulate in his
individual evolution the phylogenetic journeyings of his species.
4. That existing forms of life have been evolved from other forms,
and that these ancestral forms have been different from those
derived from them, is shown by the occasional appearance of
antecedent and abandoned types of structure among the offspring of
existing species. Occasionally a human child is born strangely
unlike its parents, but bearing an unmistakable resemblance in looks
and disposition to his great-grandfather or some other remote
ancestor. This is _atavism_, that tendency to revert to ancestral
types which is prevalent among all animals. We may think of it
figuratively as a flash of indecision when Nature hesitates for a
moment whether to adopt a new form of structure or cling to the old
and tried. Horses and mules are sometimes born with three toes on
each foot, and zebra-like stripes on their legs and shoulders; and
domestic pigeons, such as are naturally black, red, or mottled,
occasionally produce offspring with blue plumage and two black
wing-bars, like the wild rock-dove, from which all domestic breeds
have sprung. In man the cheekbone and the frontal bone of the
forehead consist normally each of a single bone. But in children and
human embryos these bones are always double, as is normally the case
in adults among some of the anthropoids and other mammals. Gills
appear regularly in the embryos of reptiles, birds, and mammals, and
human young are sometimes born with gill-slits on the neck. There
are times when, owing to inaccurate or incomplete embryological
development, these fish-like characteristics are so perfect at birth
as to allow liquids, on being swallowed, to pass out through them
and trickle down on the outside of the neck. Many muscles are
occasionally developed in man which are normal in the apes and other
mammals. As many as seven different muscular variations have been
found in a single human being, every one of which were muscles found
normally in the structure of the apes.[1]
5. Closely akin to atavism, which is the occasional persistence of
ancestral types of character, is the regular occurrence of vestigial
organs or structures, organs which in ancestral forms have definite
functions, but which in existing species, owing to changed
conditions, are rudimentary and useless. On the back of each ankle
of the horse are two splints, the atrophied remains of the second
and fourth toes. Similar vestiges of two obsolete toes are also
found just back of the wrists and ankles on all the two-toed
ungulates, such as the cow and sheep. In the body of the whale where
hind-limbs would naturally be, there are found the anatomical ruins
of these organs in the form of a few diminutive bones. The same
thing is true in the sirenians. In the Greenland whale there are
remnants of both femur and tibia in the region of the atrophied
hind-limbs. The snakes are limbless, but the pythons and boas have
internal remnants of hind-limbs and sometimes even clawed structures
representing toes. The so-called ‘glass-snake’ or
‘joint-snake’ (which is really a limbless lizard) has four
complete internal limbs. Young turtles, parrots, and whalebone
whales have teeth, but the adults of these animals are toothless.
Cows, sheep, deer, and other ruminants, never have as adults any
upper incisors, but these teeth are found in the foetal stages of
these animals just under the gums. The female frog has rudimentary
male reproductive organs, and the male has corresponding vestiges of
female organs. Similar remnants of the reproductive structures exist
in many other animals. They represent stages in the transition from
the hermaphroditism of primitive animals to the unisexuality of the
higher forms, the separation of the sex organs into those of male
and female having come about through the decay of one set of
structures in each individual.
For reasons which it is not necessary to mention here, biologists
believe that insects all originated from a common parental form,
with two pairs of wings and six legs. Insects all retain their
original allowance of legs, but in many species one or the other
pair of wings has become more or less degenerated. In the whole
order of flies the back pair of wings is represented by a couple of
insignificant knobs. In the Strepsiptera, a sub-order of beetles,
the front-wings are similarly reduced, being mere twisted filaments.
Many parasites, such as fleas and ticks, whose mode of life renders
organs of aerial locomotion unnecessary, are entirely wingless. The
insects of small isolated islands are also largely without wings,
the proportion of wingless species being much larger than among
insects inhabiting continents. This is due to their greater
liability on small land masses of being carried out to sea and
drowned, owing to the feebleness and uncertainty of insect flight.
On the island of Madeira, out of the 550 species found there, 220
species no longer have the power of flight.
