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Show [CB.A.P. V. LA "\\'"S OF VARIATION. 136 . to their uselessness, their var:lability seen1s tl b~ o~Ing havinO' no power to and therefore to natura. se ec ItOU. Th~s rudimentary check devi.a ti·o ns I·l l thmr struc ur e. . 1 f 1 f the vanous aws o Parts are left to the free pay ~· ued disuse, and to the gr.o w th, to the effe.c ts of long-con Ill tendency to reversion. . n s eoies in an extraordina~y A tnart developed ~n a Y W 't 1.. · the same part ~n r . oompar~son w~ ,~.~ degree or 'f!1'anner, ~n n,~· hl variable.-Several years allied speoMs, tends t~ be. th~g r~mark nearly to the above ago I was ~uch struc r WIW a~erhouse~ I infer also from effect, pubh~hed b~ ~r~ Professor Owen, with respect to an observation ma e Y a outanO' that he has the length of the arJ?S .ff the 0~~s~ It ~~ hopeless to con1e to a nea~ly slmi ar coni? the tr~th of this proposiattempt to con-yi~ce any o~e o arra of facts which I have tion without giVI~~ the lootg ossibly be here· introduced. collected, and whlc cann. t'p n that it is a rule of high I can only state my convif ~~veral causes of error, but I generality. I am ada~e o llowance for them. It should hope that I have rna e ue a b no means applies to any be understood that the rule 1 y ed unless it be unusually part, howe':er unusua~ly devft~p th~ same part in closely . developed !n companson b t's win is a most abnormal allied speCies. Thus, the a 1" g blit the rule would structure in the class ~h:~siaj whole O'roup of bats not .here ~ppl!,.beca~~ a;;ly only if sor:e one species having Win~s , ~t wo 1 d in some remarkable manof b~t had Its ~Ings d~fue ~t! other species of the same ner In companlson Wil' very strongly in the case of genus. The ru e app Ies h dis la ed in any un-secondary sexua~ha~acters~e:o:Kary ~clual characters, usual manner. e 1' er~, characters which are attached used by Hunter, app 1:~· o tl connected with the act of to one sex, but are no Irec f to males and females; reproduction. The rule af,p Iffer remarkable secondary but as females more rar~ y o 1 to them The h t 't pphes more rare Y · sexual c arac ers~ 1 a 1' bl in the case of secondary rule being so plainly apbp l~a ~ the great variability of sexual characters, may e ue 0 . CHAP. V.] LAWS OF VARIATION. 137 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 cirripe~ es; and I may here add, that I particularly attended to }fr. 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 s.essile 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 am.ount 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 se.ems 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 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 son1e light. In our domestic animals, if any part, or the whole animal, be neg- |