£56 RELATIONS OF THE INHABITANTS OF [Chap. XIII. 



islands, for it would have to compete with o different set of 

 organisms ; a plant for instance, would find the ground best fitted 

 for it occupied by somewhat different species in the different 

 islands, and would be exposed to the attacks of somewhat dif- 

 ferent enemies. If then it varied, natural selection would probably 

 favour different varieties in the different islands. Some species, 

 however, might spread and yet retain the same character through- 

 out the group, just as we see some species spreading widely 

 throughout a continent and remaining the same. 



The really surprising fact in this case of the Galapagos Archipe- 

 lago, and in a lesser degree in some analogous cases, is that each 

 new species after being formed in any one island, did not spread 

 quickly to the other islands. But the islands, though in sight of 

 each other, are separated by deep arms of the sea, in most cases 

 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 aro 

 tar 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 ono 

 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 intercommunication. 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 familar 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 inhabitants, 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- thrash, 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 



