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\We shall best understand the probable course of natural selection by taking the case of a country undergoing some- physical change, for instance, of climate. The proportional numbers of its inhabitants would almost immediately undergq _a_ change, andsome species might become extinct.'~"‘We may conclude, from what wé'have seenb'f the intimate and complex manner in which the inhabitants of each country are bound together, tha any change in the numerical proportions of some of the 'nhabitants, independently of the change of climate itself, would seriously affect many of the others. 31f the country were open on its borders, new forms vfll'uld certainly “immigrate, and this also would seriously disturb the relations of some of the former inhabitants. Let it be remembered how powerful the influence ofvaL single, introducedwtree 9r, mammal 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 better filled up, if some of the original inhabitants were in some manner modified; for, had the area been Open to immigration, these same places would have been seized on by intruders. In such case, every slight modification, which 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 ch pter, that a change in the conditions of life, by specially acting on the reproductive system, 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 givinga better chance of profitable variations occurring; andilpnless profita e variations do occur, natural selection can do nothingj>Not that any extreme amOunt of variability is necessa y; as man can certainly produce great results by adding up in any given direction mere individual differences, so could natural selection, but far more easily, from having incomparably longer time “for action. i or do I believe that any great physical change, as of climate, or any unusual degree of isolation to check immigration, actually necessary to produce new and unoccupied places ?( for natural selection to fill up b modifying and improving some of the varying inhabitant For as all the inhabitants of each funt are struggling together withjcely balanced m. extremely slight modifications in the M or abits 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, as long as the being continued under the same conditions of life and profited by similar means of subsistence and defence. [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 them couldanyhow 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
landI‘] And as foreigners have thus everywherembeatenssplne of the nativesrwe'rnay saréiy‘coiijgglde thin the natives might hale: ____ ___ "'with advanta e, so as to hav 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 natural selection effect? Man can act only on external and visible characters: Nature (if I may be allowed thus to personify the I natural preservation of varying and favoured individuals ' during the struggle for existence) cares nothing for appearances, except in so far as they are useful to 1 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 quadruped 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 females. lie does not rigidly destroy all inferior animals, but protects
during eachyarying-seasonmaslfar as lies in his power, all his productions. He often begins his selection by some half-monstrous form; or at least by some modification prominent enough to catch his eye, or to be plainly useful to him. Under nature, the slightest difference of structure or constitution may well turn the nicely-balanced scale in the struggle for life, and so be preserved. How fleeting are the wishes and efforts of man! how short his time! and consequently how poor will his products be, compared with those accumulated by Nature during whole geological periods! Can we wonder, then, that Nature’s productions should be far “ truer ” in character than man’s productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship 7
It may metaphorically be said that natural selection is daily and hourly scrutinising, throughout the world,%
every variation, even the slightest; rejecting that which_
is bad, preserving and adding up all that is good;‘ silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nething of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.
In order that any great amount of modification should thus in the course of time be produced, it is necessary to believe that when a variety has once arisen, it again
aries, after per aps a long interval of time ;" and that its varieties, if favourable, are again preserved, and so nwards.) That varieties more or less different from the parent-stock occasionally arise, few will deny; but that the process of variation should be thus indefinitely prolonged is an assumption, the truth of which must be judged of by how far the hypothesis accords with and explains the general phenomena of nature. On the other hand, the ordinary belief that the amount of possible variation.is a strictly limited quantity is likewise a simple assumption. 3“? Although natural selection can act only through and or the good of each being, yet characters and structures, which we are apt to consider as of very trifling importance,
may thus be acted on. When we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour of heather, and the black-grouse that of peaty earth, we must believe that these tints are of service to these birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey—so much so, that on parts of the'Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence I can see no reason to doubt that natural selection" -> might be most effective in giving the proper colour( to each kind of grouse, and in keeping that colour, when once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any particular colour would produce little effect: we should remember how essential it is in a flock of white sheep to destroy every lamb with the faintest trace of black. We have seen how in Florida the colour of the hogs, when feeding on ’the “paint root,” determines whether they shall live or die. In plants the down on the fruit and the colour of the flesh are considered by botanists as characters of the most trifling importance: yet we hear from an excellent horticulturist, Downing, that in the United States smooth-skinned fruits suffer far more from a beetle, a curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums; whereas another disease attacks yellow—fleshed peaches far more than those with other coloured flesh. If, with all the aids of art, these slight differences make a great difference in cultivating the several varieties, assuredly, in a state of nature where the trees would have to struggle with other trees and with a host of enemies, such differences would effectually settle which variety, whether a smobth or downy, a yellow or purple fleshed fruit, should succeed. In looking at many small points of difference between species, which, as far as our ignorance permits us to judge, seem quite unimportant, we must not forget that climate, food, &c., probably produce some slight and direct effect. It is, howeVer, far more necessary to bear in‘ mind that there are many unknown laws of correlation of growth, which, when one part of the organisation is modified
cocks. How low in the scale of nature the law of battle descends, I know not; male alligators have been described as fighting, bellowing, and whirling round, like Indians in a war-dance, for the possession of the females; male salmons have been seen fighting all day long; male stag-beetles sometimes bear wounds from y the huge mandibles of other males. The war is, perhaps, ‘1_ severest betWeen the males of polygamous animals, and ' these seem oftenest provided with special weapons. The lmales of carnivorous animals are already well armed; ‘ though to them and to others, special means of defence imay be given through means of sexual selection, as ‘the mane to the lion, the shoulder-pad to the boar, and the hooked jaw to the male salmon; for the shield may be as important for victory, as the sword or spear.
Amongst birds, the contest is often of a more peaceful character. All those who have attended to the subject, believe that there is the severest rivalry between the
. males of many species to attract by singing the females. .The rock-thrush of Guiana, birds of Paradise, and some others, congregate; and successive males display their gorgeous plumage and perform strange antics before the females, which, standing by as spectators, at last choose the most attractive partner. Those who have closely attended to birds in confinement well know that they often take individual preferences and dislikes: thus Sir R. Heron has described how one pied peacock was eminently attractive to all his hen birds. It may appear childish to attribute any effect to such. apparently weak means: I cannot here enter on the details necessary to support this view; but if man can. in a short time give elegant carriage and beauty to his -bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standardof beauty, might produce a marked effect. I strongly suspect that some well-known laws, with respect to the plumage of male and female birds, in comparison with the plumage of the young, can be explained on the view of plumage having been chiefly modified by sexual selection, acting when the birds have come to the breeding age or during the breeding season; the modifications thus produced being inherited at corresponding ages or