CHAPTER VII 



SPECIAL FEATURES OF ISLAND FAUNAS 



Islands present some interesting features as regards 

 their animal life which makes them worthy of special 

 study. We have already seen that mountain areas have 

 special types of animals, because they form isolated 

 regions cut o£E by the physical conditions from the 

 neighbouring districts. The special conditions in then- 

 case include both climate and topography, and in what- 

 ever region of the globe the mountains occur, if they are 

 greatly elevated, the climatic conditions are similar. 

 Islands are regions cut off from neighboiuing regions 

 by the sea, which forms a barrier to the passage of all 

 but the marine and the flying animals. Even to flying 

 animals a large expanse of sea may constitute a for- 

 midable barrier, save in the case of those with excep- 

 tional powers of flight. The consequence is that island 

 faunas form a whole, and can be studied with much 

 more ease than the animals of any other natural region. 

 The question whether a particular animal does or does 

 not occur in an island is merely a question of observa- 

 tion ; while that as to whether a form found on a 

 mountain does or does not constitute part of the moun- 

 tain fauna is a much more delicate one, involving an 

 elaborate process of reasoning. In the one case a 

 gradual migration with concurrent adaptation is pos- 

 sible from plain to moxmtain or vice versa, but no such 

 slow migration is possible in the case of islands. 



On the other hand, it is to be noted that while 



