334 KELATIONS OF THE INHABITANTS OF [CHAP. SIIL 



immigrants from the same original source, and from each other. 

 But how is it that many of the immigrants have been differently 

 modified, though only in a small degree, in islands situated 

 within sight of each other, having the same geological nature, the 

 same height, climate, &c. 1 This long appeared to me a great 

 difficulty : but it arises in chief part from the deeply-seated error 

 of considering the physical conditions of a country as the most 

 important ; whereas it cannot be disputed that the nature of the 

 other species with which each has to compete, is at least as 

 important, and generally a far more important element of success. 

 Now if we look to the species which inhabit the Galapagos 

 Archipelago, and are likewise found in other parts of the world, 

 we find that they differ considerably in the several islands. This 

 difference might indeed have been expected if the islands have 

 been stocked by occasional means of transport a seed, for 

 instance, of one plant having been brought to one island, and 

 that of another plant to another island, though all proceeding 

 from the same general source. Hence, when in former times an 

 immigrant first settled on one of the islands, or when it subse- 

 quently spread from one to another, it would undoubtedly be 

 exposed to different conditions in the different islands, for it 

 would have to compete with a 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 different 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 throughout 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 Archi- 

 pelago, 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 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 dis- 

 tribution, that they have spread from one island to the others. 

 But we often take, I think, an erroneous view of the probability 



