Chap.XIII.] islands TO THOSE OF THE MAINLAND. 193 



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 moeking-thrush 

 peculiar to Charles's 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 trans- 

 ported from Porto Santo to Madeira, yet this latter 

 island has not become colonised by the Porto Santo 

 species; nevertheless both islands have been colonised 

 by European land-shells, 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, pre- 

 occupation has probably played an important part in 

 checking the commingling of the species which inhabit 

 different districts with nearly the same physical condi- 

 tions. Thus, the south-east and south-west 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; 

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

 other animals inhabiting the great, open, and continu- 

 ous valley of the Amazons. 



