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Show ·278 HYBRIDISM. CHAP. VIII. cation by the selection of mere external differences, and not of differences in the reproductive system. In all other respects, excluding fertility, there is a close general :resemblance between hybrids and mongrels. Finally, then, the facts briefly given in this chapter do not seem to me opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties. CHAP. IX. IMPERFECTION OF GEOLOGICAL RECORD. 279 CHAPTER IX. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. On the absence of intermediate varieties at the present day- On the nature of extinct intermediate varieties ; on their numberOn the vast lapse of time, as inferred from the rate of deposition and of denudation- On the poorness of our palreontological collections - On the intermittence of geological formations - On the absence of intermediate varieties in any one formation -On the sudden appearance of groups of species - On their sudden appearance in the lowest known fossiliferous strata. IN the sixth chapter I enumerated the chief objections which might be justly urged against the views maintained in this volume. Most of them have now been discussed. One, namely the distinctness of specific forms, and their not being blended together by innumerable transitional links, is a very obvious difficulty. I assigned reasons why such links do not commonly occur at the present day, under the circumstances apparently most favourable for their presence, namely on an extensive and continuous area with graduated physical conditions. I endeavoured to show, that the life of each species depends in a more important manner on the presence of other already defined organic forms, than on climate; and, therefore, that the really governing conditions of life do not graduate away quite insensibly like heat or moisture. I endeavoured, also, to show that intermediate varieties, from existing in lesser numbers than the forms which they connect, will generally be beaten out and exterminated during the course of further modification and improvement. The main cause, however, o£ innumerable intermediate links not now occurring everywhere throughout nature de- |
