528 ISLAND LIFE 



tions to soil, climate, and organic environment, but 

 comparatively impotent and inferior beyond their own 

 domain. 



Another circumstance which makes the contest between 

 the northern and southern forms still more unequal, is the 

 much greater hardiness of the former, from having been 

 developed in a colder region, and one where alpine and 

 arctic conditions extensively prevail ; whereas the southern 

 floras have been mainly developed in mild regions to 

 which they have been altogether confined. While the 

 northern plants have been driven north or south by each 

 succeeding change of climate, the southern species have 

 undergone comparatively slight changes of this nature, 

 owing to the areas they occupy being unconnected with 

 the ice-bearing Antarctic continent. It follows, that 

 whereas the northern plants find in all these southern 

 lands a milder and more equable climate than that to 

 which they have been accustomed, and are thus often able 

 to grow and flourish even more vigorously than in their 

 native land, the southern plants would find in almost 

 every part of Europe, North America or Northern Asia, a 

 more severe and less equable climate, with winters that 

 usually prove fatal to them even under cultivation. These 

 causes, taken separately, are very powerful, but when 

 combined they must, I think, be held to be amply sufficient 

 to explain why examples of the typical southern vegetation 

 are almost unknown in the north temperate zone, while a 

 very few of them have extended so far as the northern 

 tropic.^ 



^ The fact stated in the last edition of the Origin of Species (p. 340) on 

 the authority of Sir Joseph Hooker, that Australian plants are rapidly- 

 sowing themselves and becoming naturalised on the Neilgherrie mountains 

 in the southern part of the Indian Peninsula, though an exception to the 

 rule of the inability of Australian plants to become naturalised in the 

 Northern Hemisphere, is yet quite in harmony with the hypothesis here 

 advocated. For not only is the climate of the Neilgherries more favour- 

 able to Australian plants than any part of the North Temperate zone, but 

 the entire Indian Peninsula has existed for unknown ages as an island and 

 thus possesses the "insular" characteristic of a comparatively poor and 

 less developed flora and fauna as compared with the truly " continental " 

 Malayan and Himalayan regions. Australian plants are thus enabled to 

 compete with those of the Indian Peninsula highlands with a fair chance 

 of success. 



