6] FOUNDATIONS OF MODERN DISTRIBUTIONS l6l 



Pleistocene southward migration there were three principal avenues 

 of escape, determined primarily by the prevailing direction of the 

 mountain systems : (i) along the lowlands of eastern Asia, (2) along 

 the mountain ranges of North America, and (3) down the Scan- 

 dinavian Peninsula and adjacent areas into western and central 

 Europe. The first route may have led to intermixture of northern 

 species with the original native flora and so may largely explain 

 the richness of the existing flora of China. The second route may 

 have had much the same effect in America and would, moreover, 

 explain the similarity between the floras of eastern North America 

 and eastern Asia. In the third instance, however, the great European 

 mountain ranges with axes lying east and west would have prevented 

 further southward migration, with resulting impoverishment both 

 currently and as regards possible recolonizing elements even after 

 the final Pleistocene recession, as we have already seen. However, 

 there has been some opposition to this hypothesis, and serious 

 doubts expressed, for example, as to whether southwestern France 

 was cold enough at this time to cause the postulated extermination, 

 though this might still have been effected by the persistence there 

 of closed communities, which are among the toughest barriers for 

 a migrant to cross. Nor is it known whether the great extinction 

 of plants and animals in Europe took place before the end of the 

 Pliocene or during the Pleistocene. Probably it was a gradual pro- 

 cess, each advance of rigorous conditions leading to the loss by the 

 flora of some of its less hardy elements, so that potential waves of 

 recolonizing vegetation became successively more impoverished. 



Pleistocene Persistence versus Subsequent Immigration 



The Pleistocene itself, as we have seen, was a climatically unsettled 

 period of cold spells with extensive glaciations, which were separated 

 by relatively long warm intervals (interglacials). In general the 

 plants living during the Pleistocene were specifically identical with 

 those of the present time, the chief differences exhibited by them 

 being in spatial distribution. During some at least of the inter- 

 glacials the climate appears to have been similar to that of the 

 present day — which indeed has led to the suggestion that we may 

 nowadays be merely in another interglacial. Thus more than 

 70 per cent, of the species whose remains have been identified from 

 an interglacial deposit in Germany live in the same district at present, 

 and so do most of those in a deposit near Toronto in southern 



