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Show 78 NATURAL SELECTION. (CHAP. IV. On the other hand, we may feel sure th~t. any variation in the least degree injurious would . b~ rigidly destroy.ed: This preser_va~ion of fa.vo~rable variations and the reJ ection of inJurious vanation~, .I ?all Natural Selection. Variations neither useful nor InJunous would not be aff~cted by natural selection, an~ would b~ left a fluctuating element, as perhaps "\Ve see 1n the speCies called polymor-phic. We shall best understand the probable course of na-tural selection .bY taking the c~se of a count~y undergoing some physical chang~, f~r Ins~ance, of chmate. ~he proportional numbers of Its Inhabitants wou~d alJ?-ost Immediately undergo a change, and some speCies m1ght becoine extinct. We may conclude, fron1. what. we haye seen of the intimate and cmnplex manner 1n whiCh the Inhabitants of each country are bo~nd together, that a~y change in the numerical proportions of son;te of t~e Inhabitants, independently of the change of chmate Itself, would most seriously affect many of the others. If the country were open on its ?order.s, new forr~s woul~ certainly immigrate, and th1s also w?uld ~eriously dis~urb the relations of some of the fonner Inhabitants. Let 1t be remembered how powerful the influence of a single introduced tree or marnmal has been shown to be. But in the case of an island, or of a country partly surrounded by barriers into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be ?etter filled up, if some of the original inhabitants "\vere 1n s.ome. ma~ner modified · for had the area been open to Inlmigration, these sa~e pl~ces would have been seized on by intruders. In such case every slight modification, which in the course of ages chan~ed to arise, and whic~ in any way favou:ed the individuals of any of the speCies, by better adapting them to their altered conditions, would tend to be preserved · and natural selection 'vould thus have free scope for the 'w ork of improvement. We have reason to believe, as stated in the first c?apter, that a change in the. conditions of life, by ~pecially acting on the reproductive system, causes or Increases CHAP. IV.] NATURAL ·sELECTION .. 79 variability; and in the foregoing case the conditions of life are su~:rosed to have undergone a change, and this ":o~ld man1testly be favourable to natural se] ection, by giving a better .chance o~ p:o.fitable v-ariations occurring; and unless profitable val'lations do occur, natural selection can do nothir:g ... No~ that, as I believe, any extreme amount of var1abihty 1s necessary; as man can certainly produce great results by adding up in any given direction mere individual differences, so could Nature but far more easily, from having i~compar~bly longer ti~e at her disposal. .Nor do I beheve that any great. physical change, as of chmate, or any unusual degree of Isolation to check immigration, is actually necessary to produce new and u~occupied.places for natural selection to fill up by modifying and ImErov1n~ some of the varying inhabitants. For as all t~e In~abitants of each country are struggling toge~her ,:w1th. nicely balanced forces, extremely slight modifications 1n the structure or habits of one inhabitant would often give it an advantage over 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 un~er which they ~ive, that no?e of them. could anyhow be Improved; for 1n all countnes, the natives have been so far conquered. by naturalised productions, that they have allowed formgners to take firm possession of the land And as .foreigners have thus everywhere beaten some of th~ natives, we may. safely conclude that the natives 1n1ght have been mod1fied with advantage so as to have better resisted such intruders. ' As man can .produce ~nd certainly has produced a great .result by. his methodical and unconscious means of selection, what m~~ 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 machmery of life. Man selects onl;y- for his own good · Nature only for that of the being w h1eh she tends. Every |