138 EVOLUTION AND ANIMAL LIFE 



Changes of structure at an early age may affect parts subsequently 

 developed ; and many cases of correlated variation, the nature of which 

 we are unable to understand, undoubtedly occur. Multiple parts are 

 variable in number and in structure, perhaps arising from such parts 

 not having been closely specialized 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 organization more specialized. Rudimentary 

 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 variable, because they have recently varied and 

 thus come to differ ; but we have also seen . . . that the same prin- 

 ciple 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 spe- 

 cific forms has been actively at work in that district and among 

 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 

 organization 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-con- 

 tinued 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 an ex- 

 traordinarily developed organ has become the parent of many modified 

 descendants which in 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 

 jnanner it may have been developed. Species inheriting nearly the 

 same constitution from a common parent, and exposed to similar 



