CHAP.XIIL] ISLANDS TO THOSE OF THE MAINLAND. 335 



of closely-allied species invading each other's territory, when put 

 into free intercommunication. Undoubtedly, if one species has 

 any advantage over another, it will in a very brief time wholly or 

 in part supplant it ; but if both are equally well fitted for their 

 own places, both will probably hold their separate places for 

 almost any length of time. Being familiar with the fact that 

 many species, naturalised 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 naturalised 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 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 establish- 

 ing 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 com- 

 municated 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 colonised by the 

 Porto Santo species; nevertheless both islands have been colo- 

 nised 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 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 inhabiting the great, open, and continuous valley 

 of the Amazons. 



