Chap. XII. 
OCEAKIC ISLANDS. 
401 
endemic species, wliicli 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 conditions 
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 per¬ 
fectly 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 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 
