Page - 503 - in The Origin of Species

RECAPITULATION AND CONCLUSION 503 With respect to existing forms, we should remember that we have no right to expect (excepting in rare cases) to discover directly connecting Hnks between them, but only between each and some extinct and supplanted form. Even on a wide area, which has during a long period remained continuous, and of which the climatic and other conditions of life change insensibly in proceeding from a district occupied by one species into another district occupied by a closely allied species, we have no just right to expect often to find inter- mediate varieties in the intermediate zones. For we have reason to believe that only a few species of a genus ever undergo change; the other species becoming utterly extinct and leaving no modified progeny. Of the species which do change, only a few within the same country change at the same time; and all modifications are slowly effected. I have also shown that the intermediate varieties which probably at first existed in the intermediate zones, would be liable to be supplanted by the allied forms on either hand; for the latter, from existing in greater numbers, would generally be modi- fied and improved at a quicker rate than the intermediate vari- eties, which existed in lesser numbers; so that the inter- mediate varieties would, in the long run, be supplanted and exterminated. On this doctrine of the extermination of an infinitude of connecting 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 col- lection of fossil remains afford plain evidence of the grada- tion and mutation of the forms of life? Although geological research has undoubtedly revealed the former existence of many links, bringing numerous forms of life much closer to- gether, it does not yield the infinitely many fine gradations between past and present species required on the theory ; and this is the most obvious of the many objections which may be urged against it. Why, again, do whole groups of allied species appear, though this appearance is often false, to have come in suddenly on the successive geological stages? Al- though we now know that organic beings appeared on this globe, at a period incalculably remote, long before the lowest