CREATION BY EVOLUTION
are now found. Here also many migrations back and forth between the two continents may be traced by the record of the rocks, and these migrations and counter migrations took place at widely separated geological dates. Many American mammals are the modified descendants of migrants from the Old World. Some came to America at a very ancient time; others came recently and are so little modified that naturalists disagree as to whether they should be assigned to European species or not. In short, our understanding of the distribution of existing mammals is dependent upon our knowledge of their history, and where that history is known the distribution is self-explanatory. If the doctrine of special creation is true, this knowledge is illusory, for according to that doctrine the modern species were separately created and therefore have no connection with the ancient species that look so deceptively like the ancestors of the living forms.
North America was separated from Asia at a very late date, as geological time is reckoned, and hence there is relatively little difference between the animals of the American and the Eurasiatic division of the Palaearctic region. The only large mammal that is found in the American and not in the Eurasiatic division is the musk-ox, but until a very recent period that peculiarly Arctic creature dwelt in Siberia and roamed as far westward as Great Britain, which was then a part of the mainland.
The distribution of mammals on the continents finds a satisfactory explanation in the theory of evolution and only in that theory, but the facts of island life furnish even more remarkable and more convincing testimony of the truth of that theory. Islands have been formed at different dates, some rising from the sea within the last few years, others having a history that goes back to the remotest geological
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