152 ON THE ORIGIN OF SPECIES 



ening and diminishing organs, seem to have been more 

 potent in their effects. Homologous parts tend to vary 

 in the same way, and homologous parts tend to cohere. 

 Modifications in hard parts and in external parts some- 

 times affect softer and internal parts. When one part 

 is largely developed, perhaps it tends to draw nourish- 

 ment from the adjoining parts ; and every part of the 

 structure which can be saved without detriment to the 

 individual, will be saved. Changes of structure at an 

 early age will generally affect parts subsequently de- 

 veloped ; and there are very many other correlations of 

 growth, the nature of which we are utterly unable to 

 understand. Multiple parts are variable in number and 

 in structure, perhaps arising from such parts not having 

 been closely specialised to any particular function, so 

 that their modifications have not been closely checked 

 by natural selection. It is probably from this same 

 cause that organic beings low in the scale of nature are 

 more variable than those which have their whole organ- 

 isation more specialised, and are higher in the scale. 

 Rudimentary organs, from being useless, will be disre- 

 garded by natural selection, and hence probably 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 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 any 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 — there, 

 on an average, we now find most varieties or incipient 

 species. 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 



