Discourses: Biological and Geological — Palaeontology and the Doctrine of Evolution Pt 3

Such is the further commentary which I have to offer upon the statement of the chief results of palaeontology which I formerly ventured to lay before you.

But the growth of knowledge in the interval makes me conscious of an omission of considerable moment in that statement, inasmuch as it contains no reference to the bearings of palaeontology upon the theory of the distribution of life; nor takes note of the remarkable manner in which the facts of distribution, in present and past times, accord with the doctrine of evolution, especially in regard to land animals.

That connection between palaeontology and geology and the present distribution of terrestrial animals, which so strikingly impressed Mr. Darwin, thirty years ago, as to lead him to speak of a "law of succession of types," and of the wonderful relationship on the same continent between the dead and the living, has recently received much elucidation from the researches of Gaudry, of Rutimeyer, of Leidy, and of Alphonse Milne-Edwards, taken in connection with the earlier labours of our lamented colleague Falconer; and it has been instructively discussed in the thoughtful and ingenious work of Mr. Andrew Murray "On the Geographical Distribution of Mammals."

[5] [Footnote 5: The paper "On the Form and Distribution of the Landtracts during the Secondary and Tertiary Periods respectively; and on the Effect upon Animal Life which great Changes in Geographical Configuration have probably produced," by Mr. Searles V. Wood, jun.

, which was published in the _Philosophical Magazine_, in 1862, was unknown to me when this Address was written.

It is well worthy of the most careful study.

] I propose to lay before you, as briefly as I can, the ideas to which a long consideration of the subject has given rise in my mind.

If the doctrine of evolution is sound, one of its immediate consequences clearly is, that the present distribution of life upon the globe is the product of two factors, the one being the distribution which obtained in the immediately preceding epoch, and the other the character and the extent of the changes which have taken place in physical geography between the one epoch and the other; or, to put the matter in another way, the Fauna and Flora of any given area, in any given epoch, can consist only of such forms of life as are directly descended from those which constituted the Fauna and Flora of the same area in the immediately preceding epoch, unless the physical geography (under which I include climatal conditions) of the area has been so altered as to give rise to immigration of living forms from some other area.

The evolutionist, therefore, is bound to grapple with the following problem whenever it is clearly put before him:--Here are the Faunae of the same area during successive epochs.

Show good cause for believing either that these Faunae have been derived from one another by gradual modification, or that the Faunae have reached the area in question by migration from some area in which they have undergone their development.

I propose to attempt to deal with this problem, so far as it is exemplified by the distribution of the terrestrial _Vertebrata_, and I shall endeavour to show you that it is capable of solution in a sense entirely favourable to the doctrine of evolution.

I have elsewhere[6] stated at length the reasons which lead me to recognise four primary distributional provinces for the terrestrial _Vertebrata_ in the present world, namely,--first, the _Novozelanian_, or New-Zealand province; secondly, the _Australian_ province, including Australia, Tasmania, and the Negrito Islands; thirdly, _Austro-Columbia_, or South America _plus_ North America as far as Mexico; and fourthly, the rest of the world, or _Arctogoea_, in which province America north of Mexico constitutes one sub-province, Africa south of the Sahara a second, Hindostan a third, and the remainder of the Old World a fourth.

[Footnote 6: "On the Classification and Distribution of the Alectoromorphoe;" _Proceedings of the Zoological Society_, 1868. ] Now the truth which Mr. Darwin perceived and promulgated as "the law of the succession of types" is, that, in all these provinces, the animals found in Pliocene or later deposits are closely affined to those which now inhabit the same provinces; and that, conversely, the forms characteristic of other provinces are absent.

North and South America, perhaps, present one or two exceptions to the last rule, but they are readily susceptible of explanation.

Thus, in Australia, the later Tertiary mammals are marsupials (possibly with the exception of the Dog and a Rodent or two, as at present).

