GEOGRAPHICAL DISTRIBUTION. 105 



have been introduced by man, directly or in- 

 directly) will be found to be peculiar species, 

 resembling no others found on the surface of 

 the globe, although generally exhibiting some 

 degree of affinity with those of the nearest land. 

 Mammalia, except bats, have no ordinary 

 means of crossing a broad sea ; and bats are 

 almost the onlv mammals which are met with 

 in oceanic islands. 



Batrachians and their spawn are instantly 

 killed by salt water, and, unless introduced by 

 man, are likewise absent.* Many birds and 

 insects which occur on islands are either 

 wingless or incapable of flight, a singular fact 

 which cannot be without some important 

 signification. Many of the beetles of Madeira 

 are wingless, even among those which are 

 considered specifically identical with winged 

 European species. It must not be forgotten 

 that the development of wings in beetles is 

 sometimes a variable character, even within 

 the same species ; but this consideration does 

 not affect the problem to be solved, further 

 than that the fact of a character being variable, 



* Darwin, " Origin of Species," pp. 424, 425. 



