We have endeavoured to explain how it was that the species brought by this 

 wave of migration, having entered Europe, were there destroyed, so as to leave few 

 living representatives at the present day. It remains to explain why in China and in 

 North America they survived. In whatever country they occur, these congeners of the 

 Reuverian species which now inhabit warmer latitudes, are there almost invariably mountain 

 plants. It was the mountains of China and North America that saved them. 



Geology, Palaeobotany and Palaeontology prove that during later Tertiary 

 times the climate of the Northern Hemisphere became progressively cooler. This fall of 

 temperature persisted till it reached a minimum about which it oscillated somewhat 

 rapidly, giving rise to a succession (how many we need not here discuss) of Glacial 

 and Interglacial Periods. We ourselves are now living in a warmer period succeeding 

 arctic cold. 



The problem therefore is, to explain not only how a flora managed to survive 

 in a slowly but continuously cooling Hemisphere, but how it also managed to survive 

 a succession of somewhat rapid oscillations from cold to comparative warmth. With 

 this survival, as exemplified in the Yangtse Valley, we will now deal, for it is in the 

 mountain flora of this valley that the greatest assemblage of descendants from the 

 parent stock of the Reuverian is found. The history of the survival of these plants will 

 serve as a typical example to explain the survivals in other mountainous countries. 



The fact that so much of the Tertiary flora has survived in the Yangtse Valley, 

 would by itself suggest that this valley is very ancient. In the Pliocene Period the valley* 

 floor must have been much higher than at present, and there would be free access for 

 a migrant flora to the upper reaches beyond Ichang, now to a great extent cut off from 

 the lower Yangtse Valley by a series of mountain ranges, of comparatively low eleva* 

 tion, through which the river has cut its way, giving rise to the workUrenowned gorges. 

 In fact the Yangtse Valley must have formed in one direction a continuation southward 

 of the coastal plain. Into the great area of country thus open, the stream of migration 

 poured, some members occupying, by gradual spreading, the alluvial flats, others 

 passing along the ridges. As the temperature fell, the warmstemperate species occupying 

 the valley*floor, and the less adaptable species, were successively destroyed ; the warmest 

 species to survive being therefore those which occupied the valley*floor during the 

 glaciation of the north. With returning warmth the valley^floor became too warm for 

 these species and they were forced to climb the mountain slopes, only to return to the 

 valley*bottom when the temperature again fell. 



In the way just described it is possible for a flora in a mountainous country to 

 survive changes of climate which, in a level country where free movement north and 

 south is barred, would exterminate it. For, by changing its altitudinal zone, it can, 

 within the range of a few hundred metres, find the change of climate necessary for its 

 survival — provided that the climate never becomes colder than it was when this flora 

 entered the valley. If this happens, the warmer elements must succumb. 



But not only will the vegetation of the valley*floor be saved. If the mountains 



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