Air-breathing animals—amphibians, reptiles, birds, and
mammals—have normally a pair of lungs—a right one and a left
one. But in snakes and snake-like lizards, where the body is very
slender and elongated, only one lung, sometimes the right one, and
sometimes the left, is fully developed. The right ovary is likewise
aborted in all birds, the left one yielding all the eggs. The swifts
and frigate birds live almost their whole lives long on the wing,
and the legs of these birds have grown so short and weak and
rudimentary, as a result of their constant life in the air, that
they can scarcely walk. The chimney swift is said never to alight
anywhere except on the sooty inner walls of the chimney where its
nest is. Its food consists of insects which it gathers in the air,
and the few dead twigs used in making its nest are nipped from the
tree while the bird continues its flight. The ostriches,
cassowaries, and many other birds, have, on the other hand,
developed their legs at the expense of their wings. The ostrich is
said to be able to outrun the horse, but it has no power of flight,
although it has wings and wing muscles, and even the skin-folds
covering the wings corresponding to those of birds that fly. But its
whole flying apparatus is in ruins. The rudimentary hind-toe of
birds is a vestigial organ, and so are the claws which appear on the
thumb and first finger of all young birds. So also are the rudiments
of eyes in cave crickets, fishes, and other inhabitants of total
darkness. The flounder and other so-called flat fishes swim straight
up, as ordinary fishes do, when young. But as they grow they incline
more and more to one side, and finally swim entirely on their side,
the eye on the lower side migrating around, and joining the other on
the upper side of the head.
About the first thing a human infant does on coming into the world
is to prove its arboreal origin by grasping and spitefully clinging
to everything that stimulates its palms. A little peeperless babe an
hour old can perform feats of strength with its hands and arms that
many men and women cannot equal. It can support the entire weight of
its body for several seconds hanging by its hands. Dr. Robinson, an
English physician, found as a result of sixty experiments on as many
infants, more than half of whom were less than an hour old, that
with two exceptions every babe was able to hang to the finger or to
a small stick, and sustain the whole weight of the body for at least
ten seconds. Twelve of those just born held on for nearly a minute.
At the age of two or three weeks, when this power is greatest,
several succeeded in sustaining themselves for over a minute and a
half, two for over two minutes, and one for two minutes and
thirty-five seconds. The young ape for some weeks after birth clings
tenaciously to its mother’s neck and hair, and the instinct of the
child to cling to objects is probably a survival of the instinct of
the young ape. I believe it is Wallace who relates somewhere an
incident which illustrates the instinct of the young simian to cling
to something. Wallace had captured a young ape, and was carrying it
to camp, when the little fellow happened to get its hands on the
naturalist’s whiskers, which it mistook, evidently, for the
hirsute property of its mother, and, driven by the powerful instinct
of self-preservation, it hung on to them so desperately it could
scarcely be pulled loose. Many mammals are provided with a
well-developed muscular apparatus for the manipulation of their
ears. But in man there does not exist the same necessity for
auricular detection of enemies, and while these muscles still exist,
and are capable of being used to a slight extent by occasional
individuals, they are generally so emaciated as to be useless.
Another vestigial organ in the body of man, and one of significance
from the standpoint of morphology, is the tail. The tail is an
exceedingly unpopular part of the human anatomy, most men and women
being unwilling to admit that they have such an appendage. But many
a person who has hitherto dozed in ignorance on this matter has
learned with considerable dismay, when he has for the first time
looked upon the undraped lineaments of the human skeleton, that man
actually has a tail. It consists of three or four (sometimes five)
small vertebrae, more or less fused, at the posterior end of the
spinal column. That this is really a rudimentary tail is proved
beyond a doubt by the fact that in the embryo it is highly
developed, being longer than the limbs, and is provided with a
regular muscular apparatus for wagging it. These caudal muscles are
generally represented in grown-up people by bands of fibrous tissue,
but cases are known where the actual muscles have persisted through
life.[4]
The nictitating membrane, which in birds and many reptiles consists
of a half-transparent curtain acting as a lid to sweep the eye, is
in the human eye dwindled to a small membranous remnant, draped at
the inner corner. The growth of hair over the human body surface may
be regarded, in view of the sartorial habits of man, as a vestigial
inheritance from hairy ancestors. One of the most notorious of the
vestigial organs of man is the vermiform appendix, a small slender
sac opening from the large intestine near where the large intestine
is joined by the small intestine. In some animals this organ is
large and performs an important part in the process of digestion.
But in man it is a mere rudiment, not only of no possible aid in
digestion, but the source of frequent disease, and even of death.
