Chap. XIT. OCEANIC ISLANDS. 401 



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 conditions 

 of life in the different islands, for ft would have to 

 compete with different sets of organisms : a plant, for 

 instance, would find the best-fitted ground more per- 

 fectly occupied by distinct plants in one island than 

 in another, and it woidd 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 gales of wind 

 are extraordinarily rare ; so that the islands are far 

 more effectually separated from each other than they 

 appear to be on a map. Nevertheless a good many 

 species, both those found in other parts of the world 

 and those confined to the archipelago, are common to 



