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It is a continuation of the great Arctogaea into the tropics.

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 depauperized 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.

But Austro-Columbia presents difficulties from which Australia is free; Camelidae and Tapiridae are now indigenous in South America as they are in Arctogaea; and, among the Pliocene Austro-Columbian mammals, the Austro-Columbian genera Equus, Mastodon, and Machairodus are numbered.

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

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.