ears, whether caused by the action of external influences during
mature life, or from an affection of the primary germinal vesicle, it
_tends_ to reappear in the offspring at the corresponding period of
life{474}. Hence (I may add) whatever effect training, that is the full
employment or action of every newly selected slight variation, has in
fully developing and increasing such variation, would only show itself
in mature age, corresponding to the period of training; in the second
chapter I showed that there was in this respect a marked difference in
natural and artificial selection, man not regularly exercising or
adapting his varieties to new ends, whereas selection by nature
presupposes such exercise and adaptation in each selected and changed
part. The foregoing facts show and presuppose that slight variations
occur at various periods of life _after birth_; the facts of
monstrosity, on the other hand, show that many changes take place before
birth, for instance, all such cases as extra fingers, hare-lip and all
sudden and great alterations in structure; and these when inherited
reappear during the embryonic period in the offspring. I will only add
that at a period even anterior to embryonic life, namely, during the
_egg_ state, varieties appear in size and colour (as with the
Hertfordshire duck with blackish eggs{475}) which reappear in the egg;
in plants also the capsule and membranes of the seed are very variable
and inheritable.

{474} _Origin_, Ed. i. p. 444, vi. p. 610.

{475} In _Var. under Dom._, Ed. ii. vol. I. p. 295, such eggs are
said to be laid early in each season by the black Labrador duck. In
the next sentence in the text the author does not distinguish the
characters of the vegetable capsule from those of the ovum.

If then the two following propositions are admitted (and I think the
first can hardly be doubted), viz. that variation of structure takes
place at all times of life, though no doubt far less in amount and
seldomer in quite mature life{476} (and then generally taking the form
of disease); and secondly, that these variations tend to reappear at a
corresponding period of life, which seems at least probable, then we
might _a priori_ have expected that in any selected breed the _young_
animal would not partake in a corresponding degree the peculiarities
characterising the _full-grown_ parent; though it would in a lesser
degree. For during the thousand or ten thousand