192 RELATIONS OF THE INHABITANTS OF [Chap. XIII. 



than the British Channel, and there is no reason to 

 suppose that they have at any former period been 

 continuously united. The currents of the sea are rapid 

 and sweep between the islands, and gales of wind are 

 extraordinarily rare ; so that the islands are far more 

 effectually separated from each other than they appear 

 on a map. Nevertheless some of the species, both of 

 those found in other parts of the world and of those 

 confined to the archipelago, 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 the probability of closely-allied species invading 

 each other's territory, when put into free intercommuni- 

 cation. 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 wliich become naturalised in new countries are 

 not geniirally closely allied to the aboriginal inhabitants, 

 but are very distinct forms, belonging in a large pro- 

 pnrlioii of cases, as sliown by Alph. de Candolle, to 

 di still ol genera. In tlie Gala]5agos Archipelago, many 

 ex'cn of the birds, though so well adapted for flying 

 iVoin island to island, difter on tlie diffyrent islands ; 

 thus there arc tlivee- closely -allied species of mocking- 

 tlirush, eacli confined to its own island. Now let us 

 suppose the niocking-thriish of Chatham Islaud to be 



