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Show 400 GEOGRAPHICAL DISTRIBUTION. CHAP. XII. pelago, though specifically distinct, to be clos~ly allied t o tho se of the nearest continent, we som.e times s. ee d I.s p 1a ye d on a small scale yet in a most Interesting <: ' • manner, WI' thin the limits of the same archip. elago. Thus the several. islands of the Galapagos ~rchipela?o are tenanted, as I have elsewhere shown, In a q~te marvellous manner, by very closely re~a ted spemes ; so that the inhabitants of each sep~rate Island, though mostly distinct, are related in an Incomparably closer degree to each other than to th~ i~h~bitants of .any other part of the world. And this IS JUSt what might have been expected on my view, for the islands are situated so near each other that they would almost certainly receive immigrants from the. sa~e. o~igi~al source or from each other. But tb1s dissimilanty betwe~n the endemic inhabitants of the islands may be used as an araument against my views ; for it may be asked how ha~ it happened in the several islands situated 'within sight of each other, having the same geological nature, the same height, climate, .&c., that many of the immigrants should have been diff~rently modified, though only in a small degree. . Thi.s lo~g appeared to me a great difficulty: but It ar:ses. m chief part from the deeply-seated error of consider:ng the physical conditions of a country. as the most .unportant for its inhabitants ; whereas It cannot, I think, be disputed that the nature of th~ other inhab~tants, with which each has to compete, IS at least as Important, and generally a far more imp?rtan~ element of success. Now if we look to those Inhabitants of the Galapagos Archipelago which ~re found in other parts of the world (laying on one side for the .mo~ent the endemic species, which cannot be here fairly Include~, as we are considering how they have come to be modified since their arrival), we find a considerable amount CHAP. XII. 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 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 Galapagos Archipelago, and in a lesser degree in some analogous instances, is that the new species formed in the separate islands have not quicldy 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 |