12 GEOGKAPHICAL DISTRIBUTION. 



five, in the main, widely-distributed species : two antelopes, two 

 sheep (including the musk-ox), and the bison. The question as to 

 how these animals obtained a foothold in the region which they 

 now inhabit, whether they originated there as derivatives from 

 previously- existing forms, or were introduced as migrants from 

 some land-mass lying without their domain, can only be deter- 

 mined by a reference to the still earlier fauna of not only this, but 

 of other regions as well. In the case of the bears, for example, no 

 immediate ancestors of the tribe have thus far been discovered in 

 the Western Hemisphere antedating the Post-Pliocene epoch ; on 

 the other hand, in the Eastern Hemisphere Europe the remains 

 of such animals, and of the true bears themselves, are abundant in 

 deposits of the earlier Pliocene age. Hence, the assumption appears 

 almost unavoidable that the North American fauna received its 

 ursine contingent from the Old World. The same may or may not 

 be also true of the American Bovidae ; but the determination of 

 this question is made difficult, or impossible, through the fact that 

 at least two of the genera Ovibos and Bison occur fossil in the 

 Post-Pliocene deposits, and there only, of both the Old and the 

 New World, and consequently appear in the two hemispheres as 

 being of approximately equivalent age. Yet the fact that neither 

 goats, sheep, oxen, nor antelopes have thus far been discovered 

 fossil on the North American continent, while their remains are suf- 

 ficiently abundant in the deposits of Eurasia (Europe- Asia) of Post- 

 Pliocene or even much older age, would seem to indicate that the 

 true home of the Bovidae is the Old World, whence, by gradually 

 spreading, and through the facilities afforded them in the way of a 

 northern land connection, they eventually came 'to occupy a con- 

 siderable portion of the New World as well. The giant sloth-like 

 forms, such as the Megalonyx, Megatherium, and Mylodon, which 

 in North America are associated with the remains of animals of 

 indisputably Post-Pliocene age, occur in South America in an older 

 formation, the Pliocene, and thus seemingly represent an invasion 

 of the north from the latter continent. This conclusion appears 

 further borne out by the circumstance that the Southern Hemi- 

 sphere is the home of the animals of this class, and that, with 

 scarcely a single exception (Moropus, ? Morotherium) no edentate 

 form has thus far been discovered in any North American deposit 

 antedating the period which represents the development of the 



