636 HISTORICAL GEOLOGY 



was a marked lowering of the temperature and perhaps the beginning 

 of glaciation on the higher mountains. In the English Pliocene the 

 proportion of Arctic shells rises from five per cent, in the oldest to 

 over sixty per cent, in the youngest beds. 



The disappearance of rhinoceroses and the browsing types of horses 

 and camels (those that lived largely on the leaves of shrubs and trees), 

 as well as die existence of great herds of land tortoises on the Great 

 Plains of North America, is perhaps proof of arid conditions in the 

 western interior. The disappearance on the Pacific coast of warm 

 temperate plants, as well as the character of the marine and fresh 

 water invertebrates, indicates a change to colder conditions. Evi- 

 dence is at hand showing that Japan was colder during the Pliocene 

 than during the Glacial Period (Yokohama). This has again sug- 

 gested the possibility of a " wandering pole " (p. 660) to account for 

 the Glacial Period. 



REFERENCES FOR FLORA AND CLIMATE 



Knowlton, F. H., — The Relation of Paleobotany to Geology: Am. Naturalist, Vol. 46, 

 1912, pp. 207-215. 



Knowlton, F. H., — Succession and Range of Mesozoic and Tertiary Floras: Jour. 

 Geol., Vol. 18, 1910, pp. 105-116. 



Osborn, H. F., — Age of Mammals, pp. 104; 117; 184-185; 242-244; 282-285; 

 306; 342-343. 



Schuchert, Chas., — Climates of Geologic Time: Carnegie Institution of Washing- 

 ton, Publication No. 192, 1914, pp. 263-298. 



Effects of Isolation and Migration 



During the Age of Mammals the seas were at times so expanded as 

 to isolate large areas of land, while at others they were so restricted 

 that continents now separated were then united (Figs. 561, 562, 563). 

 The mammalian life of Cuba seems to have been derived from a few 

 species that were carried there on natural rafts. Other islands were 

 doubtless populated in the same way. When the isolation was pro- 

 longed, the evolution of the animal life of the various continents took 

 place independently. When the lands were again united, widespread 

 migrations took place. This isolation and later establishment of 

 land connections occurred several times during the Tertiary. The 

 proof of the separation or reunion of great land areas is based chiefly 

 upon the dissimilarity in the one case, and the similarity in the other, 

 of the life of the past. 



