24 OUTLINE OF PLANT GEOGRAPHY 



is true of pines and firs. The rose family, buttercups and violets 

 are mostly northern types, but may occur in the temperate regions 

 of the south. It is thought that the latter may be immigrants from 

 the north, which have become more or less changed by their 

 residence in their new habitat. 



Probably of southern origin are many peculiar South American, 

 South African and Australian types. Among these may be 

 mentioned the peculiar southern conifers, the Kauri-pines 

 (Agathis), Araucaria and Podocarpus. The Myrtle family, which 

 has but few representatives in north temperate regions, is extraor- 

 dinarily developed in Australia, where the genus Eucalyptus alone 

 has over 200 species. Two extremely peculiar families, Proteaceae 

 and Casuarinaceae, the former especially abundant in Australia 

 and South Africa, the latter mainly Australian, are quite un- 

 represented in the temperate regions of the north. 



It is true that some of these, like Araucaria and Podocarpus, 

 once lived in the northern regions, from which they probably 

 disappeared owing to climatic changes. 



It is possible that the peculiar northern and southern floras 

 originated at a time when there was a more or less complete separa- 

 tion of the mainland masses of the two hemispheres. During the 

 Cretaceous, at which time many existing genera are first known, 

 there were extensive invasions of the sea resulting in the sub- 

 mersion of northern South America, North Africa and much of 

 India, thus isolating the principal southern land-masses. About 

 the same time, Western Australia was probably completely 

 isolated, and it has been thought that many peculiar Australian 

 types originated in this ancient Western Australian continent. 



Just what factors originally determined this segregation of the 

 characteristic floras of the north and south temperate zones it 

 would be difficult to say; but the subsequent differentiation is 

 undoubtedly due in large part to the very different climatic 

 conditions, and the much greater isolation of the southern land- 

 masses. 



The north temperate floras for the most part are subject to a 

 much more severe and variable climate, but readily adapt them- 

 selves to the milder climates of the south temperate; and this may 

 explain the presence of such northern types as buttercups and 

 violets in New Zealand and Australia, while such Australian forms 



