Chap. XIII.] ISLANDS TO THOSE OF THE MAINLAND. 191 



as the most important ; whereas it cannot be disputed 

 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 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 im- 

 migrant 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 

 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 



