248 Plant Life and Evolution 



so very much isolated, and under the forcing condi- 

 tions of the tropics, and the sharp struggle for ex- 

 istence, the change in species is presumably much 

 more rapid than is the case in the temperate zones. 



Present Conditions in the Southern Hemisphere. 

 The conditions in the Southern Hemisphere, at 

 present, are very different from those in the North. 

 The Antarctic continent is an absolutely barren 

 waste, with scarcely a vestige of any vegetation, and 

 it is separated completely from the three principal 

 land masses of the South Australia, South Amer- 

 ica, and South Africa. While in the course of the 

 ages which have elapsed since the three latter were 

 united, the vegetation has become very much al- 

 tered, there still are evidences of a common origin 

 for the floras, although this is by no means so 

 marked as in the Northern Hemisphere. The Arau- 

 carias of South America and Australia, and the Pro- 

 teaceae found in all three regions, are presumably 

 descendants of the common primordial flora of the 

 ancient southern continent. 



Floras of Isolated Regions. Wherever a region 

 is shut off by barriers, either mountains, desert, or 

 sea, the flora is certain to be very peculiar. In such 

 isolated regions as the Cape region of Africa, West- 

 ern Australia, or even regions like California or 

 the shores of the Mediterranean, this is very evi- 

 dent. In all of these, climatic conditions are more 

 or less similar and all of them have developed very 

 rich and peculiar floras that show some interesting 



