ORIGIN OF EXISTING FAUNAS. 179 



or less vacated by the tropical forms, and hence, in the deposits of 

 the Glacial and Post-Glacial periods, we meet with the remains of 

 the elk, reindeer, hyena, lion, giraffe, elephant, rhinoceros, and 

 hippopotamus mixed together. At about the same epoch it would 

 appear that the present rupture existing between the African and 

 European continents was effected, a separation which precluded the 

 possibility of a return migration from Africa when the more toler- 

 able climate succeeding the close of the Glacial period set in. 

 Hence the survival only of the more temperate forms of the Euro- 

 pean fauna. While Africa, therefore, retains its strictly African 

 mammalian forms, it may be considered questionable whether these 

 are not direct importations, in great part, from the region lying to 

 the north. As far as the North American continent is concerned, it 

 appears not improbable, from what has already been said with refer- 

 ence to the earlier appearance in Europe of equivalent mammalian 

 types, that a not inconsiderable portion of its later (fossil) fauna 

 was derived from the Old World. Thus, it appears likely that the 

 bears, swine, oxen, sheep, antelopes, and elephants originated in 

 the Old World, whence they were transplanted by way of some 

 land connection existing in the north into the New World. The 

 true dogs, on the other hand, seem not unlikely to have been de- 

 veloped first on the American continent ; and it is also not improb- 

 able that the ancestral line of the camel is to be traced to the West- 

 ern Hemisphere. Our paleontological knowledge of the different 

 countries, even of those more thoroughly explored, is, however, far 

 too insignificant to permit of a definite solution to the problem of 

 the origination of the various mammalian groups, and in the case 

 of most continental faunas but little can be said with positiveness 

 concerning their formation. The North American continent lacks 

 in its Tertiary fauna the giraffe, hyena, and hippopotamus; nor do 

 we find any traces of the group of the Old World monkeys, or, in 

 the greater portion of the region, of the Edentata, whose various 

 forms are now so abundantly represented in the South American 

 fauna. 



With the Post-Pliocene period the correspondence existing be- 

 tween the fossil and recent faunas of the several geographical regions 

 is in most cases further increased, and not only by the introduction 

 of many new modern genera, but by the presence, in considerable 

 number, of identical specific types. The modern fauna may now 