There are in all, according to Darwin, about eighty vestigial organs
in the human body. But these organs occur everywhere throughout the
animal kingdom. There is not an order of animals, nor of plants
either, without them. They are necessary facts growing out of
evolution. Organic structures are the result of adjustment to
surrounding conditions. The continual changes in environment to
which all organisms are exposed necessitate corresponding changes in
structure. And the vestiges found in the bodies of all animals
represent parts which in the previous existence were useful and
necessary to a complete adjustment of the organism, but which, owing
to a change of emphasis in surroundings, have become useless, and
consequently shrunken. They are the obsolete or obsolescent parts of
animal structure—parts which have been outgrown and
superseded—the ‘silent letters’ of morphology. They sustain
the same relation to the individual organism as dead or dwindling
species sustain to a fauna. They furnish indisputable proof of the
kinship and unity of the animal world.
6. It is only on the supposition that the life of the earth has
evolved step by step with the evolution of the land masses, and that
the forms of life from which existing forms were evolved were
dispersed over the earth at a time when physiographic conditions
were very different from what they are now, that it is possible to
account for the peculiar manner in which animals are distributed
over the earth. The cassowary is a flightless bird of the ostrich
order inhabiting Australia and the islands to the north of it. This
bird is found nowhere else in the world, and each area has its own
particular species. The same things are also true of the kangaroo.
It is found over a similar region, with a different species
occupying each land mass. Now, on the hypothesis of special creation
there is no thinkable reason why these animals should be divided, as
they are, into distinct species, and restricted to this particular
region. But on the hypothesis of evolution it is perfectly plain.
All of these regions at one time were united with one another, and
were subsequently submerged in part, forming islands. Each group of
animals, being isolated from every other group and subjected to
somewhat different conditions, developed a style of departure from
the original type of structure different from that of every other
group in response to the peculiar conditions operating upon it. This
has led, in the course of centuries of selection, to the formation
of distinct species such as exist to-day.
Lombock Strait, a narrow neck of water between Bali and Lombock
Island, and Macassar Strait, separating Celebes from Borneo, are
parts of a continuous passage of water which in remote times
separated two continents—an Indo-Malayan continent to which
belonged Borneo, Sumatra, Java, and the Malay Peninsula; and an
Austro-Malayan continent, now represented by Australia, Celebes, the
Moluccas, New Guinea, Solomon’s Islands, etc. Wallace first
announced this ancient boundary, and it has been called
‘Wallace’s line.’ He was led to infer its existence by the
fact which he observed as he travelled about from island to island,
that, while the faunas of these two regions are as wholes very
different from each other, the faunas of the various land patches in
each area have a wonderful similarity. Australia is a veritable
museum of old and obsolete forms of both plants and animals. Its
fauna and flora are made up prevailingly of forms such as have on
the other continents long been superseded by more specialised
species. No true mammals, excepting men and a few rats, lived in
Australia when Englishmen first went there. The most powerful
animals were the comparatively helpless marsupials. The explanation
of these remarkable facts is probably this: The Australian
continent, which formerly included New Guinea and other islands to
the north, has not been connected with the other land masses for a
very long period of time. The development upon the other continents
of the more powerful mammals, especially of the ungulates and the
carnivora, resulted in the extermination of the more helpless forms
from most of the earth’s surface. But Australia, protected by its
isolation, has retained to this day its old-fashioned forms of life,
neither land animals nor plants having been able to navigate the
intervening straits. This supposition is strengthened by the fact
that fossil remains of marsupials are to-day found scattered all
over the world, while, with the exception of the American opossums,
living marsupials are found only in Australia and its islands. There
is to-day not a single survivor of these once-numerous races in
either Europe, Asia, or Africa. Similar facts of distribution are
furnished by the lemurs—those small, monkey-like animals with fox
faces, which are sometimes called ‘half-apes,’ since they are
supposed to be the link connecting the true apes with lower forms.
