The Problems of Plant Distribution 253 



they do occur, they are, as we have seen, genera 

 which are widespread throughout the temperate 

 regions. Senecio, Vernonia, Acacia, and Ipomoea 

 are examples of some of these widespread genera. 

 Among the characteristic tropical types, like the 

 palms, aroids, bananas, etc., very few genera, even, 

 are common to both hemispheres. Sometimes an 

 Old World family is represented by an allied one in 

 the tropics of the New World. Thus, for instance, 

 the Ginger family is only found in the Old World, 

 the Canna family in the new. 



The South Temperate Zone. The south tem- 

 perate regions are very much more isolated than 

 those of the north, and for the most part have 

 very different types of vegetation, there being very 

 little in common, for instance, between the flora 

 of Argentina and that of South Africa. There are, 

 however, certain similarities between the flora of 

 the latter and some of the more temperate parts 

 of Australia, and this is true also of some parts 

 of South America, as, for example, the occurrence 

 of Proteaceae and Araucaria, which point to an an- 

 cient connection between these southern regions and 

 denote that the flora of all of these regions have 

 had a common, but very remote, origin. Unfortu- 

 nately our knowledge of the fossils of the Southern 

 Hemisphere is very incomplete, and for the present 

 the geological history of the flora must remain un- 

 satisfactory. 



Cretaceous and Tertiary Plants. There are 



