Himalaya, and Western China, may have acted as a second centre for the origin and 

 dispersal of temperate species, which radiated from these uplands when the climate 

 became colder, just as they radiated from the shores of the Arctic Sea. If such a dis* 

 persal as this took place it must have been in Miocene or Pliocene times, when the 

 northern hemisphere was cooling. 



The truth of a theory can only be tested by its ability to explain facts. Let us, in 

 recapitulation, consider how this theory, that the direction, survival, or destruction of 

 the different streams of migrants depended on the direction of the valleys, plains, and 

 mountain ranges, and the height of the mountains, is capable of explaining the facts. 



It accounts, in the first place, for the great richness of the Pliocene flora of 

 Limburg and Prussia, which reached thus far on its southward and westward journey 

 unchecked, and possibly with the loss of but few elements. By the influence of the east 

 and west mountain chains it accounts for the destruction of this rich flora of trees and 

 shrubs, which in Europe has left so few living traces of its existence. For the comparative 

 poverty of the living North European and North Asian floras, of which the members 

 are but the survivors from the destruction during the glaciation, together with return 

 migrants from the south. For the great floral wealth of the Chinese mountains, and 

 the poverty of the Szechuan Basin. For the wealth of the North American flora, which, 

 as with the Chinese, entered the country unimpeded, and in a lesser degree was saved 

 by its north and south running mountain chains. Finally, it accounts for the curious 

 fact that the trees and shrubs which were lowland plants in Pliocene Limburg, are now 

 found (they or their nearest living congeners) as mountain plants in such far*removed 

 countries as China, Japan, North America, the Himalaya, and the Caucasus, at the 

 present day. 



It is of much interest to note that Darwin, in the first edition of his Origin of 

 Species, anticipated in a general way this explanation of the reason why the living rain* 

 belt flora of China, and the corresponding flora of North America, are, partially at 

 any rate, relics of the early Pliocene floras of more northern latitudes. But in Darwin's 

 time the flora of Western China being unknown, he could reason only from the living 

 floras of Europe and North America, and the littlesknown extinct Tertiary floras. He 

 founded his conclusions upon the study of the living flora of the Arctic regions and 

 its distribution southward in isolated mountain chains. Following Edward Forbes, he 

 explained this distribution as the result of a mild period following a gradually cooling 

 one. By the cold the flora was driven south; by the returning warmth it was driven 

 back northward, and, in mountainous regions, up the mountain heights. Then by an 

 extension of his argument Darwin goes on the show that the same reasoning will 

 explain "the relationship with very little identity between the productions of North 



America and Europe" and "the singular fact that the productions of Europe and 



America during the later tertiary stages were more closely related to each other than 

 they are at the present time." He infers that during some warmer period "such as the 

 Older Pliocene period, a large number of the same plants and animals inhabited the 



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