Fossil lemurs are found in both America and Europe, but lemurs are
now extinct in both continents. Those of America were probably
exterminated by the carnivora, who are known to be very fond of
monkey meat of all kinds. The European lemurs seem to have migrated
southward into eastern Africa at a time when Madagascar formed a
part of the mainland. ‘There they have been isolated, and have
developed in a fashion comparable to that which has occurred in the
case of the Australian marsupials. Of fifty living species, thirty
are confined to Madagascar, and the lemurs are there exceedingly
numerous in individuals. Outside of Madagascar they only maintain a
precarious footing in forests or on islands, and are usually few in
number’.[3]
If the earth were peopled by migrations from Ararat, it would
require a good deal of intellectual legerdemain to show why the
sloths are confined to South America and the monotremes to Australia
and its islands. The reindeer of northern Europe and Asia, and the
elk and caribou of Arctic America, are so much alike they must have
descended from a common ancestry, and been developed into distinct
species since the separation of North America and Eurasia. The same
thing is probably also true of the puma and jaguar, who inhabit the
middle latitudes of the New World, and the lion, tiger, and leopard,
occupying like latitudes of the Old World. They all belong to the
cat family, and represent divergences from a common feline type of
structure. The camel does not exist normally outside of northern
Africa and central and western Asia. And when the camel-like llama
of South America first became known to zoologists, it was a problem
how this creature could have become separated so far from the
apparent origin of the camel family. But since then fossil camels
have been found all over both North and South America. And it has
even been suspected that perhaps America was the original home of
the camel, and that, like the horse, the camel migrated to the
eastern hemisphere at a time when the eastern and western land
masses were connected. The foxes, hares, and other mammals of the
upper Alps, also many Alpine plants, are like those of the Arctic
regions. The most probable explanation of these resemblances is that
these Alpine species climbed up into these inhospitable altitudes,
and were left stranded here on this island of cold, when their
relatives, on the return of warmth at the close of the glacial
period, retreated back to the ice-bound fastnesses around the pole.
It is for a similar reason, probably, that the flora of the upper
White Mountains resembles that of Labrador.
7. One of the strongest pieces of evidence bearing on evolution that
is furnished by any department of knowledge is that furnished by
geology. It is the evidence of the rocks. Geology is, among other
things, a history of the earth. This history has been written by the
earth itself on laminae of stone. It is from these records that we
learn incontestably the order in which the forms of life have made
their appearance on the earth.
Three-fourths of the surface of the earth is sea. Over the surface
of the remaining fourth, excepting in mountainous places, is a layer
of soil, varying from a few feet to a few hundred feet in depth.
Beneath this coverlet of soil, extending as far as man has
penetrated into the earth, is rock. Excepting in regions overflowed
by lava poured out from beneath, or along the backbones of
continents where the surface rocks have been upheaved into folds and
carried away by denudation, the rocks immediately beneath the soil,
to a thickness often of thousands of feet, are in the form of
layers, or sheets, arranged one above another. These rocks are
called sedimentary rocks, as distinguished from the unlaminated
rocks of the interior. They have been formed at the bottom of the
sea, and have, hence, all been formed since the condensation of the
oceans. They have been formed out of the detritus of continents
brought down by the rivers and the accumulated remains of animal and
vegetal forms which have slowly settled down through the waters.
They are the successive cemeteries of the dead past. Such rocks are
now forming over the floors of all oceans—forming just as they
have formed throughout the long eons of geological history. Along
the axes of ancient mountains and in deep-cut canyons the rock
layers are exposed to a thickness of thousands of feet, in some
cases thirty or forty thousand feet. Here they lie, piled up, one on
top of another, the great, broad pages upon which are written the
long, dark story of our planet. It is the mightiest and most
everlasting of all annals—the autobiography of a world. It is
possible, by studying these rock records, to know not only the kind
of life that lived in each age, but a good deal regarding the
conditions in which that life lived and passed away. Just as the
naturalist is able, from a single bone of an unknown animal, to
reconstruct the entire animal and to infer something of its
surroundings and habits of life, and as the archeologist, by going
back to the graves of deceased races and digging up the dust upon
which these races wrought, is able to tell much of their history and
characteristics, so the geologist, by studying the bones of those
more distant civilisations, the civilisations sandwiched among the
fossiliferous rocks, is able to know, not only just the kind of life
that lived in each age, but, by comparing the species of successive
strata, can construct with astonishing fulness the genealogical
outline of the entire life process. The succession of life forms as
they appear in the rocks, with a sketch of their probable genealogy,
is traced elsewhere in this chapter. It is only necessary to say
here that the order in which the forms of life appear in the
sedimentary strata is that of a gradually increasing complexity. The
invertebrates appear first; then the fishes, the lowest of the
vertebrates; after these come the amphibians; following these the
reptiles; and finally the birds and mammals.
8. There is another reason for a belief in evolution furnished by
geology, but of a somewhat different kind from that just stated. It
consists in the fact that there are found in the rocks series or
grades of structures, which fit with amazing accuracy on to the
structures of existing species. Now, this is precisely what,
according to the evolutional hypothesis, is to be expected. For, if
evolution is true, existing species represent the tops of things.
