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Show .178 DIFFICULTIES ON THEORY." CHAP. VI. polity of the country can be better ~lle~ by. some rfi t' f e one or more of Its Inhabitants. mocti ca 1on o som And such new places will depen~ on. slow chang~s of climate, or on the occasionalimmigra~wn of ntewl Inha-b 't t d probably in a still more Importan c egree, I an s, an , ' . 1 d' on some of the old inhabitants becommg slow y mo 1- fied, with the new forms thus produced and the o~d ones acting and reacting on ea~h other. So that, In any one region and at any o~e time, ~e o~ght only to see a few species presenting shght modifi~ations of structure in some degree permanent; and this assuredly we do see. Secondly, areas now continuous must often ha~e 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 c~istinct t~ rank a.s representative species. In this case, In~ermedia.te 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 generally have had a short duration. For these intermediate varieties will, from reasons already assigned (namely from what we know of the actual distr~buti?n of closely allied or representative species, and hke';Ise of acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they tend to connect. From this cause alone the interme· .CIIAP. VI. . TRANSITIONAL HABITS. 179 diate va~ieties will be liable to accidental extermination ; and dunng t~e 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 ~ggregate, 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, ~ m! theory be true, numberless intermediate varieties, hnlnng most closely all the species of the same group together, must assuredly have existed ; but the very process of natural selection constantly tends, as has been so o~ten rem~rked: to exterminate the parent-forms and the Inter~ediate hnks. Consequently evidence of their form~r exis~ence could be found only amongst fossil remains, whiCh are preserved, as we shall in a future ?hapte: attempt to show, in an extremely imperfect and Intermittent record. On. the or~gin and transitions of organic beings with peculzar habzts and structure.-! t has been asked by the opponents of such views as I hold, how, for instance, ~ land ca~nivorous .animal could have been converted ~nt~ one Wl~h. aquatic habits; for how could the animal In Its transitional state have subsisted? It would be ea~y to sh~w tha~ within the same group carnivorous animals exi~t hav1ng every intermediate grade between truly a~uatiC and strictly terrestrial habits ; and as each exists by a struggle for life, it is clear that each is well adapted in its habits to its place in nature. Look at the Mustela vison of North .c'lmerica, which has webbed feet and which resembles an otter in its fur :ort legs, and form of tail ; during summer this animai ves for and preys on fish, but during the long winter |