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Show 244 HYBRIDISM. [CHAP. VIII. tion disturbed by being compounded of. two di~tinct spe· cies, seems closely allied to that ste;nhty which S? _frequently affects pure species, whe?- t~mr ~atural conditions of life have been disturbed. This vieW IS supported by a parallelism of another kind ;-namely, that the cro.ssing of forms only slightly different is favourable ~o the vigour and fertility of their offspring; and that shght changes in the conditions of life are apparently favourable to the vigour and fertility of all organic beings. It is not surprising that the degree of difficulty in uniting tvyo species, and the degree of sterility of their hybr~d-?ffspring should generally correspond though due to distinct causes; for both depend on the 'a~ount of difference of. s.?me ki:r;d between the species which ar.e cross~d. Nor IS It sur:J;n?sing that the facility of effecting a firs~ cross, t)le fertlhty of the hybrids produced, and the capamty of bmng grafted together-though this latter capacity evidently depends on widely different circu;nstances-shou~d all ;un, to a certain extent, parallel with the sys~ematic affinity of t~e forms which are subjected to exp~riment; for systematic affinity attempts to exp:tess all lnnds of resemblance be-tween all species. . . First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, an~ the~r mongrel offspring, are very generally, but not quite universally, fertile. Nor is this nearly general .and perfect fertility surpris!ng, w~en we remember. b?w ~1able we are to argue in a circle with respect to varieties 1n a state of nature· and when we remember that the greater number of vari~ties have been produced. under domestication ~y 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 suppoTt the view, that ther~ is no fundamental distinction between species and varietws. OH..&.P, ·IX.] IMPERFECTION OF GEOLOGICAL RECORD. 245 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 number-On tho vast lapse of time, as inferred from the rate of deposition and of denudation-On the poorness of our palreontological collections-On the intflrmittence of geological formations-On the absence of intermediate varieties in any one form11tion-On the sudden appearance of groups of species-On their sudden appearance in tbe lowest known fossiliferous strata. .IN th~ sixth ~hapter I enume~ated the ~hief o bj ectio~s ~hic~ might be JUStly urged against the VIews maintained In this volume. Most of them have now been discussed. One, n~1nely the distinctness of specific forms, and their not bmng 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 un~er the circumstances apparently most favourable fo~ thmr presence, namely on an extensive and continuous area with graduated physical conditions. I endeavoured ~o show, that the life of each species depends in a more Important manner on the presence of other already defined organic forrp.s, than on climate ; and, therefore that the re~lly ~over~ing ~onditions of life do not grad~ate away quite Insensibly hke heat or moisture. I endeavoured also, to sho~ that intermediate varieties, from existing i~ lesser numbers than the forins which they connect, will generally be beaten out and exterminated during the cou.rse of further modification and improvement. The mmn cause, however, of innumerable intermediate links not now occurring everfwhere throughout nature depends on the ':"eri proc~ss o natural selection, through which new var~eties continually take the plac<:>s of and extermi· nate the1r parent-forms. But just in proportion as this |