Chapter X 



-187— 



Historical Causes 



4 species in Asia (from the Himalayas to Java), and only 2 species in 

 the Euro-African sector and these only on islands (one on Fernando Po 

 and one on Madagascar). The genus Jaegerina has disappeared al- 

 together from the mainland and is now found only on islands, its distri- 

 bution being similar to that of the genus Ravenala. One species grows 

 on Jamaica, four on Madagascar, and one on Luzon Island in the 

 Philippines. This paucity of stations within the Euro-African sector, 

 observed likewise among the higher plants, may be explained only as a 

 result of climatic changes that took place as a consequence of the 

 shifting of the tropical zone far to the south, accompanied by an ad- 

 vance of the glaciers over most of Europe and a lowering of the tem- 

 perature in central and northern Africa. 



The impoverishment of vegetation in the Euro-African sector led, in 



Fig. 31. — Area of distribution of Clelhra {Clethraceae) \ Shading = present range; black 

 dots = fossil occurrence. (Mter Irmscher). 



the case of many genera, to a complete absence of representatives in 

 Africa and the consequent formation of areas with two widely-sepa- 

 rated parts, one in South America and one in Asia. The fact that 

 genera with such areas comprise a very limited number of species 

 indicates that they are very ancient. Some genera embrace a few 

 species that have preserved their distribution in Europe but not in 

 Africa. For instance, the genus Hookeria has one species, H. acutifolia, 

 distributed in South America (Guadeloupe, Ecuador, Bolivia, Brazil) 

 and Asia (the Himalayas, Ceylon, Java) and another species, H. 

 lucens, distributed in North America and Europe. As offshoots of such 

 areas we sometimes find (as also among the higher plants) spurs ex- 

 tending to Australia and Polynesia, which attests the former existence 

 of a connection between Asia and Australasia. 



