Darwinism (1889) by Alfred Russel Wallace

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"The heel of the horse is the part commonly known as the hock.
The hinder cannon bone answers to the middle metatarsal bone of
the human foot, the pastern, coronary, and coffin bones, to the
middle toe bones; the hind hoof to the nail; as in the forefoot.
And, as in the forefoot, there are merely two splints to
represent the second and the fourth toes. Sometimes a rudiment
of a fifth toe appears to be traceable.

"The teeth of a horse are not less peculiar than its limbs. The
living engine, like all others, must be well stoked if it is to
do its work; and the horse, if it is to make good its wear and
tear, and to exert the enormous amount of force required for its
propulsion, must be well and rapidly fed. To this end, good
cutting instruments and powerful and lasting crushers are
needful. Accordingly, the twelve cutting teeth of a horse are
close-set and concentrated in the forepart of its mouth, like so
many adzes or chisels. The grinders or molars are large, and
have an extremely complicated structure, being composed of a
number of different substances of unequal hardness. The
consequence of this is that they wear away at different rates;
and, hence, the surface of each grinder is always as uneven as
that of a good millstone."[186]

We thus see that the Equidae differ very widely in structure from most
other mammals. Assuming the truth of the theory of evolution, we should
expect to find traces among extinct animals of the steps by which this
great modification has been effected; and we do really find traces of
these steps, imperfectly among European fossils, but far more completely
among those of America.

It is a singular fact that, although no horse inhabited America when
discovered by Europeans, yet abundance of remains of extinct horses have
been found both in North and South America in Post-Tertiary and Upper
Pliocene deposits; and from these an almost continuous series of
modified forms can be traced in the Tertiary formation, till we reach,
at the very base of the series, a primitive form so unlike our perfected
animal, that, had we not the intermediate links, few persons would
believe that the one was the ancestor of the other. The tracing out of
this marvellous history we owe chiefly to Professor Marsh of Yale
College, who has himself discovered no less than thirty species of
fossil Equidae; and we will allow him to tell the story of the
development of the horse from a humble progenitor in his own words.

"The oldest representative of the horse at present known is the
diminutive Eohippus from the Lower Eocene. Several species have
been found, all about the size of a fox. Like most of the early
mammals, these ungulates had forty-four teeth, the molars with
short crowns and quite distinct in form from the premolars. The
ulna and fibula were entire and distinct, and there were four
well-developed toes and a rudiment of another on the forefeet,
and three toes behind. In the structure of the feet and teeth,
the Eohippus unmistakably indicates that the direct ancestral
line to the modern horse has already separated from the other
perissodactyles, or odd-toed ungulates.

"In the next higher division of the Eocene another genus,
Orohippus, makes its appearance, replacing Eohippus, and showing
a greater, though still distant, resemblance to the equine type.
The rudimentary first digit of the forefoot has disappeared, and
the last premolar has gone over to the molar series. Orohippus
was but little larger than Eohippus, and in most other respects
very similar. Several species have been found, but none occur
later than the Upper Eocene.

"Near the base of the Miocene, we find a third closely allied
genus, Mesohippus, which is about as large as a sheep, and one
stage nearer the horse. There are only three toes and a
rudimentary splint on the forefeet, and three toes behind. Two
of the premolar teeth are quite like the molars. The ulna is no
longer distinct or the fibula entire, and other characters show
clearly that the transition is advancing.

"In the Upper Miocene Mesohippus is not found, but in its place
a fourth form, Miohippus, continues the line. This genus is near
the Anchitherium of Europe, but presents several important
differences. The three toes in each foot are more nearly of a
size, and a rudiment of the fifth metacarpal bone is retained.
All the known species of this genus are larger than those of
Mesohippus, and none of them pass above the Miocene formation.

"The genus Protohippus of the Lower Pliocene is yet more equine,
and some of its species equalled the ass in size. There are
still three toes on each foot, but only the middle one,
corresponding to the single toe of the horse, comes to the
ground. This genus resembles most nearly the Hipparion of
Europe.

