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Show 82 NATURAL SELECTION. CHAP. IV. places would have been seiz~d on b~ in~ruders. In such case, every slight modificatio~, w~ICh In the course of ages chanced to arise, and which In any way favoured the individuals of any of the species, by better adapting them to their altered conditions, would tend to be preserved ; and natural selection would thus have free scope for the work of improvement. We have reason to believe, as stated in the first chapter, that a change in the ~onditions of life, by specially acting on the rep~oductive syst~m, causes or increases variability ; and In the foregoing case the conditions of life are supposed to have undergone a change, and this would manifestly be favourable to natural selection, by giving a better chance of profitable variations occurring ; and unless profitable variations do occur, natural selection can do nothing. Not that, as I believe, any extreme amount of variability is necessa~-y ; as man can certainly pToduce great results by adcling up in any given direction mere individual differences, so could Nature but far more easily, from having incomparably longeT time at her disposal. N ~r do I believe that any great physical change, as of ?lnn~te, ?T an! unusual degTee of isolation to check ImmigratiOn, IS actually necessary to pToduce new and unocc_np~ed places for natural selection to ~ll ~p b~ modifying and improving some of the varying Inhabitants. ~or as all the inhabitants of each country are struggling together with nicely balanced force~, .extremely s.Iight modifications in the structure or habits of one Inhabitant would often give it an advantage ov~r others; and still further modifications of the same kind would often still further increase the advantage. No country can be named in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of CHAP. IV. NATURAL SELECTION. 83 them could anyhow be improved ; for in all countries, the natives have been so far conquered by naturalised productions, that they have allowed foreigners to take firm possession of the land. And as foreigners have thus everywhere beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted such intruders. As man can produce and certainly has produced a great result by his methodical and unconscious means of selection, what may not nature effect? Man can act only on external and visible characters: nature cares nothing for appearances, except in so far as they may be useful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good; Nature only for that of the being which she tends. Every selected character is fully exercised by her; and the being is placed under well-suited conditions of life. Man keeps the natives of many climates in the same country ; he seldom exercises each selected character in some peculiar and fitting manner; he feeds a long and a short beaked pigeon on the same food ; he does not exercise a long-backed or long-legged quadz: uped in any peculiar manner; he exposes sheep With long and short wool to the same climate. He does not allow the most vigorous males to struggle for. the ~emales. He does not rigidly destroy all inferwr an1m~ls, ?ut protects during each varying season, as far as lies 1n his power, all his productions. He often begins his selection by some half-monstrous form.; or at le.ast by some modification prominent enough to catch his eye, or to be plainly useful to him. Under nat_ure, the slightest difference of structure or constitutwn may well turn the nicely-balanced scale in the |