EVIDENCE FROM thIIOGRA ICA I. DISTRIBUTIoN. The present distribution (q.v.) of plants and ani mals can only be explained by reference to past geological changes in the shape and proportions of former continental masses and the resulting geo• logical extinction. We can in many cases only account for the apparently sudden appearance of groups of highly specialized animals in a given area by invoking past migrations. Thus, the camel falnity ( see CA M ELID30,) originated in western North America, where it since died out, but is still surviving in South America and Asia. So with the ox family in Eurasia, and the elephants in northeastern Africa, the mammoth (q.v.) having, migrated into, North America by way of northeastern Asia. Thil, interchange of forms between Eurasia and America, between Asia and Africa, between North and South Ameri ca, and the changes of climate and other sur roundings along the line of march, must have operated in inducing change of habits and varia tion, and, more especially by isolation, have led to the origin not only of new species and climatic varieties, hut to the beginning of new generic and family types. Australia is, in many respects,
notably in its characteristic mammals, a Jurassic continent, while Madagascar is a Tertiary island. The lima birds, peculiar to New Zealand, are the result of long ages of isolation and laek of com petition with predatory animals.
The various modes of dispersal of organisms and their colonization in remote regions also throw light on the origin of species. The study of deep-sea life is instructive in this eon neetion. It is now generally supposed that the abyssal or benthal fauna originated from shal low-water forms, and that the characters in which these animals differ from those liv ing near the coast are adaptations to life at great depths. Indeed, all the facts and conclu sions of zoogeography converge toward the view that as the different types evidently originated from this or that centre of distribution, so they had common ancestral forms.