204 DISTINCT SPECIES PRESENT [Chap. V. 



producing the lesser differences between varieties of the 

 same species, and the greater difHerences between species 

 of the same genus. Changed conditions generally in- 

 duce 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 produc- 

 ing 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 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 ad- 

 joining 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, undoubt- 

 edly occur. Multiple parts are variable in number and 

 in structure, perhaps arising from such parts not having 

 been closely specialised 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 3cale, and 

 which have their whole organisation more specialised. 

 Eudimentary 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 varia- 