They are the existing and visible parts of processes which extend
indefinitely back into the past, and whose deceased stages may
reasonably be expected to be found fossil in the earth. Considering
the youth and inexperience of paleontology and the torn and
incoherent character of the record, it is surprising that anatomists
have been able to accomplish what they have accomplished. In many
cases—notably, those of man, the snail, the crocodile, and the
horse—antecedent forms of structure have been found in almost
unbroken gradations leading back to types differing immensely from
their existing representatives. Bones and fossils of men have been
found buried beneath the alluvium of rivers, under old lava-beds,
and in caves, crusted over by the deposits of percolating waters.
Many such fossils are found in quaternary rocks, along with the
bones of animals still living and some extinct. Some of these
remains indicate unmistakable affinities with the ape. The most
celebrated of these discoveries is the fossil of an erect ape-man
(_Pithecanthropus erectus_), found by a Dutch Governor on the island
of Java in 1894. This fossil, in the shape and size of the head and
in its general structure, strikes about as near as could be the
middle between man and ape. That it is the fossil of an ambiguous
form is indicated by the fact that, when it was examined by a
company of twelve specialists at Berlin soon after its discovery,
three of them declared it to be the remains of an individual
belonging to a low variety of man; three others thought it was a
large anthropoid; while the other six held that it was neither man
nor anthropoid, but a genuine connecting link between them. It is
discussed at length by Haeckel in ‘The Last Link,’ a paper read
before the International Congress of Zoology, at Cambridge, in 1898.
‘It is,’ says the veteran biologist, ‘the much-sought
“missing link” supposed to be wanting in the chain of primates
which stretches unbroken from the lowest catarhine to the most
highly developed man.’ Associated with this fossil ape-man were
the fossils of the elephant, hyena, and hippopotamus, none of which
any longer exist in that part of the world, also the fossil remains
of two orders of animals now extinct. The genealogy of the crocodile
has been traced by Huxley, through all intermediate stages, back to
the giant reptiles of the early Tertiary.[5]And the pedigree of the
horse has been even more completely worked out by the indefatigable
Marsh. In the museum of Yale University may be seen the fossil
history of this splendid ungulate, from the time it was a clumsy
little quadruped only 14 inches high, and with four or five toes on
each foot, down to existing horses. The earliest known ancestor of
the horse, the eohippus, lived at the beginning of the Eocene epoch.
It had five toes, almost equal, on each front foot (four toes
behind), and was about the size of a fox. The orohippus, which lived
a little later, had four toes on each front-foot, and three behind.
The mesohippus, found in the Miocene, had three toes and one
rudimentary toe on each front-foot, and three toes behind. It was
about the size of a sheep. The miohippus, which is found later, had
three toes on each of its four feet, with the middle toe on each
foot larger than the other two. The pliohippus, living in the
Pliocene epoch, had one principal toe on each foot, and two
secondary toes, the two secondary toes not reaching to the ground.
It was about the size of a donkey. Existing horses have one toe on
each foot—the digit corresponding to the big middle finger—and
the ruins of two others in the form of splints on the back of each
ankle. In the embryo of the horse these splints are segmented, each
of them, into three phalanges. Fossil remains representing all
stages in the development of the horse have been found in the
regions about the upper waters of the Missouri River.
