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156 SUMMARY. 



ferences between varieties of the same species, and the 

 greater diffences between species of the same genus. Changed 

 conditions generally induce mere fluctuating variability, 

 but 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 producing constitutional peculiarities, and use in 

 strengthening, and disuse in weakening and diminishing 

 organs, appear in many cases to have been potent in their 

 effects. Homologous parts tend to vary in the same man- 

 ner, and homologous parts tend to cohere. Modifications 

 in hard parts and in external parts sometimes affect softer 

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

 haps 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. Mutiple 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 bfjing useless, are not regulated by 

 natural selection, and hence ^u'e 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 char- 

 acters, 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 

 among these species, we now find, on an average, most 



