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Show 404 RECAPITULATION. [CHAP. XIV. more important objections relate to questions on which we are confessedly ignorant; nor do we know how ignorant we are. We do not know all the possible transitional gradations betw·een the shnplest and the most perfect organs ; it cannot be pretended that we know all the varied 1neans of Distribution during the long lapse of years, or that we know how in1perfect the Geological Record is. Grave as these several difficulties are, in my judgment they do not overthrow the theory of descent with modification. Now let us turn to the other side of the argument. Under do1nestication we see 1nuch variability. This seems to be mainly due to the reproductive systmn being eminently susceptible to changes in the conditions of life; so that this system, when not rendered impotent, fans to reproduce offspring exactly like the parent-form. Variability is governed by many complex laws,-by correlation of growth, by use and disuse, and by the direct action of the physical conditions of life. There is much difficulty in ascertaining how much modification our domestic productions have undergone ; but we 1nay safely infer that the amount has been large, and that modifications can be inherited for long periods. As long as the conditions of life remain the same, we have reason to believe that 1nodification, which has already been inherited for many generations, may continue to be inherited for an ahnost infinite nu1nber of generations. On the othe1~ hand we have evidence that variability, when it has once come into play, does not wholly cease; for new varieties are still occasionally produced by our most anciently domesticated productions. Man does not actually produce variability ; he only unintentionally exposes organic beings to new conditions of life, and then nature acts on the organisation, and causes variability. But man can and does select the variations given to him by nature, and thus accumulate them in any desired 1nanner. He thus adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it unconsciously by preserv- CHAP. XIV.] RECAPITULATION. 405 . ing tte individuals most useful to him at the time without any t ou~h~ of altering the breed. It is certain' that he ~an la~e Y 1nflu_ence tll.e character of a breed by selecting Ill; eac successrv~ g:eneration, individual differences s~ sh~ht as to be qurte Inappreciable by an uneducated eye This pr?cess of selection has been the great agency in th~ production of the most distinct and useful domestic breeds. That many of the breeds produced by 1nan have to a lar e ~xtent. the character of natural species, is shown by tEe 1~e~tncable ~o~bts whether very many of them are varieties or aboriginal species. There is no ob-yious reason why the principles which have acted so efficiently under domestication should not ~av:e .acted under nature. In the preservation of favoured Individuals and .races, during the constantly-recurrent Struggl~ for Existence, w.e see the most powerful and ~ver:actrng means of selection: The struggle for existence lnevitabl~ fol~ows fron1 the high geometrical ratio of increase ~hich IS ?ommon to all organic beings. This high rate of 1~crease IS pr?ved by calculation, by the effects of a su~ce~sion of pecu~Iar seasons, and by the results of natu~ a~Isatron, as explained in the third chapter. More in~ Ividuals ~re bo~n than c~n pos~ibl:y s~rvive. A grain In the ~alance w1l~ determrne wluch Indrvidual shall live and wh~ch shall dre,-which variety or species shall increase m J?-Umber, and w~ic~ .shali decrease, or finally beco~e extinct. A~ the Indrvrduals of the same species come In all respects Into the closest competition with each other, t~e s!ruggle will generally be most severe between t~em ; It Will be almost equally severe between the varieties ?f the same species, and next in severity between the speCies of the same genus. But the struggle will often be very severe between beings most remote in the scale of nature: The slightest advantage in one being, at any age or dur1~&' any season, over those with which it comes into competitwn, or better adaptation in however slight a degree to the surrounding physical conditions will turn the oalance. ' With animals having separate sexes there will in most cases be a st:ruggle betwee:p the males for possession of the |