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When we see any part or organ developed in a remarkable degree or manner in any species； the fair presumption is that it is of high importance to that species； nevertheless the part in this case is eminently liable to variation。 Why should this be so？ On the view that each species has been independently created； with all its parts as we now see them； I can see no explanation。 But on the view that groups of species have descended from other species； and have been modified through natural selection； I think we can obtain some light。 In our domestic animals； if any part； or the whole animal； be neglected and no selection be applied； that part （for instance； the comb in the Dorking fowl） or the whole breed will cease to have a nearly uniform character。 The breed will then be said to have degenerated。 In rudimentary organs； and in those which have been but little specialized for any particular purpose； and perhaps in polymorphic groups； we see a nearly parallel natural case； for in such cases natural selection either has not or cannot come into full play； and thus the organisation is left in a fluctuating condition。 But what here more especially concerns us is； that in our domestic animals those points； which at the present time are undergoing rapid change by continued selection； are also eminently liable to variation。 Look at the breeds of the pigeon； see what a prodigious amount of difference there is in the beak of the different tumblers； in the beak and wattle of the different carriers； in the carriage and tail of our fantails； &c。； these being the points now mainly attended to by English fanciers。 Even in the sub…breeds； as in the short…faced tumbler； it is notoriously difficult to breed them nearly to perfection； and frequently individuals are born which depart widely from the standard。 There may be truly said to be a constant struggle going on between； on the one hand； the tendency to reversion to a less modified state； as well as an innate tendency to further variability of all kinds； and； on the other hand； the power of steady selection to keep the breed true。 In the long run selection gains the day； and we do not expect to fail so far as to breed a bird as coarse as a common tumbler from a good short…faced strain。 But as long as selection is rapidly going on； there may always be expected to be much variability in the structure undergoing modification。 It further deserves notice that these variable characters； produced by man's selection； sometimes become attached； from causes quite unknown to us； more to one sex than to the other； generally to the male sex； as with the wattle of carriers and the enlarged crop of pouters。
Now let us turn to nature。 When a part has been developed in an extraordinary manner in any one species； compared with the other species of the same genus； we may conclude that this part has undergone an extraordinary amount of modification； since the period when the species branched off from the common progenitor of the genus。 This period will seldom be remote in any extreme degree； as species very rarely endure for more than one geological period。 An extraordinary amount of modification implies an unusually large and long…continued amount of variability； which has continually been accumulated by natural selection for the benefit of the species。 But as the variability of the extraordinarily…developed part or organ has been so great and long…continued within a period not excessively remote； we might； as a general rule； expect still to find more variability in such parts than in other parts of the organisation； which have remained for a much longer period nearly constant。 And this； I am convinced； is the case。 That the struggle between natural selection on the one hand； and the tendency to reversion and variability on the other hand； will in the course of time cease； and that the most abnormally developed organs may be made constant； I can see no reason to doubt。 Hence when an organ； however abnormal it may be； has been transmitted in approximately the same condition to many modified descendants； as in the case of the wing of the bat； it must have existed； according to my theory； for an immense period in nearly the same state； and thus it comes to be no more variable than any other structure。 It is only in those cases in which the modification has been comparatively recent and extraordinarily great that we ought to find the generative variability； as it may be called； still present in a high degree。 For in this case the variability will seldom as yet have been fixed by the continued selection of the individuals varying in the required manner and degree； and by the continued rejection of those tending to revert to a former and less modified condition。
The principle included in these remarks may be extended。 It is notorious that specific characters are more variable than generic。 To explain by a simple example what is meant。 If some species in a large genus of plants had blue flowers and some had red； the colour would be only a specific character； and no one would be surprised at one of the blue species varying into red； or conversely； but if all the species had blue flowers； the colour would become a generic character； and its variation would be a more unusual circumstance。 I have chosen this example because an explanation is not in this case applicable； which most naturalists would advance； namely； that specific characters are more variable than generic； because they are taken from parts of less physiological importance than those commonly used for classing genera。 I believe this explanation is partly； yet only indirectly； true； I shall； however； have to return to this subject in our chapter on Classification。 It would be almost superfluous to adduce evidence in support of the above statement； that specific characters are more variable than generic； but I have repeatedly noticed in works on natural history； that when an author has remarked with surprise that some important organ or part； which is generally very constant throughout large groups of species； has differed considerably in closely…allied species； that it has； also； been variable in the individuals of some of the species。 And this fact shows that a character； which is generally of generic value； when it sinks in value and becomes only of specific value； often becomes variable； though its physiological importance may remain the same。 Something of the same kind applies to monstrosities： at least Is。 Geoffroy St。 Hilaire seems to entertain no doubt； that the more an organ normally differs in the different species of the same group； the more subject it is to individual anomalies。
On the ordinary view of each species having been independently created； why should that part of the structure； which differs from the same part in other independently…created species of the same genus； be more variable than those parts which are closely alike in the several species？ I do not see that any explanation can be given。 But on the view of species being only strongly marked and fixed varieties； we might surely expect to find them still often continuing to vary in those parts of their structure which have varied within a moderately recent period； and which have thu