In Austro-Columbia, the later Tertiary fauna exhibits numerous and varied forms of Platyrrhine Apes, Rodents, Cats, Dogs, Stags, _Edentata_, and Opossums; but, as at present, no Catarrhine Apes, no Lemurs, no _Insectivora_, Oxen, Antelopes, Rhinoceroses, nor _Didelphia_ other than Opossums.

And in the widespread Arctogaeal province, the Pliocene and later mammals belong to the same groups as those which now exist in the province.

The law of succession of types, therefore, holds good for the present epoch as compared with its predecessor.

Does it equally well apply to the Pliocene fauna when we compare it with that of the Miocene epoch?

By great good fortune, an extensive mammalian fauna of the latter epoch has now become known, in four very distant portions of the Arctogaeal province which do not differ greatly in latitude.

Thus Falconer and Cautley have made known the fauna of the sub-Himalayas and the Perim Islands; Gaudry that of Attica; many observers that of Central Europe and France; and Leidy that of Nebraska, on the eastern flank of the Rocky Mountains.

The results are very striking.

The total Miocene fauna comprises many genera and species of Catarrhine Apes, of Bats, of _Insectivora_; of Arctogaeal types of _Rodentia_; of _Proboscidea_; of equine, rhinocerotic, and tapirine quadrupeds; of cameline, bovine, antilopine, cervine, and traguline Ruminants; of Pigs and Hippopotamuses; of _Viverridoe_ and _Hyoenidoe_ among other _Carnivora_; with _Edentata_ allied to the Aretogaeal _Oryeteropus_ and _Manis_, and not to the Austro-Columbian Edentates.

The only type present in the Miocene, but absent in the existing, fauna of Eastern Arctogaea, is that of the _Didelphidoe_, which, however, remains in North America.

But it is very remarkable that while the Miocene fauna of the Arctogaeal province, as a whole, is of the same character as the existing fauna of the same province, as a whole, the component elements of the fauna were differently associated.

In the Miocene epoch, North America possessed Elephants, Horses, Rhinoceroses, and a great number and variety of Ruminants and Pigs, which are absent in the present indigenous fauna;

Europe had its Apes, Elephants, Rhinoceroses, Tapirs, Musk-deer, Giraffes, Hyaenas, great Cats, Edentates, and Opossum-like Marsupials, which have equally vanished from its present fauna; and in Northern India, the African types of Hippopotamuses, Giraffes, and Elephants were mixed up with what are now the Asiatic types of the latter, and with Camels, and Semnopithecine and Pithecine Apes of no less distinctly Asiatic forms.

In fact the Miocene mammalian fauna of Europe and the Himalayan regions contains, associated together, the types which are at present separately located in the South-African and Indian sub-provinces of Arctogaea.

Now there is every reason to believe, on other grounds, that both Hindostan, south of the Ganges, and Africa, south of the Sahara, were separated by a wide sea from Europe and North Asia during the Middle and Upper Eocene epochs.

Hence it becomes highly probable that the well-known similarities, and no less remarkable differences between the present Faunae of India and South Africa have arisen in some such fashion as the following.

Some time during the Miocene epoch, possibly when the Himalayan chain was elevated, the bottom of the nummulitic sea was upheaved and converted into dry land, in the direction of a line extending from Abyssinia to the mouth of the Ganges.

By this means, the Dekhan on the one hand, and South Africa on the other, became connected with the Miocene dry land and with one another.

The Miocene mammals spread gradually over this intermediate dry land; and if the condition of its eastern and western ends offered as wide contrasts as the valleys of the Ganges and Arabia do now, many forms which made their way into Africa must have been different from those which reached the Dekhan, while others might pass into both these sub-provinces.

That there was a continuity of dry land between Europe and North America during the Miocene epoch, appears to me to be a necessary consequence of the fact that many genera of terrestrial mammals, such as _Castor, Hystrix, Elephas, Mastodon, Equus, Hipparion, Anchitherium, Rhinoceros, Cervus, Amphicyon, Hyoenarctos_, and _Machairodus_, are common to the Miocene formations of the two areas, and have as yet been found (except perhaps _Anchitherium_) in no deposit of earlier age.

