GEOGRAPHICAL DISTRIBUTION 36S 



through 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, belong- 

 ing 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 mocking- 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 an- 

 nually more eggs are laid and young birds hatched than can pos- 

 sibly 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. WoUaston have communicated to me a remarkable fact bear- 

 ing 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 al- 

 though 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. From these considera- 

 tions 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 con- 

 tinent, 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 butter- 

 flies and other animals inhabiting 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 the source 

 whence colonists could have been most easily derived, together 

 with their subsequent modification, is of the widest application 



