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Show 462 RECAPITULATION. CHAP. XIV. even to conjecture how this could have been effected. Yet as we have reason to believe that some species have ret:ined the same specific form for very long periods, enormously long as measured by rears, t?o m~ch ~tress ought not to be laid on t~e occasional wid~ diffusw~ of the same species ; for during very long peno~s of ~Ime there will always be a good chance for wide migration by many means. A broken or inte:ru~ted range may often be accounted for by the extinctiOn of the species in the intermediate regions. It cannot be denied that we are as yet very ignorant of the full extent of the various climatal and geographical changes which have affected the earth during modern periods ; and such changes will obviously have greatly facilitated migration. As an example, I have attempted to show how potent has been the influence of the Glacial period on the distribution both of the same and of representative species throughout the world. We are as yet profoundly ignorant of the many occasional means of transport. With respect to distinct species of the same genus inhabiting very distant and isolated regions, as the process of modification bas necessarily been slow, all the means of migration will have been possible during a very long period; and consequently the difficulty of the wide diffusion of species of the saine genus is in some degree lessened. As on the theory of natural selection an interminable number of intermediate forms must have existed, linking together all the species in each group by gradations as fine as our present varieties, it may be asked, Why do we not see these linking forms all around us ? Why are not all organic beings blended together in an inextricable chaos? With respect to existing forms, we should remember that we have no right to expect (~xcepting in rare cases) to discover directly connectlllg CHAP. XIV. RECAPITULATION. 463 link.s betwde en them, but only between each and 8 orne extinct an supplanted form. Even on a wide h . h h d . area, w Ic as unng a long period remained continu d f h . h . ous, an o w IC the chmate and other conditions of li£ change i~sen.sibly in going from a district occupied b; on~ specie~ Into another district occupied by a closely allied. spemes, .we hav~ ~o j?st right to expect often to find Intermediate varieties In the intermediate zone. For we hav~ reason to believe that only a few species are undergoing change at any one period ; and all changes are slowly effected. I have also shown that the ~nterme?iate var~eties which will at first probably exist In the Intermediate zones, will be liable to be supplanted by the allied forms on either hand ; and the latter, fr_om existi~g in greater numbers, will generally be modified and Improved at a quicker rate than the intermediate varieties, which exist in lesser numbers · so that the intermediate varieties will, in the long run: be supplanted and exterminated. On this doctrine of the extermination of an infinitude of c_onnecting links, between the living and extinct inhabitants of the world, and at each successive period between the extinct and still older species, why is not every geological formation charged with such links ? Why does not every collection of fossil remains afford plain evidence of the gradation and mutation of the forms of life ? We meet with no such evidence and this is t?e most obvious and forcible of the many ~bjections which may be urged against my theory. Why, again, d~ whole groups of allied species appear, though certamly they often falsely appear, to have come in suddenly on the several geological stages ? Why do we not find gre~t piles of strata beneath the Silurian system, stored With the remains of the progenitors of the Silurian groups of fossils ? For certainly on my theory such |
