442 ORIGIN OF SPECIES 



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 dis- 

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

 subsequently spread from one to another, it would undoubt- 

 edly be exposed to different conditions in the different islands, 

 for it would have to compete with a different set of organ- 

 isms; 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 

 Archipelago, 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 effect- 

 ually 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, 



