chap, vi.] MAMMALIA OF THE OLD WORLD. 109 



The natural division of our subject therefore is into geological 

 periods. We first go back to the Post-Pliocene period, which 

 includes that of the caves and gravels of Europe containing flint 

 implements, and extends back to the deposit of the glacial drift 

 in the concluding phase of the glacial epoch. Next we have 

 the Pliocene period, divided into its later portion (the Newer 

 Pliocene) which includes the Glacial epoch of the northern 

 hemisphere ; and its earlier portion (the Older Pliocene), repre- 

 sented by the red and coralline crag of England, and deposits of 

 similar age in the continent. During this earlier epoch the 

 climate was not very dissimilar from that which now prevails ; 

 but we next get evidence of a still earlier period, the Miocene, 

 when a warmer climate prevailed in Europe, and the whole 

 fauna and flora were very different. This is perhaps the most 

 interesting portion of the tertiary deposits, and furnishes us 

 with the most valuable materials for our present study. 

 Further back still we have the Eocene period, with apparently 

 an almost tropical climate in Europe ; and here we find a clue 

 to some of the most puzzling facts in the distribution of living 

 animals. Our knowledge of this epoch is however very im- 

 perfect; and we wait for discoveries that will elucidate some 

 of the mystery that still hangs over the origin and migrations 

 of many important families. Beyond this there is a great chasm 

 in the geological record as regards land animals ; and we have to 

 go so far back into the past, that when we again meet with mam- 

 malia, birds, and land-reptiles, they appear under such archaic 

 forms that they cease to have any local or geographical signi- 

 ficance, and we can only refer them to wide-spread classes 

 and orders. For the purpose of elucidating geographical distri- 

 bution, therefore, it is, in the present state of our knowledge, 

 unnecessary to go back beyond the tertiary period of geology. 



The remains of Mammalia being so much more numerous and 

 important than those of other classes, we. shall at first confine 

 ourselves almost exclusively to these. What is known of the 

 birds, reptiles, and fishes of the tertiary epoch will be best 

 indicated by a brief connected sketch of their fossils in all parts 

 of the globe, which we shall give in a subsequent chapter. 



