Chapter X 



—185— 



Historical Causes 



zone, that the continents were at one time in contact with one another. 

 Thus, if we take the southern hemisphere, we find a number of dis- 

 continuous areas, embracing South America and Australia, Africa and 

 Austraha, or South America, Africa, and Australia. The fact that 

 these areas do not include any territory in the northern hemisphere 

 cannot be regarded as accidental and indicates that these areas con- 

 stitute parts of once-continuous areas. This connection between the 

 continents, for instance, in the case of South America and Australia, 

 must have been achieved by the shortest possible route, i.e., by way of 

 Antarctica. Hence we may conclude that the continents of the 

 southern hemisphere were at one time connected, not only South 



Fig. 30. — Shifting of the North Pole in its relation to Europe: i, in the Miocene; 

 2-4, in the Pliocene; (4, Kansan); 5, during the Giinz glaciation; 6, during the 

 Mindelglaciation; 7, during the Riss glaciation; 8, during the Wurni glaciation; 

 9, during the Baltic advance of the icesheet. (After Koppen and Wegener). 



America, Australia, and New Zealand being in contact, but also South 

 Africa. 



On the basis of these fundamental features of the areas of angio- 

 sperms and the conclusions which we have drawn, we may now take 

 up the problem of the genesis of these areas. From what has been said 

 it follows that there existed a period in the history of many families 

 of plants when their areas embraced very extensive territories, lying 

 either within the boreal, north temperate, and tropical zones or within 

 the south temperate zone. In the former case it is necessary to 

 assume a direct connection between America, Euro-Africa, and Asia; 

 in the latter case it must be assumed that it was possible for species to 

 migrate directly from South America to Australia and also for an 



