, can be the effect of any
corresponding difference in their exposure. We are led (as Mueller has
remarked) to the same conclusion, when we see in the same litter,
produced by the same act of conception, animals considerably different.

{194} This corresponds to _Origin_, Ed. i. p. 10, vi. p. 9.

As variation to the degree here alluded to has been observed only in
organic beings under domestication, and in plants amongst those most
highly and long cultivated, we must attribute, in such cases, the
varieties (although the difference between each variety cannot possibly
be attributed to any corresponding difference of exposure in the
parents) to the indirect effects of domestication on the action of the
reproductive system{195}. It would appear as if the reproductive powers
failed in their ordinary function of producing new organic beings
closely like their parents; and as if the entire organization of the
embryo, under domestication, became in a slight degree plastic{196}. We
shall hereafter have occasion to show, that in organic beings, a
considerable change from the natural conditions of life, affects,
independently of their general state of health, in another and
remarkable manner the reproductive system. I may add, judging from the
vast number of new varieties of plants which have been produced in the
same districts and under nearly the same routine of culture, that
probably the indirect effects of domestication in making the
organization plastic, is a much more efficient source of variation than
any direct effect which external causes may have on the colour, texture,
or form of each part. In the few instances in which, as in the
Dahlia{197}, the course of variation has been recorded, it appears that
domestication produces little effect for several generations in
rendering the organization plastic; but afterwards, as if by an
accumulated effect, the original character of the species suddenly gives
way or breaks.

{195} _Origin_, Ed. i. p. 8, vi. p. 10.

{196} For _plasticity_ see _Origin_, Ed. i. pp. 12, 132.

{197} _Var. under Dom._, Ed. ii. I. p. 393.

_On Selection._

We have hitherto only referred to the first appearance in individuals of
new peculiarities; but to make a race or breed, something more is
generally{198} requisite than such peculiarities (except in the case of
the peculiarities being the direct effect of constantly surrounding
conditions) should be inheritable,--namely t