420 



RELATIONS OF THE INHABITANTS OF 



ant 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 subsequently spread from one to another, it would 

 undoubtedly be exposed to different conditions in the dif- 

 ferent islands, for it would have to compete with a differ- 

 ent 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 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 cjuickly to the other islands. 

 But the islands, though in sight of each other, are sepa- 

 rated 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 m.ay infer from the present 

 minner 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 inter- 

 communication. Undoubtedly, if one species has any 



