ISLANDS TO THOSE OF THE MAINLAND. 421 



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, naturalized 

 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 inhabi- 

 tants, 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 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 0. 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 possesses many distinct but representative 

 species of land-shells, some of which live in crevices of 

 stone; and although large quantities of stone are annually 

 t];ansported 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 considerations I 

 think we need not greatly marvel at the endemic species 

 which inhabit the several islands of the Galapagos Archi- 

 pelago not having all spread from island to island. On 

 the same continent, also, preoccupation has probably 

 played an important part in checking the commingling of 

 the species which inhabit different districts with nearly the 



