204 DISTINCT SPECIES PEESENT [Chap. V. 



producing the lesser differences between varieties of tile 

 same species, and the greater differences 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 

 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 special- 

 ised for any particular function, so that their modifica- 

 tions 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 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 



