126 THE THEORY OF EVOLUTION 



arms of the sea; no such animal can pass the English 

 Channel or Bering Strait. Next in importance is 

 climate and especially temperature, though hu- 

 midity also plays an effective part. As we have seen, 

 similarity of climate is, of itself, entirely powerless 

 to bring about similarity of animals, but as a passive 

 barrier, climatic differences are most effective in 

 limiting the spread of organisms, many of which are 

 utterly unable to pass an isothermal line. In minor 

 degree, large rivers, mountain-chains and the other 

 great topographic features frequently act as barriers. 

 Still another kind of barrier is pre-occupation by 

 some other species, it may be a competing species 

 which leaves no place in the economy of nature 

 which the newcomer can fill, or it may be an enemy 

 or infectious disease which destroys the immigrants 

 as fast as they can arrive. The Tse-tse flies of Africa 

 close the area of their range against the entrance of 

 most of the domestic animals by infecting them with 

 a fatal disease. 



/ Many attempts have been made to divide the 

 lands of the globe into zoological regions, which 

 shall express the degrees of likeness and difference 

 in their animal inhabitants. This cannot be satis- 

 factorily done by using the same scheme for all 

 kinds of organisms, for the geological dates of the 

 origin and dispersal of the various types of land 

 animals are so far apart in time, that each found 

 such very different geographical arrangements, and 

 frequently also climatic conditions, that the possi- 



