INHABITANTS OF ISLANDS 443 



are common to the several islands; and we may infer from 

 their present manner of distribution, that they have spread 

 from one island to the others. But we often take, I think, an 

 erroneous view of tlie probability of closely-allied species in- 

 vading each other's territory, when put into free intercom- 

 munication. Undoubtedly, if one species has any advantat^^e 

 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 prob"bly hold their separate places for al- 

 most any length of ♦^ime. Being familiar with the fact that 

 many species, naturalised tlirough 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 dis- 

 tinct 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 Chat- 

 ham 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 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 spe- 

 cies. From these considerations I think we need not greatly 



