CiiAi'. XII. ISLANDS TO THOSE OF THE MAIN-LAND. 2G5 



period boon continuously uuitccL The currcuts of the sea arc 

 rapid and sweep across the archipela;^o, and gales of wind arc 

 extraordinarily rare ; so that the islands are far more ciTcct- 

 iially separated from each other than they appear on a map. 

 Nevertlieless, some of tlie species, both those foimd in other 

 parts of tlic world and tliose confined to the archii)elag-o, are 

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

 present manner of distribution that they have spread frona one 

 island to the othei-s. But we often take, I think, au erroneous 

 view of the probability of closely-allied species invading each 

 other's territory, when put into free intercommunication. Un- 

 doubtedly, 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 proba- 

 bly will hold their jilaces and keep separate for almost any 

 length of time. Being familiar with the fact that many spe- 

 cies, naturalized through man's agency, have spread with as- 

 tonishing raj)idity over wide areas, we are apt to infer that 

 most species would thus spread ; but we should remember that 

 the speci(;s 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 CandoUe, to distinct genera. In the 

 (jrulapagos Archipelago, many even of the birds, though so 

 well adapted for Hying from island to island, are distinct on 

 each ; thus there are three closely-allied species of mocking- 

 thrush, each confined to its own island. Now let us suppose 

 the mocking-thrush of Cliatham Island to be blown to Charles 

 Island, which has its own mocking-thrush ; why should it suc- 

 ceed in establishing itself there ? We may safely infer that 

 Charles Island is well stocked with its own species, for aiuiu- 

 ally more eggs are laid and yv>ung birds hatched, than can 

 possibly be reared ; and we may infer that the mocking-tlirush 

 l)eculiar to Charles Island is at least as well fitted for its home 

 as is the species pecuhar 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 annuallv transported 

 from Porto Santo to Madeira, yet this latter island has not be- 

 come coloniz(vl by the Porto Santo species : nevertheless, both 

 islands have been colunized by European land-shells, which no 



