When the question is asked, “How does one arrive at the conclusion that there are fossil faunas of Atlantic or Pacific, European, Asiatic, or South American origins?” we paleontologists are guided in our answer by the principles made use of by zoogeographers, the students of the continuous or discontinuous distribution of living animals. We know, for example, that at present humming-birds are to be found only in America, and chiefly in Central and South America; that mammals with solid horns or, better, with antlers are common to all the Northern Hemisphere, but that those with hollow horns, like the antelopes, are in greatest abundance in Africa south of the Sahara Desert. Africa is also known for its elephants, giraffes, rhinoceroses, hippopotami, lions, monkeys, and apes; and yet to the east of this land, on the great continental island of Madagascar, none of these mammals are present, but . . .