202 THE ORIGIN OF SPECIES 



of mocking-thrush, each confined to its own island. Mow 

 let us suppose the mocking-thrush of Chatham Island to 

 be blown to Charles Island, which has its own mocking- 

 thrush; why should it succeed in establishing itself there/ 

 We may safely infer that Charles Island is well stocked 

 with its own species, for annually more eggs are laid and 

 young birds hatched than can possibly be reared; and we 

 may infer that the mocking-thrush peculiar to Charles 

 Island is at least as well fitted for its home as is the 

 species peculiar to Chatham Island. Sir C. Lyell and 

 Mr. Wollaston have communicated to me a remarkable 

 fact bearing on this subject; namely, that Madeira and 

 the adjoining islet of Porto Santo possess many distinct 

 but representative species of land-shells, some of which 

 live in crevices of stone; and although large quantities 

 of stone are annually transported from Porto Santo to 

 Madeira, yet this latter island has not become colonized 

 by the Porto Santo species; nevertheless both islands 

 have been colonized by European land-sbells, which no 

 doubt had some advantage over the indigenous species. 

 From these considerations I think we need not greatly 

 marvel at the endemic species which inhabit the several 

 islands of the Galapagos Archipelago not having all 

 spread from island to island. On the same continent, 

 also, preoccupation has probably played an important 

 part in checking the commingling of the species which 

 inhabit different districts with nearly the same physical 

 conditions. Thus, the southeast and southwest corners of 

 Australia have nearly the same physical conditions, and 

 are united by continuous land, yet they are inhabited by 

 a vast number of distinct mammals, birds, and plants; 

 ao it is, according to Mr. Bates, with the butterflies and 



