This is what might have been expected, for islands situated so near to each other would almost necessarily receive immigrants from the same original source, and from each other.

But how is it that many of the immigrants have been differently modified, though only in a small degree, in islands situated within sight of each other, having the same geological nature, the same height, climate, etc?
This long appeared to me a great difficulty: but it arises in chief part from the deeply-seated error of considering the physical conditions of a country 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 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.