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Show 160 DIFFICULTIES ON THEORY. (CHAP. VI. slow changes of climate, or on the o?cas1o~al immigration of new inhabitants and, probably, In a still more Important degree, on so~e of the old inhabitants becoming slowly modified, with the new forms thus produced and the old ones acting and reacting on each other. So that, in any one region and at any one time, we ought only to see a few species presenting slight modifications of structure in some degree permanent; and this assuredly we see: Secondly, areas now continuous must often have existed within the recent period in isolated portions, in which many forms, more especially amongst the classes which unite for each birth and wander much, may have separately been rendered sufficiently distinct to rank as representative species. In. this case, intermediate varieties between the several representative species and their common parent, must formerly have existed in each broken portion of the land, but these links will have been supplanted and exterminated during the process of natural selection, so that they will no longer exist in a living state. Thirdly, when two or more varieties have been formed in different portions of a strictly continuous area, intermediate varieties will, it is probable, at first have been formed in the intermediate zones, but they will e;enerally have had a short duration. For these intermediate varieties will, from reasons already assigned (namely from what we know of the actual distribution of closely allied or representative species, and likewise of acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they tend to connect. From this cause alone the intermediate varieties will be liable to accidental extermination; and during the process of further modification through natural selection, they will almost certainly be beaten and supplanted by the forms which they connect; for these from existing in greater numbers will, in the aggregate, present more variation, and thus be further improved through-natural selection and gain further advantages. Lastly, looking not to any one time, but to all time, if CHAP. VLJ TRANSITIONAL HABITS. 161 ~y theory be true, numberless intermediate varieties, linkIng most closely all the species of the saine group together must assured~y have existed ; but the very process of natural selection constantly tends, as has been so often remar~ 0d, ~o exterminate the parent-forms and the inter~ nediate hnks. Consequently evidence of their fomer exIstence could be found only amongst fossil remains which are pr~served, as we sh.all in a future chapter atte~pt to show, In an extremely Imperfect and intermittent record. On the origin and transitions Q( organic beings with peculiar habits and ~tructure.-It has been asked by the opponent~ of such yiews as I hold, how, for instance, a land carnivorous animal could have been converted into one with aquatic habits ; for how could the animal in its transitional ~ta~e have subsisted? It would be easy to show that Within the same group carnivorous animals exist havi~g every int~rmedia~e grade between truly aquatic and strwtly ~err~st~Ial habits ; and as each exists by a struggle for hfe; It IS clear that each is well adapted in its habits to its place in nature. Look at the Mustela vison of North America, which has webbed feet and which resmnbles an otter in its fur short leO's and form of tail; during summer this animal dives fo~ 'and preys on fish, but during: the long winter it leaves the frozen w~ters, and preJ:s like other pole-cats on mice and land ainmals. If a different case had been taken and it had b.een asked how an insect~vorous q:nadruped could posSibly have been converted I.nto a flying bat, the question . would have been far :rr;ore dlfficu~t, and .I could have given no answer. Yet I think such difficulties have very little weight. Here, as on other occasions, I lie under a heavy disadvantage, for out of the many striking cases which I have .c?llected, ! can give only one or two instances of transitional habits and structures in closely allied species of the same genus ; and of diversified habits either constant or occasional, in the same species. And it seems to m~ that nothing less than a long list of such cases is suffiCient to lessen the difficulty in any particular case like that of the bat. ' |