"In the Pliocene we have the last stage of the series before
reaching the horse, in the genus Pliohippus, which has lost the
small hooflets, and in other respects is very equine. Only in
the Upper Pliocene does the true Equus appear and complete the
genealogy of the horse, which in the Post-Tertiary roamed over
the whole of North and South America, and soon after became
extinct. This occurred long before the discovery of the
continent by Europeans, and no satisfactory reason for the
extinction has yet been given. Besides the characters I have
mentioned, there are many others in the skeleton, skull, teeth,
and brain of the forty or more intermediate species, which show
that the transition from the Eocene Eohippus to the modern Equus
has taken place in the order indicated"[187] (see Fig. 33).

[Illustration: FIG. 33.--Geological development of the horse tribe
(Eohippus since discovered).]

Well may Professor Huxley say that this is demonstrative evidence of
evolution; the doctrine resting upon exactly as secure a foundation as
did the Copernican theory of the motions of the heavenly bodies at the
time of its promulgation. Both have the same basis--the coincidence of
the observed facts with the theoretical requirements.

_Development of Deer's Horns._

Another clear and unmistakable proof of evolution is afforded by one of
the highest and latest developed tribes of mammals--the true deer. These
differ from all other ruminants in possessing solid deciduous horns
which are always more or less branched. They first appear in the Middle
Miocene formation, and continue down to our time; and their development
has been carefully traced by Professor Boyd Dawkins, who thus summarises
his results:--

"In the middle stage of the Miocene the cervine antler consists
merely of a simple forked crown (as in Cervus dicroceros), which
increases in size in the Upper Miocene, although it still
remains small and erect, like that of the roe. In Cervus
Matheroni it measures 11.4 inches, and throws off not more than
four tines, all small. The deer living in Auvergne in the
succeeding or Pliocene age, present us with another stage in the
history of antler development. There, for the first time, we see
antlers of the Axis and Rusa type, larger and longer, and more
branching than any antlers were before, and possessing three or
more well-developed tines. Deer of this type abounded in
Pliocene Europe. They belong to the Oriental division of the
Cervidae, and their presence in Europe confirms the evidence of
the flora, brought forward by the Comte de Saporta, that the
Pliocene climate was warm. They have probably disappeared from
Europe in consequence of the lowering of the temperature in the
Pleistocene age, while their descendants have found a congenial
home in the warmer regions of Eastern Asia.

"In the latest stage of the Pliocene--the Upper Pliocene of the
Val d'Arno--the Cervus dicranios of Nesti presents us with
antlers much smaller than those of the Irish elk, but very
complicated in their branching. This animal survived into the
succeeding age, and is found in the pre-glacial forest bed of
Norfolk, being described by Dr. Falconer under the name of
Sedgwick's deer. The Irish elk, moose, stag, reindeer, and
fallow deer appear in Europe in the Pleistocene age, all with
highly complicated antlers in the adult, and the first
possessing the largest antlers yet known. Of these the Irish elk
disappeared in the Prehistoric age, after having lived in
countless herds in Ireland, while the rest have lived on into
our own times in Euro-Asia, and, with the exception of the last,
also in North America.

"From this survey it is obvious that the cervine antlers have
increased in size and complexity from the Mid-Miocene to the
Pleistocene age, and that their successive changes are analogous
to those which are observed in the development of antlers in the
living deer, which begin with a simple point, and increase in
number of tines till their limit of growth be reached. In other
words, the development of antlers indicated at successive and
widely-separated pages of the geological record is the same as
that observed in the history of a single living species. It is
also obvious that the progressive diminution of size and
complexity in the antlers, from the present time back into the
early Tertiary age, shows that we are approaching the zero of
antler development in the Mid-Miocene. No trace of any
antler-bearing ruminant has been met with in the lower Miocenes,
either of Europe or the United States."[188]

_Progressive Brain-Development._

The three illustrations now given sufficiently prove that, whenever the
geological record approaches to completeness, we have evidence of the
progressive change of species in definite directions, and from less
developed to more developed types--exactly such a change as we may
expect to find if the evolution theory be the true one. Many other
illustrations of a similar change could be given, but the animal groups
in which they occur being less familiar, the details would be less
interesting, and perhaps hardly intelligible. There is, however, one
very remarkable proof of development that must be briefly noticed--that
afforded by the steady increase in the size of the brain. This may be
best stated in the words of Professor Marsh:--

