CHAP. V.] ANALOGOUS VARIATIONS. 123 



variation, the nature of which we are unable to understand, un- 

 doubtedly occur. Multiple parts are variable in number and in 

 structure, perhaps arising from such parts not having been closely 

 specialised for any particular function, so that their modifications 

 have not been closely checked by natural selection. It follows 

 probably from this same cause, that organic beings low in the 

 scale are more variable than those standing higher in the scale, 

 and which have their whole organisation more specialised. Rudi- 

 mentary organs, from being useless, are not regulated by natural 

 selection, and hence are variable. Specific characters that is, the 

 characters which have come to differ since the several species of 

 the same genus branched off from a common parent are more 

 variable than generic characters, or those which have long been 

 inherited, and have not differed within this same period. In these 

 remarks we have referred to special parts or organs being still vari- 

 able, because they have recently varied and thus come to differ ; 

 but we have also seen in the second chapter that the same principle 

 applies to the whole individual; for in a district where many 

 species of a genus are found that is, where there has been much 

 former variation and differentiation, or where the manufactory of 

 new specific forms has been actively at work in that district and 

 amongst these species, we now find, on an average, most varieties. 

 Secondary sexual characters are highly variable, and such characters 

 differ much in the species of the same group. Variability in the 

 same parts of the organisation has generally been taken advantage 

 of in giving secondary sexual differences to the two sexes of the 

 same species, and specific differences to the several species of the 

 same genus. Any part or organ developed to an extraordinary 

 size or in an extraordinary manner, in comparison with the same 

 part or organ in the allied species, must have gone through an 

 extraordinary amount of modification since the genus arose ; and 

 thus we can understand why it should often still be variable in 

 a much higher degree than other parts ; for variation is a long- 

 continued and slow process, and natural selection will in such 

 cases not as yet have had time to overcome the tendency to further 

 variability and to reversion to a less modified state. But when a 

 species with any extraordinarily-developed organ has become the 

 parent of many modified descendants which on our view must be 

 a very slow process, requiring a long lapse of time in this case, 

 natural selection has succeeded in giving a fixed character to the 

 organ, in however extraordinary a manner it may have been 

 developed. Species inheriting nearly the same constitution from 

 a common parent, and exposed to similar influences, naturally 

 tend to present analogous variations, or these same species may 

 occasionally revert to some of the characters of their ancient pro- 

 genitors. Although new and importaat modifications may not 



