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by migration. In other words scientific thought is distinctly opposed to a dual origin for genera.
To illustrate what I mean by an example. The probable ancestor of the Agriotherium, Arctotherium branch of the Homicyonina is Ursavus. The types of this existed in Europe in the Middle Miocene and certain species in America have been referred to the same genus. It is assumed that migration is responsible for this. The next member of the phylum which has been recognized is Indarctos. This genus is distributed in the Lower Pliocene almost continuously from Europe to North America. Again this distribution is thought to be due to migration. When, however, we meet a more advanced type, Agriotherium in Europe and India, and another equally advanced type Arctotherium in America, both probably derivable more or less directly from Indarctos, it is accepted without further question that Agriotherium arose from one species of Indarctos in Europe or Asia and Arctotherium from another species of Indarctos in America. If, on the contrary, it had been the case that Agriotherium and Arctotherium were generically inseparable, palæontologists would at once have demurred to a separate origin, but would have contended that the occurrence of Arctotherium in America was due to a third migration. Is it, however, logical to base this difference in thought on the mere chance that the variation of the evolving form has been in the one case little, in the other great?
In early days one thought of evolution as proceeding in a regular sequence from one individual or at any rate one species to another. Now, students of evolution think of progress not as being in regular straight lines but as a series of anastomosing lines forming a network. The individuals of a group confined to one feeding ground vary and interbreed and the result in the course of years is the production of a new species. Where the members of a species become separate and keep to different feeding grounds another species of the genus may arise. The final outcome is that many species may exist in the same part of the world, but there is no reason to suppose that these have all sprung from a single original species. I see nothing strange in supposing that a particular community in any genus may wander so far from the original hordes, that it never returns to them but goes on developing apart. If the environment is very different from that which it has left, as must often be the case, then a new type arises, which is separable generically from that
evolved in the original centre of distribution, always supposing that the change is not so prejudicial to the migrant as to exterminate it. Where, on the other hand, the environment is not appreciably altered it seems quite natural that the differences between the species developing in the two areas should not possess generic value. More especially is this likely to be the case where the family is an ancient one and, as is common in ancient families, has little tendency to vary. Under such conditions I cannot refuse to believe. that the trend of evolution may follow the existing tendencies which have been impressed upon it through the ages and that two different species which cannot be separated generically may arise in different parts of the world.
This concatenation of circumstances seems to be completely attained in the later members of the Equidae. In North America their evolution has been very gradual and has followed in the main the same course in all the later branches of the family. Merriam (1919, p. 558) and Lewis (1937, p. 196) have stressed the fact that advanced species of Merychippus cannot always be distinguished from its descendant Hipparion. Moreover the open steppe conditions favourable to the habit of these animals are likely to have been approximately the same everywhere throughout the Holarctic region in the later Coenozoic epoch. I will go so far as to say that for all we know to the contrary this type of evolution may have been the rule rather than the exception. In very few cases, however, could it possibly be susceptible of proof. In this connection it is of great interest to find that Hopwood (1937, p. 909), from the facts which he has observed concerning the geological history and distribution of the Zebrine and Caballine types of Equus inclines to the view that these two groups evolved independently from the animals grouped under Pliohippus and Plesippus.
In the case of two groups of species which had an independent origin, such as I have suggested for the American and Asiatic species. of Hipparion, it might be supposed, in fact the contrary is almost inconceivable, that they should differ in certain ways from one another, and that these differences would be perpetuated in all the succeeding species of each group. It might also happen that evolution though following the course usual in the family was (1) in some parts of the anatomy faster and (2) in others stower than among the species which remained in the original centre of distribution. The curious circumstance might even result (3) that the new genus actually
developed earlier in the new area than in the old one. The first two of the possibilities suggested have certainly come about in the case of the Old World species of Hipparion, in the precocious complication of the enamel folds in the teeth on the one hand and in the retarded atrophy of the lateral metapodials on the other. It was this, no doubt, that induced Matthew many years ago to suggest that Hipparion of Europe was evolved separately from Neohipparion of the New World. The third possibility will be demonstrated to have taken place in the evolution of Hipparion from Merychippus if my views on the correlation of the Siwalik stages are accepted.
The failure so far to find Merychippus in Central Asia is no argument against a dual origin for Hipparion, since Tortonian deposits are still unknown in that region. Even its absence as well as that of Hipparion from the Tung Gur beds of supposed Sarmatian age is understandable, because that seems to be a forest rather than a plains fauna.
APPENDIX ON THE CORRELATION OF CERTAIN TERTIARY DEPOSITS OF INDIA AND EUROPE.
BY A. TINDELL HOPWOOD, D.Sc., F.L.S, Department of Geology, British Museum (Natural History).
