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Show 150 LAWS OF VARIATION. CHAP. v. rudimentary parts are left to the free play of. the va:ious laws of growth, to the effects ~f long-continued disuse, and to the tendency to reversion. .A part developed in any species ~n an extraordina~y degree or manner, in aomp.arison w~th the same part zn allied speaies, tends to be hzghly varzable.-Several years ago I was much struck with a remark, nearly to. the above effect, published by Mr. Waterhouse. I I~er also from an observation made by Professor Owen, with respect to the length of the arms of the ourang-outan?', 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 ~he long array ~f 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 cha~acters 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 rar~ly . offer remarkable secondary sexual characters, it apphes CHAP. V. LAWS OF VARIATION. 151 more :arely to them. The rule being so plainly applicable m the case of secondary sexual characters, may be due to the great variability of these characters whether o:r n~t displayed in any unusual manner-of ;hich fact I think there can be little doubt. But that our rule is not con.:fined to secondary sexual characters is clearly shown m the case of hermaphrodite cirripedes ; and I rmay her~ add, that I pa;rt~cularl! attended to Mr. "aterhouse s remark, whilst Investigating this Order and I am fully convinced that the rule almost invari~ ably ho~ds goo~ with cirripedes. I shall, in my future work, give a hst of the more remarkable cases · I will ~ere only briefly give one, as it illustrates the ~ule in I~s l.argest application. The opercular valves of sessile cur1pedes (rock barnacles) are, in every sense of the word, ve~ important structures, and they differ extremely httle even in different genera · but in the several species of one genus, Pyrgoma, 'these valves present a marvellous amount of diversification : the ~omologous valves in the different species being somet~ mes wholly unlike in shape; and the amount of variation in the i~di~iduals of several of the species is so ?r~at, t~at It Is no exaggeration to state that the Varieties differ more from each other in the characters o.f these important valves than do other species of distmct 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 thi~ cl~ss. I cann~t make out that it applies to plants, and this would seriously have shaken my belief in its truth h~d not the great variability in plants made it particular!; drfficult 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 |