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e immigration of better adapted forms having been checked。 But the action of natural selection will probably still oftener depend on some of the inhabitants becoming slowly modified； the mutual relations of many of the other inhabitants being thus disturbed。 Nothing can be effected； unless favourable variations occur； and variation itself is apparently always a very slow process。 The process will often be greatly retarded by free intercrossing。 Many will exclaim that these several causes are amply sufficient wholly to stop the action of natural selection。 I do not believe so。 On the other hand； I do believe that natural selection will always act very slowly； often only at long intervals of time； and generally on only a very few of the inhabitants of the same region at the same time。 I further believe； that this very slow； intermittent action of natural selection accords perfectly well with what geology tells us of the rate and manner at which the inhabitants of this world have changed。
Slow though the process of selection may be； if feeble man can do much by his powers of artificial selection； I can see no limit to the amount of change； to the beauty and infinite complexity of the coadaptations between all organic beings； one with another and with their physical conditions of life； which may be effected in the long course of time by nature's power of selection。
Extinction
This subject will be more fully discussed in our chapter on Geology； but it must be here alluded to from being intimately connected with natural selection。 Natural selection acts solely through the preservation of variations in some way advantageous； which consequently endure。 But as from the high geometrical powers of increase of all organic beings； each area is already fully stocked with inhabitants； it follows that as each selected and favoured form increases in number； so will the less favoured forms decrease and become rare。 Rarity； as geology tells us； is the precursor to extinction。 We can； also； see that any form represented by few individuals will； during fluctuations in the seasons or in the number of its enemies； run a good chance of utter extinction。 But we may go further than this； for as new forms are continually and slowly being produced； unless we believe that the number of specific forms goes on perpetually and almost indefinitely increasing； numbers inevitably must become extinct。 That the number of specific forms has not indefinitely increased； geology shows us plainly； and indeed we can see reason why they should not have thus increased； for the number of places in the polity of nature is not indefinitely great； not that we have any means of knowing that any one region has as yet got its maximum of species。 probably no region is as yet fully stocked； for at the Cape of Good Hope； where more species of plants are crowded together than in any other quarter of the world； some foreign plants have become naturalised； without causing； as far as we know； the extinction of any natives。
Furthermore； the species which are most numerous in individuals will have the best chance of producing within any given period favourable variations。 We have evidence of this； in the facts given in the second chapter； showing that it is the common species which afford the greatest number of recorded varieties； or incipient species。 Hence； rare species will be less quickly modified or improved within any given period； and they will consequently be beaten in the race for life by the modified descendants of the commoner species。
From these several considerations I think it inevitably follows； that as new species in the course of time are formed through natural selection； others will become rarer and rarer； and finally extinct。 The forms which stand in closest competition with those undergoing modification and improvement； will naturally suffer most。 And we have seen in the chapter on the Struggle for Existence that it is the most closely…allied forms； varieties of the same species； and species of the same genus or of related genera； which； from having nearly the same structure； constitution； and habits； generally come into the severest competition with each other。 Consequently； each new variety or species； during the progress of its formation； will generally press hardest on its nearest kindred； and tend to exterminate them。 We see the same process of extermination amongst our domesticated productions； through the selection of improved forms by man。 Many curious instances could be given showing how quickly new breeds of cattle； sheep； and other animals； and varieties of flowers； take the place of older and inferior kinds。 In Yorkshire； it is historically known that the ancient black cattle were displaced by the long…horns； and that these 'were swept away by the short…horns' （I quote the words of an agricultural writer） 'as if by some murderous pestilence。'
Divergence of Character
The principle； which I have designated by this term； is of high importance on my theory； and explains； as I believe； several important facts。 In the first place； varieties； even strongly…marked ones； though having somewhat of the character of species as is shown by the hopeless doubts in many cases how to rank them yet certainly differ from each other far less than do good and distinct species。 Nevertheless； according to my view； varieties are species in the process of formation； or are； as I have called them； incipient species。 How； then； does the lesser difference between varieties become augmented into the greater difference between species？ That this does habitually happen； we must infer from most of the innumerable species throughout nature presenting well…marked differences； whereas varieties； the supposed prototypes and parents of future well…marked species； present slight and ill…defined differences。 Mere chance； as we may call it； might cause one variety to differ in some character from its parents； and the offspring of this variety again to differ from its parent in the very same character and in a greater degree； but this alone would never account for so habitual and large an amount of difference as that between varieties of the same species and species of the same genus。
As has always been my practice； let us seek light on this head from our domestic productions。 We shall here find something analogous。 A fancier is struck by a pigeon having a slightly shorter beak； another fancier is struck by a pigeon having a rather longer beak； and on the acknowledged principle that 'fanciers do not and will not admire a medium standard； but like extremes；' they both go on （as has actually occurred with tumbler…pigeons） choosing and breeding from birds with longer and longer beaks； or with shorter and shorter beaks。 Again； we may suppose that at an early period one man preferred swifter horses； another stronger and more bulky horses。 The early differences would be very slight； in the course of time； from the continued selection of swifter horses by some breeders； and of stronger ones by others； the differences would become greater； and would be noted as forming two sub…breeds； finally； after the lapse of centuries； the sub…breeds would become converted into two well…established and dis