E. V. Wulff 



-32- 



Historical Plant Geography 



prised the Iberian Peninsula (Spain, 44 species; Portugal, 20 species), 

 Morocco (52 species), and Algeria (44 species), which at one time 

 formed a united region. This conclusion has been visualized by a map 

 drawn by Szymkiewicz (1933) on the basis of these same data.* 



Another example is provided by a table given by Szymkiewicz 

 himself, where the number of species are given for regions located at 

 approximately the same latitude from the Iberian peninsula to Japan. 

 For each genus the figures in bold-faced type indicate the region or 

 regions in which the genus is represented by the greatest diversity of 

 species. 



Similarly, we may take the distribution of wild species of Nicoiiana, 

 a genus including, according to data of a study of the geography of this 

 genus made by Grabovetskaya (1937), a total of 76 species. They are 

 distributed as follows: North America, 12 (of which 7 are endemic); 

 Central America, 14 (7 endemic); South America, 43 (39 endemic); 

 Austraha, 14 (all endemic). On the basis of these data the center of 

 the area of this genus must be regarded as South America. 



This method of determining the location of the centers of origin of 

 genera, and on the basis of the latter, the centers of development of 

 floras, is at the present time generally accepted, but it should, never- 

 theless, be emphasized that this method is only relatively rehable, in 

 many cases leading undoubtedly to incorrect conclusions. Most genera 

 of angiosperms originated in the Cretaceous period, some probably even 

 earlier, in the Jurassic. Having attained at the end of the Cretaceous 

 and beginning of the Tertiary periods a very wide distribution, most of 

 them had at that time a considerably more hmited intrageneric differen- 

 tiation than at present. Intensive processes of species-formation and 

 the initiation of geographical series of species (see below) were not 

 begun until later, in the second half of the Tertiary period. These 

 were induced by climatic changes, particularly decrease in humidity, 

 and geomorphological changes— mountain-forming processes {e.g., the 



* Discrepancies between the figures on the map and those in Shirjaev's table arise 

 from the fact that for the map Szymkiewicz used only the number of species, whereas 

 Shirjaev included subspecies as well. 



