134 THE ORIGIN OF SPECIES 



to vary in the same manner, and homologous parts tend to cohere. 

 Modifications in hard parts and in external parts sometimes affect 

 softer and internal parts. When one part is largely developed, per- 

 haps if tends to draw nourishment from the adjoining parts; and 

 every part of the structure which can be saved without detriment 

 will be saved. 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, un- 

 doubtedly 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. 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 

 variable, 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 manufac- 

 tory of new specific forms has been actively at work — in that dis- 

 trict 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. Vari- 

 ability 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 ex- 

 traordinary 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 



