admit the difference
of our races <to be> due to the differences of their original stocks, so
much must we give up of the amount of variation produced under
domestication. But this appears to me unimportant, for we certainly know
in some few cases, for instance in the Dahlia, and potato, and rabbit,
that a great number of varieties have proceeded from one stock; and, in
many of our domestic races, we know that man, by slowly selecting and by
taking advantage of sudden sports, has considerably modified old races
and produced new ones. Whether we consider our races as the descendants
of one or several wild stocks, we are in far the greater number of cases
equally ignorant what these stocks were.

_Limits to Variation in degree and kind._

Man's power in making races deends, in the first instance, on the stock
on which he works being variable; but his labours are modified and
limited, as we have seen, by the direct effects of the external
conditions,--by the deficient or imperfect hereditariness of new
peculiarities,--and by the tendency to continual variation and
especially to reversion to ancestral forms. If the stock is not variable
under domestication, of course he can do nothing; and it appears that
species differ considerably in this tendency to variation, in the same
way as even sub-varieties from the same variety differ greatly in this
respect, and transmit to their offspring this difference in tendency.
Whether the absence of a tendency to vary is an unalterable quality in
certain species, or depends on some deficient condition of the
particular state of domestication to which they are exposed, there is no
evidence. When the organization is rendered variable, or plastic, as I
have expressed it, under domestication, different parts of the frame
vary more or less in different species: thus in the breeds of cattle it
has been remarked that the horns are the most constant or least variable
character, for these often remain constant, whilst the colour, size,
proportions of the body, tendency to fatten &c., vary; in sheep, I
believe, the horns are much more variable. As a general rule the less
important parts of the organization seem to vary most, but I think there
is sufficient evidence that every part occasionally varies in a slight
degree. Even when man has the primary requisite variability he is
necessarily checked by the health and life of the stock he is working
on: thus he has already made pigeons with such s