192 RELATIONS OF THE INHABITANTS OP [Chap. XIII, 



wider 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 intercom- 

 munication. Undoubtedly, if one species has any ad- 

 vantage 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 sepa- 

 rate places for almost any length of time. Being«famil- 

 iar 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 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 



