Chapter IX — 153^ On Migration 



in the same state as, let us say, in the Carboniferous period, with the 

 same distribution of climatic zones, the same climatic conditions, and 

 the same distribution of land and sea, there is no doubt that the earth's 

 vegetation would not have attained, simply by the slow processes of 

 natural selection, that stage in its evolution at which it now finds 

 itself. Instead of the angiosperms, having an anatomical structure 

 adapted to intense light and moderately humid habitat conditions, 

 perhaps even to-day there would still be flourishing Lepidodendron and 

 Sigillaria, and the animal world would be represented not by mammals 

 and man but by gigantic reptiles. 



The great geological revolutions that have periodically occurred, 

 inducing intensified mountain-formative processes and accompanied by 

 the advance and regression of seas and the shifting of continents as 

 regards their position in relation to the poles, have served as the cause 

 of changes in the location of the climatic zones. These latter changes, 

 in turn, have led to ever new changes in the distribution of plants and 

 animals, producing an enormous effect on their development and 

 evolution. 



During long ages, extending to the very close of the Tertiary period, 

 the distribution of the climatic zones favored the development of the 

 vegetation of the northern hemisphere. In contrast to the com- 

 paratively uniform development of the flora of the northern hemi- 

 sphere, the development of the flora of the southern hemisphere was 

 time and again disturbed. Chief among such disturbances was the 

 great southern glaciation in the Permo-Carboniferous epoch (perhaps 

 also a second later one), the principal center of development of which 

 lay in South Africa. These glaciations, as well as subsequent climatic 

 changes during the Quaternary period, constitute one of the reasons 

 for the poverty of the flora of tropical Africa as compared with Asia 

 and South America. 



A second cause of the more uneven course of development of the 

 vegetation of the southern hemisphere was the separation of the conti- 

 nents (accompanied by a decrease in rainfall), which began as far back 

 as the Mesozoic era. This was necessarily reflected in the number of 

 species in the flora of Africa, since, besides being cut off from the 

 vegetation of America, Africa was almost cut off from the rich vege- 

 tation of Europe and Asia, due to the existence in the region of the 

 Sahara in the Cretaceous period of a great sea and in later times of a 

 great desert that barred the way to species migrating from the north. 



In the early part of the Quaternary period the fate of South Africa 

 was shared by Europe and North America, which continents had pre- 

 viously been characterized by an exceptional wealth of plant forms. 

 At the present time Europe is one of the poorest continents as regards 

 the number of species in its flora. This may be ascribed to the fact 

 that the flora destroyed during the Ice Age had here, as compared with 

 North America, very little possibihty of becoming restored. The 

 stocking of this continent with new vegetation could proceed only from 

 the south, from the Mediterranean Basin, and from the east, from 

 northern and southwestern Asia, which regions at that time also had 

 greatly impoverished floras. The path from the west, from North 

 America, was closed, since the Atlantic Ocean had already been formed; 



