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ave its nutriment wasted in building up an useless structure。 I can thus only understand a fact with which I was much struck when examining cirripedes； and of which many other instances could be given： namely； that when a cirripede is parasitic within another and is thus protected； it loses more or less completely its own shell or carapace。 This is the case with the male Ibla； and in a truly extraordinary manner with the Proteolepas： for the carapace in all other cirripedes consists of the three highly…important anterior segments of the head enormously developed； and furnished with great nerves and muscles； but in the parasitic and protected Proteolepas； the whole anterior part of the head is reduced to the merest rudiment attached to the bases of the prehensile antennae。 Now the saving of a large and complex structure； when rendered superfluous by the parasitic habits of the Proteolepas； though effected by slow steps； would be a decided advantage to each successive individual of the species； for in the struggle for life to which every animal is exposed； each individual Proteolepas would have a better chance of supporting itself； by less nutriment being wasted in developing a structure now become useless。
Thus； as I believe； natural selection will always succeed in the long run in reducing and saving every part of the organisation； as soon as it is rendered superfluous； without by any means causing some other part to be largely developed in a corresponding degree。 And； conversely； that natural selection may perfectly well succeed in largely developing any organ； without requiring as a necessary compensation the reduction of some adjoining part。
It seems to be a rule； as remarked by Is。 Geoffroy St Hilaire； both in varieties and in species； that when any part or organ is repeated many times in the structure of the same individual （as the vertebrae in snakes； and the stamens in polyandrous flowers） the number is variable； whereas the number of the same part or organ； when it occurs in lesser numbers； is constant。 The same author and some botanists have further remarked that multiple parts are also very liable to variation in structure。 Inasmuch as this 'vegetative repetition；' to use Prof。 Owen's expression； seems to be a sign of low organisation； the foregoing remark seems connected with the very general opinion of naturalists； that beings low in the scale of nature are more variable than those which are higher。 I presume that lowness in this case means that the several parts of the organisation have been but little specialised for particular functions； and as long as the same part has to perform diversified work； we can perhaps see why it should remain variable； that is； why natural selection should have preserved or rejected each little deviation of form less carefully than when the part has to serve for one special purpose alone。 In the same way that a knife which has to cut all sorts of things may be of almost any shape； whilst a tool for some particular object had better be of some particular shape。 Natural selection； it should never be forgotten； can act on each part of each being； solely through and for its advantage。
Rudimentary parts； it has been stated by some authors； and I believe with truth； are apt to be highly variable。 We shall have to recur to the general subject of rudimentary and aborted organs； and I will here only add that their variability seems to be owing to their uselessness； and therefore to natural selection having no power to check deviations in their structure。 Thus rudimentary parts are left to the free play of the various laws of growth； to the effects of long…continued disuse； and to the tendency to reversion。
A part developed in any species in an extraordinary degree or manner； in comparison with the same part in allied species； tends to be highly variable。
Several years ago I was much struck with a remark； nearly to the above effect； published by Mr Waterhouse。 I infer also from an observation made by Professor Owen； with respect to the length of the arms of the ourang…outang； that he has come to a nearly similar conclusion。 It is hopeless to attempt to convince any one of the truth of this proposition without giving the long array of facts which I have collected； and which cannot possibly be here introduced。 I can only state my conviction that it is a rule of high generality。 I am aware of several causes of error； but I hope that I have made due allowance for them。 It should be understood that the rule by no means applies to any part； however unusually developed； unless it be unusually developed in comparison with the same part in closely allied species。 Thus； the bat's wing is a most abnormal structure in the class mammalia； but the rule would not here apply； because there is a whole group of bats having wings； it would apply only if some one species of bat had its wings developed in some remarkable manner in comparison with the other species of the same genus。 The rule applies very strongly in the case of secondary sexual characters； when displayed in any unusual manner。 The term； secondary sexual characters； used by Hunter； applies to characters which are attached to one sex； but are not directly connected with the act of reproduction。 The rule applies to males and females； but as females more rarely offer remarkable secondary sexual characters； it applies more rarely to them。 The rule being so plainly applicable in the case of secondary sexual characters； may be due to the great variability of these characters； whether or not displayed in any unusual manner of which fact I think there can be little doubt。 But that our rule is not confined to secondary sexual characters is clearly shown in the case of hermaphrodite cirripedes； and I may here add； that I particularly attended to Mr。 Waterhouse's remark； whilst investigating this Order； and I am fully convinced that the rule almost invariably holds good with cirripedes。 I shall； in my future work； give a list of the more remarkable cases； I will here only briefly give one； as it illustrates the rule in its largest application。 The opercular valves of sessile cirripedes （rock barnacles） are； in every sense of the word； very important structures； and they differ extremely little even in different genera； but in the several species of one genus； Pyrgoma； these valves present a marvellous amount of diversification： the homologous valves in the different species being sometimes wholly unlike in shape； and the amount of variation in the individuals of several of the species is so great； that it is no exaggeration to state that the varieties differ more from each other in the characters of these important valves than do other species of distinct genera。
As birds within the same country vary in a remarkably small degree； I have particularly attended to them； and the rule seems to me certainly to hold good in this class。 I cannot make out that it applies to plants； and this would seriously have shaken my belief in its truth； had not the great variability in plants made it particularly difficult to compare their relative degrees of variability。
When we see any part or organ developed in a remarkable degree or manner in any species； the fair pr