It is an important fact that the types of structure forming any
series grow more and more generalised as the distance from the
present increases, and that different lines of development, when
traced back into the past, often converge in types which combine the
main characters of various existing groups. The horses,
rhinoceroses, and tapirs, great as are the differences among them
now, can be traced back step by step through fossil forms, their
differences gradually becoming less marked, until ‘the lines
ultimately blend together, if not in one common ancestor, at all
events into forms so closely alike in all essentials that no
reasonable doubt can be held as to their common origin.’ ‘The
four chief orders of the higher mammals—the primates, ungulates,
carnivora, and rodents—seem to be separated by profound gulfs,
when we confine our attention to the representatives of to-day. But
these gulfs are completely closed, and the sharp distinctions of the
four orders are entirely lost, when we go back and compare their
extinct predecessors of the Cenozoic period, who lived at least
three million years ago. There we find the great sub-class of the
placentals, which to-day comprises more than two thousand five
hundred species, represented by only a small number of insignificant
pro-placentals, in which the characters of the four divergent orders
are so intermingled and toned down that we cannot in reason do other
than consider them as the precursors of those features. The oldest
primates, the oldest ungulates, the oldest carnivora, and the oldest
rodents, all have the same skeletal structure and the same typical
dentition (forty-four teeth) as these pro-placentals; all are
characterised by the small and imperfect structure of the brain,
especially of the cortex, its chief part, and all have short legs
and five-toed, flat-soled (plantigrade) feet. In many cases among
these oldest placentals it was at first very difficult to say
whether they should be classed with the primates, ungulates,
carnivora, or rodents, so very closely and confusedly do these four
groups, which diverge so widely afterwards, approach each other at
that time. Their common origin from a single ancestral group follows
incontestably’.[6]
9. Man is the most powerful and influential of animals. He rules the
world—rules it with a sovereignty more despotic and extensive than
that hitherto exercised by any other animal. Many races of beings
are, and have been for centuries, completely dominated by him. These
races, during their long subjection, have been changed and
transformed by man in a wonderful manner through his control of
their power to breed. All domestic animals have come from wild
animals; they have been derived by a process of selective evolution
conducted by man himself. By continually choosing as the progenitors
of each generation those with qualities best suited to his whims and
purposes, man has evolved races as different from each other in
appearance and structure, and as different from the original
species, as many groups which, in the wild state, constitute
distinct species; indeed, man has in some cases created entirely new
species, both of plants and animals—species that breed true and
are what biologists call ‘good’—by his own selections.
There are something over 150 different varieties of the domestic
pigeon. Some of these varieties—as many as a dozen, Mr. Darwin
thinks—differ from each other sufficiently to be reckoned, if they
are considered solely with reference to their structures, as
entirely distinct species. The carrier, for instance, the giant of
the pigeons, measures 17 inches from bill-tip to the end of its
tail, and has a beak 1 3/4 inches long. Around each eye is a large
dahlia-like wattle, and another large wattle is on the beak, giving
the beak the appearance of having been thrust through the kernel of
a walnut. The tumbler is small, squatty, and almost beakless. It has
the preposterous habit of rising high in the air and then tumbling
heels over head. The roller, one of the many varieties of the
tumbler, descends to the ground in a series of back somersaults,
executed so rapidly that it looks like a falling ball. The runt is
large, weighing sometimes as much as the carrier. The fantail has
thirty or forty feathers in its tail, while all other varieties have
only twelve or fourteen, the normal number for birds. The trumpeter,
so named on account of its peculiar coo, has an umbrella-like hood
of feathers covering its head and face, and its feet are so heavily
feathered that they look like little wings. In the correct specimens
of this variety the feathers have to be clipped from the face before
the birds can see to feed themselves. The pouter has the absurd
habit of inflating its gullet to a prodigious size, and the Jacobin
wears a gigantic ruff. The homing pigeon has such a strong
attachment for its cote that it will travel hundreds of miles,
sometimes as many as 1,400 miles, in order to reach the home from
which it has been separated. But it is not simply in their colour,
size, habits, and plumage, that pigeons vary. There are
corresponding differences in their structures, in the number of
their ribs and vertebrae, in the shape and size of the skull, in the
bones of the face, in the development of the breast-bone, and in the
length of the neck, legs, and bill. Pigeons also differ in the shape
and size of their eggs, and in their dispositions and voice.
‘There is,’ says Huxley in summing up his discussion of the
great variety in these birds, ‘hardly a particular of either
internal economy or external shape which has not by selective
breeding been perpetuated and become the foundation of a new
race’.[7]
All of the 150 different varieties of domestic pigeons have been
evolved by human selection during the past three or four thousand
years from the blue rock-doves which to-day inhabit the seacoast
countries of Europe.
