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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 chapter； 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 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 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 incomparably longer time at her disposal。 Nor do I believe that any great physical change； as of climate； or any unusual degree of isolation to check immigration； is actually necessary to produce new and unoccupied places for natural selection to fill up by modifying and improving some of the varying inhabitants。 For as all the inhabitants of each country are struggling together with nicely balanced forces； extremely slight modifications in 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 under which they live； that none of 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 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。 He does not rigidly destroy all inferior animals； but protects during each varying season； as far 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？
It may 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 nothing of these slow changes in progress； until the hand of time has marked the long lapses 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。
Although natural selection can act only through and for 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。 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 smooth or downy； a yellow or purple fleshed fruit； should succeed。
In looking at many small points of difference between species； w