242 ISLAND LIFE 



case of continents, and their peculiar species and groups 

 are usually well defined and strictly limited in range. 

 Again, their relations with other lands are often direct 

 and simple, and even when more complex are far easier to 

 comprehend than those of continents ; and they exhibit 

 besides certain influences on the forms of life and certain 

 peculiarities in their distribution which continents do not 

 present, and whose study offers many points of interest. 



In islands we have the facts of distribution pre- 

 sented to us, sometimes in their simplest forms, in other 

 cases becoming gradually more and more complex ; and we 

 are therefore able to proceed step by step in the solution 

 of the problems they present. But as in studying these 

 problems we have necessarily to take into account the 

 relations of the insular and continental faunas, we also get 

 some knowledge of the latter, and acquire besides so much 

 command over the general principles which underlie all 

 problems of distribution, that it is not too much to say 

 that when we have mastered the difficulties presented by 

 the peculiarities of island life we shall find it comparatively 

 easy to deal with the more complex and less clearly de- 

 fined problems of continental distribution. 



Classification of Islands ivitli Reference to Distribution. — 

 Islands have had two distinct modes of origin — they have 

 either been separated from continents of which they are 

 but detached fragments, or they have originated in the 

 ocean and have never formed part of a continent or any 

 large mass of land. This difference of origin is funda- 

 mental, and leads to a most important difference in their 

 animal inhabitants ; and we may therefore first distinguish 

 the two classes — oceanic and continental islands. 



Mr. Darwin appears to have been the first writer who 

 called attention to the number and importance, both from 

 a geological and biological point of view, of oceanic 

 islands. He showed that with very few exceptions all the 

 remoter islands of the great oceans were of volcanic or 

 coralline formation, and that none of them contained 

 indigenous mammalia or amphibia. He also showed the 

 connection of these two phenomena, and maintained 

 that none of the islands so characterised had ever formed 



