ISLANDS TO THOSE OF THE MAINLAND. 396 



man's agency, have spread with astonishing rapidity over 

 wide areas, we are apt to infer that most species would thus 

 spread; but we should remember that the species which 

 become naturalized in new countries are not generally closely 

 allied to the aboriginal inhabitants, but are very distinct 

 forms, belonging in a large proportion of cases, as shown by 

 Alph. de Candolle, to distinct genera. In the Galapagos 

 Archipelago, many even of the birds, though so well adapted 

 for flying from island to island, differ on the different 

 islands ; thus there are three closely allied species of mock- 

 ing-thrush, each confined to its own island. Now 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 pos- 

 sess 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-shells, 

 which no doubt had some advantage over the indigenous 

 species. Prom 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 conditions. 

 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 inhabit- 

 ing the great, open, and continuous valley of the Amazons. 



The same principle which governs the general character 

 of the inhabitants of oceanic islands, namely, the relation to 