"The real progress of mammalian life in America, from the
beginning of the Tertiary to the present, is well illustrated by
the brain-growth, in which we have the key to many other
changes. The earliest known Tertiary mammals all had very small
brains, and in some forms this organ was proportionally less
than in certain reptiles. There was a gradual increase in the
size of the brain during this period, and it is interesting to
find that this growth was mainly confined to the cerebral
hemispheres, or higher portion of the brain. In most groups of
mammals the brain has gradually become more convoluted, and thus
increased in quality as well as quantity. In some also the
cerebellum and olfactory lobes, the lower parts of the brain,
have even diminished in size. In the long struggle for existence
during Tertiary time the big brains won, then as now; and the
increasing power thus gained rendered useless many structures
inherited from primitive ancestors, but no longer adapted to new
conditions."

This remarkable proof of development in the organ of the mental
faculties, forms a fitting climax to the evidence already adduced of the
progressive evolution of the general structure of the body, as
illustrated by the bony skeleton. We now pass on to another class of
facts equally suggestive of evolution.

_The Local Relations of Fossil and Living Animals._

If all existing animals have been produced from ancestral forms--mostly
extinct--under the law of variation and natural selection, we may expect
to find in most cases a close relation between the living forms of each
country and those which inhabited it in the immediately preceding epoch.
But if species have originated in some quite different way, either by
any kind of special creation, or by sudden advances of organisation in
the offspring of preceding types, such close relationship would not be
found; and facts of this kind become, therefore, to some extent a test
of evolution under natural selection or some other law of gradual
change. Of course the relationship will not appear when extensive
migration has occurred, by which the inhabitants of one region have been
able to take possession of another region, and destroy or drive out its
original inhabitants, as has sometimes happened. But such cases are
comparatively rare, except where great changes of climate are known to
have occurred; and we usually do find a remarkable continuity between
the existing fauna and flora of a country and those of the immediately
preceding age. A few of the more remarkable of these cases will now be
briefly noticed.

The mammalian fauna of Australia consists, as is well known, wholly of
the lowest forms--the Marsupials and Monotremata--except only a few
species of mice. This is accounted for by the complete isolation of the
country from the Asiatic continent during the whole period of the
development of the higher animals. At some earlier epoch the ancestral
marsupials, which abounded both in Europe and North America in the
middle of the Secondary period, entered the country, and have since
remained there, free from the competition of higher forms, and have
undergone a special development in accordance with the peculiar
conditions of a limited area. While in the large continents higher forms
of mammalia have been developed, which have almost or wholly
exterminated the less perfect marsupials, in Australia these latter have
become modified into such varied forms as the leaping kangaroos, the
burrowing wombats, the arboreal phalangers, the insectivorous
bandicoots, and the carnivorous Dasyuridae or native cats, culminating
in the Thylacinus or "tiger-wolf" of Tasmania--animals as unlike each
other as our sheep, rabbits, squirrels, and dogs, but all retaining the
characteristic features of the marsupial type.

Now in the caves and late Tertiary or Post-Tertiary deposits of
Australia the remains of many extinct mammalia have been found, but all
are marsupials. There are many kangaroos, some larger than any living
species, and others more allied to the tree-kangaroos of New Guinea; a
large wombat as large as a tapir; the Diprotodon, a thick-limbed
kangaroo the size of a rhinoceros or small elephant; and a quite
different animal, the Nototherium, nearly as large. The carnivorous
Thylacinus of Tasmania is also found fossil; and a huge phalanger,
Thylacoleo, the size of a lion, believed by Professor Owen and by
Professor Oscar Schmidt to have been equally carnivorous and
destructive.[189] Besides these, there are many other species more
resembling the living forms both in size and structure, of which they
may be, in some cases, the direct ancestors. Two species of extinct
Echidna, belonging to the very low Monotremata, have also been found in
New South Wales.

