168 
LAWS OF VAEIATION. 
Chap. Y. 
climate and food, &c., seem to have induced some slight 
modifications. Habit in producing constitutional dif¬ 
ferences, and use in strengthening and disuse in weak¬ 
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 organi¬ 
sation more specialised, and are higher in the scale. 
Kudimentary 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; 
