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-Plocene 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. 
