| Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part
→has varied. But whenever we have the means of instituting a comparison, the same laws appear to have acted in producing the lesser differences between varieties of the 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
and use in
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
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
→OMIT
will be saved. Changes of structure at an early age
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 specialised
any particular function, so that their modifications have not been closely checked by natural selection. It
probably from this same
that organic beings low in the scale
are more variable than those
→standing higher in the scale, and which
have their whole organisation more
Rudimentary organs, from being useless,
→are not regulated
by natural selection, and hence
are variable. Specific
that is, the characters which have come to differ since the several species of the same genus branched off from a common
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
that the same principle applies to the whole individual; for in a district where many species of
genus are
that is, where there has been much former variation and differentiation, or where the manufactory of new specific forms has been actively at
→in that district and amongst these species,
we now
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