364 THE ORIGIN OF SPECIES 



portant, 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 in- 

 stance, 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 im- 

 migrant first settled on one of the islands, or when it subsequently 

 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 favor different varieties in the 

 different islands. Some species, however, might spread and yef 

 retain the same character throughout the group, just as we see 

 some species spreading widely throughout a continent and re- 

 maining 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 deep 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 the 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 in- 

 vading each other's territory, when put into free intercommunica- 

 tion. 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 prob- 

 ably hold their separate places for almost any length of tim.e. 

 Being familiar with the fact that many species, naturahzed 



