96 STUDIES, SCIENTIFIC AND SOCIAL CHAP. 



was no doubt earlier than that of the forests of (Eninghen 

 (though both are usually termed Miocene), the northern 

 plants having migrated southward owing to the lowering 

 of the mean temperature. As the severer cold of the 

 Glacial epoch came on, the same species could only live 

 by migrating still farther south ; and then, when the cold 

 period had passed away, they moved back again, and 

 many of them now occupy the same countries as they did 

 before the Glacial epoch. 



And now we arrive at the explanation of the exceptional 

 poverty of Europe. If we look at a good map or large 

 globe, we shall see that in North America the Alleghany 

 Mountains run north and south, and the lowlands east 

 and west of them extend uninterruptedly to Florida, to 

 Texas, and to the Gulf of Mexico. There was, therefore 

 nothing to prevent the southward migration of the flora, 

 when the mountains were covered with snow and ice, 

 and its return afterwards. But in Europe the geo- 

 graphical conditions are very different. There is a great 

 chain of mountains, the Alps and Pyrenees, running in an 

 east and west direction, and farther south a great sea, the 

 Mediterranean, also running east and west. As the 

 Glacial epoch came on, the icy mantle crept southward 

 from the Arctic Ocean and downward from the mountain 

 heights, thus preventing the plants of Central Europe 

 from migrating southward, and destroying all that were 

 not capable of enduring a very severe climate, or which 

 did not also exist south of the Alps. But here, too, the 

 Mediterranean prevented any southern migration; and 

 being crowded into a diminished area between the 

 mountains and the sea, many species must have perished. 

 When the cold passed away, the survivors spread northwards 

 and rapidly covered the whole country, but their greatly 

 diminished numbers and the prevalence of a few hardy 

 species over very wide areas, sufficiently attest the severe 

 ordeal they have passed through. 



The correctness of this explanation can hardly be 

 doubted, more especially as it equally serves to explain the 

 superior riches of Eastern Asia. For here we find a far 



