The distribution of those animals living near the coast depends very largely on local features such as the nature of the bottom, the salinity, the acidity of the water, the temperature and tem perature range, the nature of the currents, and other factors which can only be determined in each individual case. It is therefore impossible to deal with them in this article, but an account will be found in the article DISTRIBUTION OF ANIMALS.
In the foregoing account it has been assumed that the great continents have been stable in the sense that they contain within themselves great areas, such as Mongolia, Eastern Canada, West Australia and South and Central Africa, which have been dry land for a period vastly greater than that within which the adaptive radiation and distribution of the mammals has taken place. The truth of this assumption is abundantly demonstrated by geological observations, but the further belief that these old land masses have always occupied the same positions with respect to one another and to the poles has been seriously questioned by A. Wegener.
Wegener holds that in Upper Carboniferous times the whole of the dry land of the world was concentrated into one great continent, subdivided to some extent into independent areas by shallow seas, but essentially continuous. The south pole lay within this land mass in what is now Portuguese West Africa.
The present continents have arisen from this mass by an actual horizontal displacement, the present coasts, or more accurately the margins of the continental shelfs which face one another across the Atlantic and Indian oceans having formerly been in actual contact. By such drift not only have the Americas become torn away from the western coast of Europe, Africa and Antarc tica and Asia from its eastern coast, but the island festoons, New Zealand, the Antilles, Madagascar, etc., have been detached from the neighbouring coast of a continent. These great movements naturally produced great pressures on those continental margins which lay in the direction of motion, and led to the uprising of the coastal mountain ranges which are a marked feature of the earth's morphology.
The continental drift did not take place freely, the large land masses twisted round with respect to one another and became deformed, so that their margins in many cases no longer fit accurately, and small land bridges between the continents per sisted long after the main masses had become widely separated.
This remarkable view has been accepted by some geologists and has a great mass of geological evidence behind its basal assumption.
Wegener was first led to the investigation of its possibility by a consideration of the evidence from the distribution of animals for the former existence of a land bridge between Africa and Brazil. Wegener's view has the very great merit of explaining simply the remarkable facts of the distribution of that peculiar flora, the Glossopteris flora, which in South America, the Falkland Islands, South Africa, Madagascar, India and Australia occurs in late Carboniferous and Permian rocks, usually in association with boulder clays and other evidences of an ice-bound land and arctic climate. It accounts satisfactorily for the occurrence of the little fresh-water reptile Mesosaurus in Brazil and South Africa, and for similarities in the Triassic reptilian faunas of those regions and the extreme resemblance between the land and shallow sea faunas of North America and Europe in Carbonif erous times.
But a continuation of these and other land bridges into late Tertiary times which has been suggested on the evidence of the present distribution of land snails, earthworms and similar groups, seems to be negatived by the distribution of mammals, where, as W. D. Matthew has shown, the existing distribution of land masses together with a few former land bridges, such as that which connected Asia and America through the Aleutian Islands, are sufficient to account for all the observed facts. The evidence of mammals far outweighs in value that of every other group, because for them alone have we any considerable knowl edge of the history of the individual groups. (D. M. S. W.)