338 HISTORICAL PALEONTOLOGY. 



with the bones of numerous Mammals, such as Camels, Rhino- 

 ceroses, Mastodons, Elephants, the Horse, Stag, &c. 



As regards the life of the Pliocene period, there are only 

 two classes of organisms to which our attention need he 

 directed namely, the Shell-fish and the Mammals. So far as 

 the former are concerned, we have to note in the first place 

 that the introduction of new species of animals upon the globe 

 went on rapidly during this period. In the Older Pliocene 

 deposits, the number of shells of existing species is only from 

 40 to 60 per cent; but in the Newer Pliocene the pro- 

 portion of living forms rises to as much as from 80 to 

 95 per cent. Whilst the Molluscs thus become rapidly mod- 

 ernized, the Mammals still all belong to extinct species, 

 though modern generic types gradually supersede the more 

 antiquated forms of the Miocene. In the second place, there 

 is good evidence to show that the Pliocene period was one in 

 which the climate of the northern hemisphere underwent a 

 gradual refrigeration. In the Miocene period, there is evi- 

 dence to show that Europe possessed a climate very similar 

 to that now enjoyed by the Southern United States, and cer- 

 tainly very much warmer than it is at present. The presence 

 of Palm-trees upon the land, and of numerous large Cowries, 

 Cones, and other shells of warmer regions in the sea, sufficiently 

 proves this. In the Older Pliocene deposits, on the other 

 hand, northern forms predominate amongst the Shells, though 

 some of the types of hotter regions still survive. In the Newer 

 Pliocene, again, the Molluscs are such as almost exclusively 

 inhabit the seas of temperate or even cold regions ; whilst if 

 we regard deposits like the " Bridlington Crag " and " Chilles- 

 ford beds " as truly referable to this period, we meet at the 

 close of this period with shells such as nowadays are distinct- 

 ively characteristic of high latitudes. It might be thought 

 that the occurrence of Quadrupeds such as the Elephant, 

 Rhinoceros, and Hippopotamus, would militate against this 

 generalization, and would rather support the view that the 

 climate of Europe and the United States must have been a 

 hot one during the later portion of the Pliocene period. We 

 have, however, reason to believe that many of these extinct 

 Mammals were more abundantly furnished with hair, and more 

 adapted to withstand a cool temperature, than any of their 

 living congeners. We have also to recollect that many of 

 these large herbivorous quadrupeds may have been, and 

 indeed probably were, more or less migratory in their habits; 