What is true of pigeons is also true largely of most of the other
races associated with man—of cats, cattle, horses, sheep, swine,
goats, fowls, and the like. All varieties of the domestic
chicken—the clumsy Cochin with its feather-duster legs, the tall
and stately Spanish, the great-crested Minorca, the Dorking with its
matchless; comb and wattle, the almost combless Polish, the blue
Andalusian, the gigantic Brahma, the tiny Bantam, the Wyandottes in
all colours (black, white, buff, silver, and golden), the
magnificent Plymouth Rocks, and the exceedingly pugnacious
Game-cock—these and dozens of other varieties, all flightless,
have come from the jungle-bird whose morning clarion still greets
Aurora from the wilds of distant India. The dog is a civilised wolf,
and the wild-boar is the progenitor of the oleaginous swine. The
Merino and South Down breeds of sheep have come from the same stock
in the last century and a half. In 1790 a lamb was born on the farm
of Seth Wright in Massachusetts. It had a long body and short, bowed
legs. It was noticed that this lamb could not follow the others over
the fences. The owner thought it would be a good thing if all his
sheep were like it. So he selected it to breed from. Some of its
offspring were like it, and some were like the ordinary sheep. By
continual selection of those with long bodies and short legs the
ancon breed of sheep was finally produced. In 1770 in a herd of
Paraguay cattle a hornless male calf appeared, and from this
individual in a similar way came the stock of Muleys. The occasional
appearance of horned calves and lambs among the offspring of
hornless breeds of cattle and sheep are examples of atavism
indicating the presence of a vestigial tendency to breed true to
their horned ancestors. The Hereford cattle originated as a distinct
variety about 1769 through the careful selections of a certain
Englishman by the name of Tompkins. All domesticated quadrupeds,
except the elephant, have come from wild species with erect ears,
the ears acting as funnels to harvest the sound-waves. But there are
few of them in which there is not one or more varieties with
drooping ears—cats in China, horses in parts of Russia, sheep in
Italy, cattle in India, and pigs, dogs, and rabbits in all
long-civilised lands. We are so accustomed to seeing dogs and pigs
with pendent ears that we are surprised to know there are varieties
with erect ears. The goldfish is a carp, and in its native haunts in
the waters of China it has the colour of the carp. The golden hue
seen in the occupants of our aquaria has been given to this fish by
the Chinese through the continual selection of certain kinds. The
goldfish, almost as much as the pigeon, has been the sport of
fanciers, and the strangest varieties have resulted. Some have
outlandishly long fins, while others have no dorsal fin at all. Some
are streaked and splotched with gold and scarlet; others are pure
albinos. One of the most monstrous varieties has a three-lobed
tail-fin, and its eyeballs, without sockets, are on the outside of
its head. All of our common barnyard fowls—turkeys, ducks, geese,
and chickens—are flightless, but the varieties from which the
domesticated forms have come all have functional wings, two of these
varieties crossing continents in their annual migrations.
Not only animals, but plants also, many of them, have been greatly
changed by man in his efforts to adapt them to his uses as food,
ornamentation, and the like. On the seaside cliffs of Chili and Peru
may still be found growing the wild-potato—the small, tough,
bitter ancestor of the mammoth Burbank, Peerless, Early Rose, and
the nearly two hundred other varieties of this matchless tuber found
in the gardens of civilised man. The cabbage, kale, cauliflower, and
kohlrabi are all modifications of the same wild species (_Brassica
oleracea_), the cauliflower being the developed flower, kohlrabi the
stalk, and kale and cabbage the leaves. The peach and the almond,
Darwin thinks, have also come from a common ancestral drupe, the
peach being the developed fruit, and the almond the seed. There are
nearly 900 different varieties of apples, varying in the most
wonderful manner in size, colour, flavour, texture, and shape, but
all of them probably derived from the little, sour, inedible Asiatic
crab. The many times ‘double’ roses of our gardens have come
from the five-petalled wild-rose of the prairies. The cultivated
varieties of viburnum and hydrangea have showy corymbs of infertile
flowers only, but the wild forms from which the domestic varieties
have been derived have only a single marginal row of showy infertile
flowers surrounding a mass of inconspicuous fertile flowers. It has
been due to their efforts to please men that bananas, pineapples,
and oranges have got into the habit of neglecting to produce seeds.
There are certain species of grapes that are seedless, also seedless
sugar-cane, and a seedless apple has just been announced by
horticulturists. The development of domesticated plants is only in
its infancy, and it is probably impossible even for the most agile
imagination to dream of the miracles the horticulturist is destined
to work in the ages to come. There is every reason to believe that
seedless varieties of all our common fruits will ultimately be
produced, and that in size, flavour, nutrient constituents, and
appearance, they will be developed into forms utterly different from
existing varieties. Just within the last few years the U.S.
Department of Agriculture has developed a cotton-plant immune to the
bacterial diseases of the soil, which had completely driven the
cotton-raising industry out of large districts of the South. The
cultivation of many of the cereals has gone on so long, and has
proceeded so far, that their origin is lost in antiquity.
