SUMMARY. 147 



sometimes they cause direct and definite effects ; and these 

 may become strongly marked in the course of time, though 

 we have not sufficient evidence on this head. Habit in pro- 

 ducing constitutional peculiarities, and use in strengthening 

 and disuse in weakening and diminishing organs, appear in 

 many cases to have been potent in their effects. Homol- 

 ogous parts tend to vary in the same manner, and homolo- 

 gous parts tend to cohere. Modifications in hard parts and 

 in external parts sometimes affect softer and internal parts. 

 When one part is largely developed, perhaps it 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, 

 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 char- 

 acters 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 manufactory of 

 new specific 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 vari- 

 able, and such characters differ much in the species of the 

 same group. Variability in the same parts of the organiza- 

 tion has generally been taken advantage of in ' giving sec- 

 ondary sexual differences to the two sexes of the same 

 species, and specific differences to the several species of the 



