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Show 124 DARWINISM OIIAr. characters of a species-and so few oven among naturalists have ever compared, accurately, considerable numbers of individuals, that the conception of variability as a general characteristic of all dominant and widespread species, large in its amount and affecting, not a few, but considerable masses of the individuals which make up the species, will be to many entirely new. Equally important is the fact that the variability exteods to every organ and every structure, external and internal; while perhaps most important of all is the independent variability of these several parts, each one varying without any constant or even usual dependence on or correlation with, other parts. No doubt there is some s;1ch correlation in the differences that exist between species and species-more developed wings usually accompanying smaller fee~ and vice versa-but this is, generally, a useful adaptation wh1ch has been brought about by natural selection, and docs not apply to the individual variability which occurs within the species. I~ is because these facts of variation are so important and so. httlc understood, that they have been discussed in what w1ll seem to ~orne readers wearisome and unnecessary detail. ~any n.aturahsts, however, will hold that even more evidence IS reqmred ;. and more, to almost any amount, could easily have been ~1ven. The character and variety of that already adduced Will, however, I trust, convince most readers that the fa~ts are ~s stated; while they have been drawn from a suffiCiently Wide area to indicate a general principle throuO'h-out nature. o If, now, ':e fully .rc~lise these facts of variation, along with those of rapid mult1phcation and the stru()'()'le for existence most of_.t~e difficulties in the way of co~preh~~ding how specie~ have ongmated through natural selectiOn will disappear. For whenever, through changes of climate or of altitude or of the .nature of the soil, or of the are~ of the count;y, any specJes are exposed to new dangers ahd have to maintain themselves and provide for the safety' of their offsprio()' under new and more arduous conditions, then, in the variability of ~11 p~rts, organs, and structures, no less than of habits and mthe.l iJhO0 'ence we have th f . . . ' . • e means o producmg modifications w Ic will certamly brmg the species into harmony with its v NATURAL SELECTION 125 new conditions. And if we remember that all such physicn1 chan()'es are slow and gradual in their operation, we shall sec that ~he amount of variation which we know occurs in every new ()'eneration will be quite sufficient to enable modification and ~daptation to go on at the same rate. Mr. Da,rwin was rather inclined to exaggerate the neccs. :-try slowness of the action of natural selection; but with the knowledge w • now possess of the great amount and range of individun I variation, there seems no difficulty in an amount of changr, quite equivalent to that which usually di tinO'ni. hcs n,lliecl species, sometimes taking :pl.acc in less ~han a century, shon ~d any rapid change of conditiOns nccc:.Itatc an c~ually ra~)l(l adaptation. This may often hav~ occurred, mther to Immigrants into a new land, or to residents whose country ~as been cut off by subsidence from a larger and more vaned area over which they had formerly roamed. When no chanO'e of conditions occurs, species may remain unchanged for very long periods, and thus produce that appearance of stability of spe~ies which is even now often adduced as an argument agamst evolution by natural selection, but which is really quite in harmony with it. . On the principles, and by the light of the facts, now bnefly summarised, we have been able, in the present chapter, to indicate how natural selection acts, how divergence of character is set up, how adaptation to conditions at various perio?s of life has been effected, how it is that low forms of hfe continue to exist, what kind of circumstances arc most favourable to the formation of new species, and, lastly, to what extent the advance of organisation to higher types is produced by natural selection. We will ~ow pass o~ to c~nsider some of the more important objectiOns and d1fficult10s which have been advanced by eminent naturalists. |