Chapter X 



—181— 



Historical Causes 



of studies of the flora of Malaysia, and Kryshtofovich (1933), on the 

 basis of investigations of the flora of the Philippines, likewise arrived 

 at the same conclusions. Even for regions as far north as northern 

 Japan and Sakhalin we must conclude, on the basis both of paleo- 

 botanic data and of data of investigations of the present-day flora, that 

 the composition of the vegetation has remained practically unchanged 

 since Tertiary times, although here climatic variations were consider- 

 ably more marked than farther south. 



Turning now to the tropics of the New World, we do not find such 

 constancy as regards climatic conditions; nevertheless, there were no 

 such great climatic changes as took place in the more northern lati- 

 tudes of North America and Europe. 



The circumstances that we have just mentioned throw considerable 

 light on the numerical composition of floras. Those regions of the 

 earth that have been least subjected to climatic changes— southeastern 



Fig. 27. — Same as in Fig. 25, but in the Eocene. (After Koppen and Wegener). 



subtropical and tropical Asia and tropical South America — are charac- 

 terized by the richest and most constant floras, that have developed 

 with comparatively little alteration since Tertiary times. The floras 

 of these regions comprise from 20,000 to 45,000 species (Wulff, 1934). 

 The tropical flora of Africa, on the other hand, which has not enjoyed 

 such an even tenure of existence, is poorer, consisting of about 10,000- 

 15,000 species. We find a relative wealth of species in the Mediterra- 

 nean floras and also in those of the southeastern and southwestern 

 sections of the United States of America, which — though hav- 

 ing undergone considerable impoverishment, expressed in the loss 

 chiefly of hydrophytic species — have stiU preserved a number of an- 

 cient. Tertiary elements. This is particularly marked in the above- 

 mentioned sections of the United States and also in the southwestern 

 part of the Iberian peninsula, the Istranja Dagh region in the Balkans, 

 the northern coast region of Asia Minor, and western Transcaucasia, 

 where, thanks to specific orographic conditions, these ancient elements 



