360 ON THE ORIGIN OF SPECIES 



important for its inhabitants ; whereas it cannot, I think, 

 be disputed that the nature of the other inhabitants, 

 with which each has to compete, is at least as import- 

 ant, and generally a far more important element of 

 success. Now if we look to those inhabitants of the 

 Galapagos Archipelago which are found in other parts 

 of the world (laying on one side for the moment the 

 endemic species, which cannot be here fairly included, 

 as we are considering how they have come to be modi- 

 fied since their arrival), we find a considerable amount 

 of difference in the several islands. This difference 

 might indeed have been expected on the view of the 

 islands having 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. Hence when in former times an 

 immigrant settled on any one or more of the islands, or 

 when it subsequently spread from one island to another, 

 it would undoubtedly be exposed to different condi- 

 tions of life in the different islands, for it would have 

 to compete with different sets of organisms : a plant 

 for instance, would find the best-fitted ground more 

 perfectly occupied by distinct plants in one island than 

 in another, and it 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 on continents some species 

 spreading widely and remaining the same. 



The really surprising fact in this case of the Gala- 

 pagos Archipelago, and in a lesser degree in some 

 analogous instances, is that the new species formed in 

 the separate islands have not quickly spread 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 across the archipelago, and 