Whether or not it is possible for new varieties and species to be
evolved is a question, therefore, which does not need to depend for
reply wholly upon theory. It is known to have taken place; and the
process by which the different varieties of domestic animals and
plants have been evolved—domestic selection—is not different in
principle from the process of natural selection, the chief operation
by which life in general, both plant and animal, is assumed to have
been evolved.
10. There are other reasons for a belief in organic evolution, but
the last one I shall mention is the fact that the theory of organic
evolution harmonises with the known tendencies of the universe as a
whole. The organic kingdoms of the earth—animals and plants—are
as truly parts of the terrestrial globe as the inorganic kingdom is;
and as such they share in, and are actuated by, the same great
tendency or instinct as that which actuates the whole. Nine-tenths
of the substance of all animals and plants is oxygen, hydrogen,
carbon, and nitrogen—the very elements which make up the entire
ocean and air, and enter largely into the composition of the
continents. The human body, which has essentially the same chemical
composition as the bodies of animals in general, is made up of four
solids, five gases, and seven metals—in all, sixteen elements of
the something like seventy which constitute the entire planet. ‘In
the past, man appeared to be a creature foreign to the earth, and
placed upon it as a transitory inhabitant by some incomprehensible
power. The more perfect insight of the present day sees man as a
being whose development has taken place in accordance with the same
laws as those that have governed the development of the earth and
its entire organisation—a being not put upon the earth
accidentally by an arbitrary act, but produced in harmony with the
earth’s nature, and belonging to it as do the flowers and the
fruits to the tree which bears them.’ Animals are not outside of,
nor distinct from, the universe, as one might suspect who has
listened much to the recital of tradition so long accepted as
science. They are more or less detached portions of the planet earth
which move over its surfaces and through its fluids and multiply,
but which in their phenomena obey the same laws of chemistry and
physics as those in accordance with which the rest of the universe
acts. Animals are moulds through which digressing matters from the
soil, sea, and sky pass on rounds of eternal itineracy.
Now, the earth as a planet is in process of evolution. Not many
things are more certain than this. The earth has come out of fire.
It has _grown_ to be what it is. Its mountains, valleys, plains,
seas, shores, islands, lakes, rivers, and continents—these were
not always here. They have been evolved. Not only the earth, but the
entire family of spheres of which the earth is a member—the solar
system—are all evolving. Mr. Spencer never did anything more
profound than when he demonstrated in his ‘Law and Cause of
Progress’ the universal migration of things from a condition of
homogeneity toward a condition of greater and greater heterogeneity.
The whole universe, or as much of it as can be examined by
terrestrial instruments, has probably evolved out of the same
primordial matters. The organic part of the earth has evolved,
therefore, and is destined to continue to evolve, because it is a
part of a whole whose habit or ambition it is to evolve.
The evidence is overwhelming. The theory of organic evolution is
sustained by a mass of facts not less authoritative and convincing
than that which supports the Copernican theory of the worlds.
Evolution is, in fact, a doctrine so apparent that it only needs to
be honestly and intelligently looked into to be accepted
unreservedly. It is, indeed, _more_ than a _doctrine_. It is a
_known_ fact. It is a _necessary effect_ of the _conditions known to
exist_ among the animals and plants of the earth. If beings _vary_
among themselves generation after generation, if only the _fittest_
of each generation _survive_ and if the survivors tend to _transmit_
to their offspring the qualities of their superiority (and the
animals and plants of the earth are known to do continually all of
these things), then it follows _with mathematical certainty_ that
evolution is going on, and that it will continue to go on as long as
these conditions continue. It is inevitable. It could not be
otherwise. We would _know_ that evolution were going on among
organisms where these conditions existed, even though we had never
observed it.
The boldest and most enthusiastic opponents of evolution have always
been those with the least information about it. But the evidence is
accumulating so rapidly, and is being drawn up in such unanswerable
array, that, if it is not already the case, it will not be many
years before it will be an intellectual reproach for anyone to
discredit, or to be known to have discredited, this splendid and
inspiring revelation.
1. Darwin: _Descent of Man_, 2nd edit.; London, 1874.
2. Huxley: _Man’s Place in Nature_; New York, 1883.
3. Thompson: _Outlines of Zoology_, 3rd edit.; Edinburgh, 1899.
4. Drummond: _Ascent of Man_; New York, 1894.
5. See table of geological ages, at the end of the chapter.
6. Haeckel: _The Riddle of the Universe_; New York, 1901.
7. Huxley: _On the Origin of Species_, lecture iv.