Whether this connection took place by the east, or by the west, or by both sides of the Old World, there is at present no certain evidence, and the question is immaterial to the present argument; but, as there are good grounds for the belief that the Australian province and the Indian and South-African sub-provinces were separated by sea from the rest of Arctogaea before the Miocene epoch, so it has been rendered no less probable, by the investigations of Mr. Carrick Moore and Professor Duncan, that Austro- Columbia was separated by sea from North America during a large part of the Miocene epoch.

It is unfortunate that we have no knowledge of the Miocene mammalian fauna of the Australian and Austro-Columbian provinces; but, seeing that not a trace of a Platyrrhine Ape, of a Procyonine Carnivore, of a characteristically South-American Rodent, of a Sloth, an Armadillo, or an Ant-eater has yet been found in Miocene deposits of Arctogaea, I cannot doubt that they already existed in the Miocene Austro-Columbian province.

Nor is it less probable that the characteristic types of Australian Mammalia were already developed in that region in Miocene times.

But Austro-Columbia presents difficulties from which Australia is free;

_Cantelidoe_ and _Tapirdoe_ are now indigenous in South America as they are in Arctogaea; and, among the Pliocene Austro-Columbian mammals, the Arctogaeal genera _Equus, Mastodon,_ and _Machairodus_ are numbered.

Are these Postmiocene immigrants, or Praemiocene natives?

Still more perplexing are the strange and interesting forms _Toxodon, Macrauchenia, Typotherium_, and a new Anoplotherioid mammal (_Homalodotherhon_) which Dr. Cunningham sent over to me some time ago from Patagonia.

I confess I am strongly inclined to surmise that these last, at any rate, are remnants of the population of Austro-Columbia before the Miocene epoch, and were not derived from Arctogaea by way of the north and east.

The fact that this immense fauna of Miocene Arctogaea is now fully and richly represented only in India and in South Africa, while it is shrunk and depauperised in North Asia, Europe, and North America, becomes at once intelligible, if we suppose that India and South Africa had but a scanty mammalian population before the Miocene immigration, while the conditions were highly favourable to the new comers.

It is to be supposed that these new regions offered themselves to the Miocene Ungulates, as South America and Australia offered themselves to the cattle, sheep, and horses of modern colonists.

But, after these great areas were thus peopled, came the Glacial epoch, during which the excessive cold, to say nothing of depression and ice-covering, must have almost depopulated all the northern parts of Arctogaea, destroying all the higher mammalian forms, except those which, like the Elephant and Rhinoceros, could adjust their coats to the altered conditions.

Even these must have been driven away from the greater part of the area; only those Miocene mammals which had passed into Hindostan and into South Africa would escape decimation by such changes in the physical geography of Arctogaea.

And when the northern hemisphere passed into its present condition, these lost tribes of the Miocene Fauna were hemmed by the Himalayas, the Sahara, the Red Sea, and the Arabian deserts, within their present boundaries.

Now, on the hypothesis of evolution, there is no sort of difficulty in admitting that the differences between the Miocene forms of the mammalian Fauna and those which exist at present are the results of gradual modification; and, since such differences in distribution as obtain are readily explained by the changes which have taken place in the physical geography of the world since the Miocene epoch, it is clear that the result of the comparison of the Miocene and present Faunae is distinctly in favour of evolution.

Indeed I may go further.

I may say that the hypothesis of evolution explains the facts of Miocene, Pliocene, and Recent distribution, and that no other supposition even pretends to account for them.

It is, indeed, a conceivable supposition that every species of Rhinoceros and every species of Hyaena, in the long succession of forms between the Miocene and the present species, was separately constructed out of dust, or out of nothing, by supernatural power; but until I receive distinct evidence of the fact, I refuse to run the risk of insulting any sane man by supposing that he seriously holds such a notion.