PEESIDENTIAL ADDEESS. 17 



length in the " Origin of Species" ; as the " Intercrossing 

 of Individuals," " Extinction Caused by Natural Selec- 

 tion jf" " Divergence and Convergence of Character," 

 " Laws of Variation," " Use and Disuse of Parts," and a 

 host of other subjects. 



In considering the distribution of forms of life over the 

 globe, Darwin lays stress on two great facts, viz., that 

 climatical and other physical conditions of a continent, to 

 a very small extent, account for the structural character 

 and form of its inhabitants, but that natural barriers of 

 any kind, or obstacles to free migration, have a most 

 important bearing on the productions of various regions. 



Thus, though America presents almost every variety of 

 climate and temperature, and a parallel condition extends 

 throughout the Old World, the fauna of the two are 

 widely different ; and if small areas of the Old World are 

 hotter than any in the New, these are not inhabited by a 

 fauna different from that of the surrounding districts. 

 The inhabitants of Australia, Africa, and South America, 

 though under the same latitude, are extremely different. 



Great Britain possesses the same quadrupeds as the 

 rest of Europe, because they were doubtless once united, 

 while Europe, Australia, and South America do not possess 

 one mammal common to all, because, so far as we can 

 tell, they were never united. And so ad infinitum. 

 Islands separated by deep channels from the mainland 

 for ages, possess a different fauna. Floras, we have seen, 

 are more uniform, doubtless accounted for by the fact 

 that seeds being extremely hardy, are carried about by 

 winds, sea currents, icebergs, and often by birds and 

 insects. Kemarkable instances are reported by Darwin 

 of plants spread by these agencies. 



Nothing has tended more to exemplify the truth of the 

 Darwinian theory of development than the advances in 



