294 RELATIONS OF THE INHABITANTS OF 



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 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 yet retain the same character throughout the group, just 

 as we see some species spreading widely throughout a con- 

 tinent 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 Chan- 

 nel, 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 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 

 familiar with the fact that many species, naturalized through 