Next to Australia, South America possesses the most remarkable
assemblage of peculiar mammals, in its numerous Edentata--the sloths,
ant-eaters, and armadillos; its rodents, such as the cavies and
chinchillas; its marsupial opossums, and its quadrumana of the family
Cebidae. Remains of extinct species of all these have been found in the
caves of Brazil, of Post-Pliocene age; while in the earlier Pliocene
deposits of the pampas many distinct genera of these groups have been
found, some of gigantic size and extraordinary form. There are
armadillos of many types, some being as large as elephants; gigantic
sloths of the genera Megatherium, Megalonyx, Mylodon, Lestodon, and many
others; rodents belonging to the American families Cavidae and
Chinchillidae; and ungulates allied to the llama; besides many other
extinct forms of intermediate types or of uncertain affinities.[190] The
extinct Moas of New Zealand--huge wingless birds allied to the living
Apteryx--illustrate the same general law.

The examples now quoted, besides illustrating and enforcing the general
fact of evolution, throw some light on the usual character of the
modification and progression of animal forms. In the cases where the
geological record is tolerably complete, we find a continuous
development of some kind--either in complexity of ornamentation, as in
the fossil Paludinas of the Hungarian lake-basins; in size and in the
specialisation of the feet and teeth, as in the American fossil horses;
or in the increased development of the branching horns, as in the true
deer. In each of these cases specialisation and adaptation to the
conditions of the environment appear to have reached their limits, and
any change of these conditions, especially if it be at all rapid or
accompanied by the competition of less developed but more adaptable
forms, is liable to cause the extinction of the most highly developed
groups. Such we know was the case with the horse tribe in America, which
totally disappeared in that continent at an epoch so recent that we
cannot be sure that the disappearance was not witnessed, perhaps caused,
by man; while even in the Eastern hemisphere it is the smaller
species--the asses and the zebras--that have persisted, while the larger
and more highly developed true horses have almost, if not quite,
disappeared in a state of nature. So we find, both in Australia and
South America, that in a quite recent period many of the largest and
most specialised forms have become extinct, while only the smaller types
have survived to our day; and a similar fact is to be observed in many
of the earlier geological epochs, a group progressing and reaching a
maximum of size or complexity and then dying out, or leaving at most but
few and pigmy representatives.

_Cause of Extinction of Large Animals._

Now there are several reasons for the repeated extinction of large
rather than of small animals. In the first place, animals of great bulk
require a proportionate supply of food, and any adverse change of
conditions would affect them more seriously than it would smaller
animals. In the next place, the extreme specialisation of many of these
large animals would render it less easy for them to be modified in any
new direction suited to changed conditions. Still more important,
perhaps, is the fact that very large animals always increase slowly as
compared with small ones--the elephant producing a single young one
every three years, while a rabbit may have a litter of seven or eight
young two or three times a year. Now the probability of favourable
variations will be in direct proportion to the population of the
species, and as the smaller animals are not only many hundred times more
numerous than the largest, but also increase perhaps a hundred times as
rapidly, they are able to become quickly modified by variation and
natural selection in harmony with changed conditions, while the large
and bulky species, being unable to vary quickly enough, are obliged to
succumb in the struggle for existence. As Professor Marsh well observes:
"In every vigorous primitive type which was destined to survive many
geological changes, there seems to have been a tendency to throw off
lateral branches, which became highly specialised and soon died out,
because they were unable to adapt themselves to new conditions." And he
goes on to show how the whole narrow path of the persistent Suilline
type, throughout the entire series of the American tertiaries, is
strewed with the remains of such ambitious offshoots, many of them
attaining the size of a rhinoceros; "while the typical pig, with an
obstinacy never lost, has held on in spite of catastrophes and
evolution, and still lives in America to-day."

