230 THE ORIGIN OF SPECIES 



in the course of time, tliough we have not sufficient evi- 

 dence on this heiul. Habit in producing constitutional 

 peculiarities and use in strengthening and disuse in weak- 

 ening and diminishing organs, appear in many cases to 

 have been potent in their effects. Homologous parts tend 

 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, 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 be- 

 ings low in the scale are more variable than those stand- 

 ing higher in the scale, and which have their whole or- 

 ganization 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 



