178 GEOLOGICAL DISTEIBUTIOK. 



Pliocene, deposits of the Siwalik Hills of India we have the true 

 hippopotamus, bison, bear, and elephant, forms which do not make 

 their appearance until a somewhat later date in Western Europe, 

 Pliocene, or Post-Pliocene. Similarly, the genera Cervus, Hystrix, 

 Pelis, Hipparion, and Mastodon, which appear in Western Europe 

 in the Miocene period, are still wanting in the American continent, 

 making their first appearance ia the Pliocene. And, likewise, the 

 tme bears and oxen in Europe antedate the American forms by one 

 period. It may be stated, as a general rule, that where identical 

 genera of living forms occur in the deposits of both the Old and the 

 New World, those of the Old World are the more ancient ; and the 

 same probably holds good, although to a less extent, with families. 

 From these differences in the dates of appearance of certain animal 

 groups, their presence or absence, we are led to discuss the probable 

 origin of our existing faunas, or portions of them. 



The existence in Western Europe in Miocene, and especially in 

 Pliocene, times of a fauna consisting of forms which still inhabit 

 the region, and of others as are only to be found at the present time 

 on the continents of Africa and Asia, may appear at first sight 

 somewhat singular. But when we reflect that the climate, during 

 the whole or the greater portion of this period, was probably very 

 much more uniform and warmer than it is at the present time, and 

 possibly not very different from what it now is in the region of the 

 Tropics, the apparent singularity in great measure disappears. 

 Some of the tropical forms, as the giraffe and rhinoceros, may have 

 been indigenous to the region, while others, whose development in 

 South-Central Asia appears to have taken place at an earlier period, 

 not improbably represent immigrants from the heart of that con- 

 tinent. It is practically certain, moreover, that direct land commu- 

 nication existed during a considerable portion of this period with 

 the continent of Africa, with which, consequently, there would 

 have been effected a general interchange of forms. Indeed, it is 

 much more singular that Europe no longer retains its more charac- 

 teristic African forms; but it must be recollected that, with the 

 advent of the Glacial period, an era of cold set in, and that with 

 this inclement climate a general retreat southward took place, the 

 more tropically constituted animals passing over into the conti- 

 nent of Africa, or suffering extermination by the cold. The most 

 northerly animals, passing southward, occupied the region now more 