_Indications of General Progression in Plants and Animals._

One of the most powerful arguments formerly adduced against evolution
was, that geology afforded no evidence of the gradual development of
organic forms, but that whole tribes and classes appeared suddenly at
definite epochs, and often in great variety and exhibiting a very
perfect organisation. The mammalia, for example, were long thought to
have first appeared in Tertiary times, where they are represented in
some of the earlier deposits by all the great divisions of the class
fully developed--carnivora, rodents, insectivora, marsupials, and even
the perissodactyle and artiodactyle divisions of the ungulata--as
clearly defined as at the present day. The discovery in 1818 of a single
lower jaw in the Stonesfield Slate of Oxfordshire hardly threw doubt on
the generalisation, since either its mammalian character was denied, or
the geological position of the strata, in which it was found, was held
to have been erroneously determined. But since then, at intervals of
many years, other remains of mammalia have been discovered in the
Secondary strata, ranging from the Upper Oolite to the Upper Trias both
in Europe and the United States, and one even (Tritylodon) in the Trias
of South Africa. All these are either marsupials, or of some still lower
type of mammalia; but they consist of many distinct forms classed in
about twenty genera. Nevertheless, a great gap still exists between
these mammals and those of the Tertiary strata, since no mammal of any
kind has been found in any part of the Cretaceous formation, although in
several of its subdivisions abundance of land plants, freshwater shells,
and air-breathing reptiles have been discovered. So with fishes. In the
last century none had been obtained lower than the Carboniferous
formation; thirty years later they were found to be very abundant in the
Devonian rocks, and later still they were discovered in the Upper Ludlow
and Lower Ludlow beds of the Silurian formation.

We thus see that such sudden appearances are deceptive, and are, in
fact, only what we ought to expect from the known imperfection of the
geological record. The conditions favourable to the fossilisation of any
group of animals occur comparatively rarely, and only in very limited
areas; while the conditions essential for their permanent preservation
in the rocks, amid all the destruction caused by denudation or
metamorphism, are still more exceptional. And when they are thus
preserved to our day, the particular part of the rocks in which they lie
hidden may not be on the surface but buried down deep under other
strata, and may thus, except in the case of mineral-bearing deposits, be
altogether out of our reach. Then, again, how large a proportion of the
earth consists of wild and uncivilised regions in which no exploration
of the rocks has been yet made, so that whether we shall find the
fossilised remains of any particular group of animals which lived during
a limited period of the earth's history, and in a limited area, depends
upon at least a fivefold combination of chances. Now, if we take each of
these chances separately as only ten to one against us (and some are
certainly more than this), then the actual chance against our finding
the fossil remains, say of any one order of mammalia, or of land plants,
at any particular geological horizon, will be about a hundred thousand
to one.

It may be said, if the chances are so great, how is it that we find such
immense numbers of fossil species exceeding in number, in some groups,
all those that are now living? But this is exactly what we should
expect, because the number of species of organisms that have ever lived
upon the earth, since the earliest geological times, will probably be
many hundred times greater than those now existing of which we have any
knowledge; and hence the enormous gaps and chasms in the geological
record of extinct forms is not to be wondered at. Yet, notwithstanding
these chasms in our knowledge, if evolution is true, there ought to have
been, on the whole, progression in all the chief types of life. The
higher and more specialised forms should have come into existence later
than the lower and more generalised forms; and however fragmentary the
portions we possess of the whole tree of life upon the earth, they ought
to show us broadly that such a progressive evolution has taken place. We
have seen that in some special groups, already referred to, such a
progression is clearly visible, and we will now cast a hasty glance over
the entire series of fossil forms, in order to see if a similar
progression is manifested by them as a whole.

_The Progressive Development of Plants._

Ever since fossil plants have been collected and studied, the broad fact
has been apparent that the early plants--those of the Coal
formation--were mainly cryptogamous, while in the Tertiary deposits the
higher flowering plants prevailed. In the intermediate secondary epoch
the gymnosperms--cycads and coniferae--formed a prominent part of the
vegetation, and as these have usually been held to be a kind of
transition form between the flowerless and flowering plants, the
geological succession has always, broadly speaking, been in accordance
with the theory of evolution. Beyond this, however, the facts were very
puzzling. The highest cryptogams--ferns, lycopods, and
equisetaceae--appeared suddenly, and in immense profusion in the Coal
formation, at which period they attained a development they have never
since surpassed or even equalled; while the highest plants--the
dicotyledonous and monocotyledonous angiosperms--which now form the bulk
of the vegetation of the world, and exhibit the most wonderful
modifications of form and structure, were almost unknown till the
Tertiary period, when they suddenly appeared in full development, and,
for the most part, under the same generic forms as now exist.