The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

descent--the lines of which are in most cases perfectly clear

and obvious--from the simple and unornamented Vivipara

achatinoides of the Congerien-Schichten (the lower division of

the series of strata). It is interesting to notice that a large

portion of these unquestionably derived forms depart so widely

from the type of the genus Vivipara, that they have been

separated on so high an authority as that of Sandberger, as a

new genus, under the name of Tulotoma. And hence we are led to

the conclusion that a vast number of forms, certainly exhibiting

specific distinctions, and according to some naturalists,

differences even entitled to be regarded of generic value, have

all a common ancestry."

It is, as Professor Judd remarks, owing to the exceptionally favourable

circumstances of a long-continued and unbroken series of deposits being

formed under physical conditions either identical or very slowly

changing, that we owe so complete a record of the process of organic

change. Usually, some disturbing elements, such as a sudden change of

physical conditions, or the immigration of new sets of forms from other

areas and the consequent retreat or partial extinction of the older

fauna, interferes with the continuity of organic development, and

produces those puzzling discordances so generally met with in geological

formations of marine origin. While a case of the kind now described

affords evidence of the origin of species complete and conclusive,

though on a necessarily very limited scale, the very rarity of the

conditions which are essential to such completeness serves to explain

why it is that in most cases the direct evidence of evolution is not to

be obtained.

is afforded by Professor Huxley's researches on fossil crocodiles. The

gap between the existing crocodiles and the lizards is very wide, but as

we go back in geological time we meet with fossil forms which are to

some extent intermediate and form a connected series. The three living

genera--Crocodilus, Alligator, and Gavialis--are found in the Eocene

formation, and allied forms of another genus, Holops, in the Chalk. From

the Chalk backward to the Lias another group of genera occurs, having

anatomical characteristics intermediate between the living crocodiles

and the most ancient forms. These, forming two genera Belodon and

Stagonolepis, are found in a still older formation, the Trias. They have

characters resembling some lizards, especially the remarkable Hatteria

of New Zealand, and have also some resemblances to the

Dinosaurians--reptiles which in some respects approach birds.

Considering how comparatively few are the remains of this group of

animals, the evidence which it affords of progressive development is

remarkably clear.[184]
Among the higher animals the rhinoceros, the horse, and the deer afford

good evidence of advance in organisation and of the filling up of the

gaps which separate the living forms from their nearest allies. The

earliest ancestral forms of the rhinoceroses occur in the Middle Eocene

of the United States, and were to some extent intermediate between the

rhinoceros and tapir families, having like the latter four toes to the

front feet, and three to those behind. These are followed in the Upper

Eocene by the genus Amynodon, in which the skull assumes more distinctly

the rhinocerotic type. Following this in the Lower Miocene we have the

Aceratherium, like the last in its feet, but still more decidedly a

rhinoceros in its general structure. From this there are two diverging

lines--one in the Old World, the other in the New. In the former, to

which the Aceratherium is supposed to have migrated in early Miocene

times, when a mild climate and luxuriant vegetation prevailed far within

the arctic circle, it gave rise to the Ceratorhinus and the various

horned rhinoceroses of late Tertiary times and of those now living. In

America a number of large hornless rhinoceroses were developed--they

are found in the Upper Miocene, Pliocene, and Post-Pliocene

formations--and then became extinct. The true rhinoceroses have three

toes on all the feet.[185]

_The Pedigree of the Horse Tribe._
Yet more remarkable is the evidence afforded by the ancestral forms of

the horse tribe which have been discovered in the American tertiaries.

The family Equidae, comprising the living horse, asses, and zebras,

differ widely from all other mammals in the peculiar structure of the

feet, all of which terminate in a single large toe forming the hoof.

They have forty teeth, the molars being formed of hard and soft material

in crescentic folds, so as to be a powerful agent in grinding up hard

grasses and other vegetable food. The former peculiarities depend upon

modifications of the skeleton, which have been thus described by

Professor Huxley:--

"Let us turn in the first place to the fore-limb. In most

quadrupeds, as in ourselves, the fore-arm contains distinct

bones, called the radius and the ulna. The corresponding region

in the horse seems at first to possess but one bone. Careful

observation, however, enables us to distinguish in this bone a

part which clearly answers to the upper end of the ulna. This is

closely united with the chief mass of the bone which represents

the radius, and runs out into a slender shaft, which may be

traced for some distance downwards upon the back of the radius,

and then in most cases thins out and vanishes. It takes still

more trouble to make sure of what is nevertheless the fact, that

a small part of the lower end of the bone of a horse's fore-arm,

which is only distinct in a very young foal, is really the lower

extremity of the ulna.

'cannon bone' answers to the middle bone of the five metacarpal

bones which support the palm of the hand in ourselves. The

pastern, coronary, and coffin bones of veterinarians answer to

the joints of our middle fingers, while the hoof is simply a

greatly enlarged and thickened nail. But if what lies below the

horse's 'knee' thus corresponds to the middle finger in

ourselves, what has become of the four other fingers or digits?

We find in the places of the second and fourth digits only two

slender splintlike bones, about two-thirds as long as the cannon

bone, which gradually taper to their lower ends and bear no

finger joints, or, as they are termed, phalanges. Sometimes,

small bony or gristly nodules are to be found at the bases of

these two metacarpal splints, and it is probable that these

represent rudiments of the first and fifth toes. Thus, the part

of the horse's skeleton which corresponds with that of the human

hand, contains one overgrown middle digit, and at least two

imperfect lateral digits; and these answer, respectively, to the

third, the second, and the fourth fingers in man.
"Corresponding modifications are found in the hind limb. In

ourselves, and in most quadrupeds, the leg contains two distinct

bones, a large bone, the tibia, and a smaller and more slender

bone, the fibula. But, in the horse, the fibula seems, at first,

to be reduced to its upper end; a short slender bone united with

the tibia, and ending in a point below, occupying its place.

Examination of the lower end of a young foal's shin-bone,

however, shows a distinct portion of osseous matter which is the

lower end of the fibula; so that the, apparently single, lower

end of the shin-bone is really made up of the coalesced ends of

the tibia and fibula, just as the, apparently single, lower end

of the fore-arm bone is composed of the coalesced radius and

ulna.
"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.

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.

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).]

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._

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.

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._

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._

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 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.

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