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, Ked, 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 an 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. 