Dr. Pilgrim has asked me to put together a few notes on the correlation of the Middle and Upper Tertiary deposits of India and Europe, basing my arguments on the Proboscidea. Two of the Indian stages, the Dhok Pathan and the Pinjor, are directly comparable with European deposits on the evidence of the fossil Proboscidea they contain, and a third the Fatehjang, also yields results which are valuable as an indication, even though the comparisons are less precise. I have not discussed the work of other authors because it seemed preferable to attempt an independent solution, and also because adequately so to do would take up more space than is at my disposal in an appendix.
1. General considerations on migration.
The period of the first appearance of immigrant forms is of primary importance in correlation studies (cf. Depéret, 1905, 1906). Under favourable conditions, the time needed for migration is of no geological consequence, even though animals such as the Bovinae,
which inhabit wooded or forested areas, will tend to travel less rapidly than others such as the Equidae, which dwell on grassy plains. It must, however, be difficult to apply the principle of immigrant forms when dealing with areas at or near the centre of evolution of a given group. For example, the genera Bos, Elephas, and Equus are all immigrants in the European area and one may properly employ the time of their first appearance as marking the beginning of a new phase of geological time (cf. Hopwood, 1935). But in India these same genera illustrate very vividly the disadvantages attaching to any attempt to employ them in this manner. The first two evolved in or near to northern India and probably appear in the Indian deposits at an earlier date than elsewhere, i.e., Bos and Elephas probably mark the end of the Pliocene rather than the beginning of the Indian Pleistocene. Equus, however, is an immigrant in the same area, and the strata in which unequivocal remains of that genus are known to appear for the first time may be regarded as the base of the Pleistocene in India, but support should be looked for among other groups.
In the final analysis, animal migration seems to have its origin in the search for food. This one factor influences migration in two chief ways. First, the local climatic conditions may change, and the animals follow their retreating food supply. On the eastern part of the Serengeti plains there are many thousands of antelopes during the rainy season when food is abundant. When the rains cease, the ground dries, the supply of grass diminishes, and antelopes become progressively scarcer as the main herds follow their food in its retreat to the West. Such migrations are usually seasonal, but if the change in conditions is due to geological phenomena they are likely to be permanent. In any event, they are not as a rule of the same importance as those next to be discussed.
The second class of migrations comprises those due to direct competition for food. It is axiomatic that a given amount of food will suffice for a given population, and that if the population exceeds that limit, either the whole population goes short, or else the surplus must seek supplies elsewhere. The second solution obtains under natural conditions. The ultimate results of such migrations may manifest themselves in more ways than one, and they affect correlation so closely that further discussion may be helpful.
In a well-balanced community migration is unlikely to take place so long as the natural checks and balances remain undisturbed,
and evolution will continue in situ as it were. If however, one of the component groups evolves so rapidly as to exceed its former food requirements, then so far as that group is concerned the area will become overpopulated, and some members of the group must move farther afield or perish. Under such circumstances, fossil remains of the group will be restricted to the original area up to the time when migration took place, and they will there be found in deposits of earlier date than in those areas to which they migrated. An example of this phenomenon is afforded by the mastodontoid genus Choerolophodon Schlesinger [Synconolophus Osborn.] This genus evolved quietly throughout the Kamlial and Chinji stages, and typical teeth, though rare, are found in the latter. In Dhok Pathan times, it suddenly became a dominant form. Teeth of Choerolophodon are the commonest proboscidean fossils found in that stage, and it is very significant that the genus appears suddenly in the Pontian deposits of Maragha, Samos, and Pikermi.
The ultimate fate of the migrants depends more on the physical conditions they encounter than on anything else. If the conditions are favourable, they will travel rapidly and multiply quickly according to their nature; but if the conditions are not favourable, if they find their way barred by climatic or physical obstacles, or if the land they seek to enter already supports animals capable of competing with them, then progress will be slow. Consequently the time-lag between the appearance of the genus at its centre of evolution and its arrival at the limit of its ultimate area of distribution will increase, and it may even come about that when migration is at an end the survivors are no longer recognisable as being of the same kind as those which originally set out on the journey. As examples of the first possibility may be mentioned the spread of the African elephant from an unknown centre over the greater part of that continent in post-Pluvial times, and the speed with which the horse recolonised North America after it had been introduced by Europeans. As an example of the second, slower, sequence of events, the Miocene invasion of Europe by Equidae of American origin, and their transformation on the way from the Oligocene Miohippus to the familiar Anchitherium of the Burdigalian and Vindobonian.
Sometimes there is every appearance of swift migration, a temporary check, and then a renewal of migration, with the result that beds of the same age may have a false appearance of being