152 UNUSUALLY DEVELOPED PARTS Chap. V. 



A Part developed in any F^pccks in an extraordinary Degree 

 or Manner, in comparison with the same Part in allied 

 JS^)ccic^, tends to he highly variable. 



Several years ago I was much struck by a rcmaik, to the 

 above effect, made by Mr, Waterliouse ; Prof. Owen, also, 

 seems to have come to a nearly similar conclusion. It is hope- 

 less to attempt to convince any one of the truth of the aljovc 

 j)roposition witliout 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 wing of the 

 bat is a most abnormal structure in the class of mammalia ; but 

 the rule would not apply here, because the whole group of bats 

 possesses wings ; it would apply only if some one species had 

 wings develojoed in a remarkable manner in comparison with 

 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 Avith the act of repro- 

 duction. The nde applies to males and females ; but as females 

 more rarely offer remarkable secondary sexual characters, it ap- 

 plies 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 Avhich 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 ; I particularly attended to ^Ir. \Vaterhouse's remark, 

 while investigating this order, and I am fully convinced that 

 the rule almost invariably holds good. I shall, in a future; 

 work, give a list of all the more remarkable cases ; I will lierc 

 only 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 

 diller extremely little even in different genera ; but in the several 

 species of one genus, Pyrgoma, these valves present a marvel- 

 lous amount of diversification ; the homologous valves in the 



