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Show 4.18 COXCLUDING REMARKS. CnAr . xxvm. liable to slight d viations of structure and to monstrosities,. than d. · 1d as wJCloly-specios living under their natural con Iti?nR, . al . . . . .1 tl whwh ml1abrt rcstnctcd ran o·mo- species vary more t lan .lose d b b · b'l' · 1 depends on chano-o nTeas, we may infer that vana 1 rty mam Y b . . f 1.r W ust not overlook the effects of tho condJtwns o 11e. e m . l b . t' f the characters den ved from both parontR, 1mequa corn ma wn o < •• · t i' . . pr·oo-onitors Changed cond1t10ns have nor reverswn o o1mer b • . · 1 t doncy to render tho reproductive organs more ftn espeCla en . l or oss 1· mpo t en t , a s shown in the chapter de. voted to th.J S .s.u b-ject; and these organs consequently often fa1l to. t~·ansmrt fmth-f'nlly the parental characters. Cl~an~ed condJtwns also act directly and definitely on the orgamsatwn, so that all or nearly all the individuals of the same species thu~ exposed becom.(~ modified in the same mann r; but why tlns or that part IS especm· 1 1 y ..,.rrected we can seldom or never say. In most cases, ull' . . . however, of the direct action of changed eond1twns, mdope~d-t l of the indirect variability caused by the reproductive en y . . h l . organs being affected, indefinite moch:ficatwns are t e resu t ;. m nearly the same manner as exposur~ t~ ~old or. the ~bsorpt1on of the same poison affects different md1v~duals m varwus :Vays. \Ve have reason to suspect that an habitual exce s of highly nutritious food, or an excess relatively to the wear and tear of the organisation from exercise, is a powerful exciting cause of variaLility. When we see the symmetrical and complex outgrowths, caused by a minute atom of .the poison o~ a gall-insect, wo may believe that slight changes m the chemwal nature of the sap or blood would lead to extraordinary modifications of structure. Tho increased use of a muscle with its various attacqed parts, :mel tho increased activity of a gland or other organ, lcacl to their increa od development. Disuse has a contrary effect. ·with domesticated productions organs sometimes become rudimentary through abortion; but we have no reason to suppose that this· has ever followed from mere disuse. vVith natural species, on the contrary, many organs appear to have been rendered rudimentary through disuse, aided by the principle of the economy of growth, and by the hypothetical principle discussed in the last chapt r, namely, the final de truction of the germs or gemmules of such . useless parts. This difference may be partly C' I!AP. XXVIII. CONCJ.. . UDIKG REMARKS. 4.19 accounted for by disuse having acted on domestic forms for an insufficient length of time, and partly from their exemption ~rom any severe struggle for existence, entailing rigid economy m the development of each part, to which all species under nature are subjected. Nevertheless the law of compensation or balancement apparently affects, to a certain extent, our domesticated productions. vVe must not exaggerate the importance of the definite action of changed conditions in modifying all the individuals of the same species in the same manner, or of use and disuse. As every part of the organisation is highly variable, and as varia~ io.ns are so. easily selected, both consciously and unconsciously, rt IS very cbfficult to distinguish between the effects of the selection of indefinite variations, and the direct action of the conditions of life. ] or instance, it is possible that the feet of our water-dogs, and of the American dogs which have to travel much over the snow, may have become partially webbed from the stim nlns of widely extending their toes; but it is far more probable that the weLbing, like the membrane between the toes of certain pigeons, spontaneously appeared and was afterwards increased by the best. swimmers and the best snow-travellers being preserved dunng many generations. A fancier who wished to decrease the size of his bantams or tumbler-pigeons would never think of starving them, but would select the smallest indiv. iduals which spontaneously appeared. Quadrupeds are sometllues born destitute of hair, and hairless breeds have been form ed, ?ut there i~ n~ reason to believe that this is caused by a hot climate. vVIthm the tropics heat often causes sheep to lo e their fl eeces, and on the other hand wet and cold act as a direct stimulus to the growth of Lair; it is, however, possible tltat these changes may merely be an exaggeration of the regular ye~rly change of coat; and who will pretend to decide how far tins yearly change, or the thick fur of arctic animals or as I may ad.d their white colour, is due to the direct acti~n of a severe climate, and how far to the preservation of the b t _ t oc t e d m. a·1 v1· a ua1 s d urm. g a long succession of generationse s? pro Of all t~1e laws governing variability, that of correlation is the most Important. In many cases of slight deviations of structure as well as of grave monstrosities, we cannot even 2 E 2 |