42 DISTRIBUTION OF ANIMALS. [PART I. 
among forms implying a climate very little different from the 
present ; and our own Crag formation furnishes evidence of a 
gradual refrigeration of climate; since its three divisions, the 
Coralline, Red, and Norwich Crags, show a decreasing number 
of southern, and an increasing number of northern species, as we 
approach the Glacial epoch. Still later than these we have the 
shells of the drift, almost all of which are northern and many 
of them arctic species. Among the mammalia indicative of 
cold, are the mammoth and the reindeer. In gravels and cave- 
deposits of Post-Pliocene date we find the same two animals, 
which soon disappear as the climate approached its present con- 
dition; and Professor Forbes has given a list of fifty shells 
which inhabited the British seas before the Glacial epoch and 
inhabit it still, but are all wanting in the glacial deposits. The 
whole of these are found in the Newer Pliocene strata of Sicily 
and the south of Europe, where they escaped destruction during 
_ the glacial winter. 
There are also certain facts in the distribution of plants, which 
are so well explained by the Glacial epoch that they may be said 
to give an additional confirmation to it. All over the northern 
hemisphere within the glaciated districts, the summits of lofty 
mountains produce plants identical with those of the polar 
regions. In the celebrated case of the White Mountains in New 
Hampshire, United States (latitude 45°), all the plants on the 
summit are arctic species, none of which exist in the lowlands 
for near a thousand miles further north. It has also been re- 
marked that the plants of each mountain are more especially 
related to those of the countries directly north of it. Thus, 
those of the Pyrenees and of Scotland are Scandinavian, and 
those of the White Mountains are all species found in Labrador. 
Now, remembering that we have evidence of an exceedingly 
mild and uniform climate in the arctic regions during the 
Miocene period and a gradual refrigeration from that time, it is 
evident that with each degree of change more and more hardy 
plants would be successively driven southwards; till at last the 
plains of the temperate zone would be inhabited by plants, which 
were once confined to alpine heights or to the arctic regions. 
