us {443} fancy animals, cannot
positively tell, until some time after the animal has been born, what its
merits or form will ultimately turn out. We see this plainly in our own
children; we cannot always tell whether the child will be tall or short, or
what its precise features will be. The question is not, at what period of
life any variation has been caused, but at what period it is fully
displayed. The cause may have acted, and I believe generally has acted,
even before the embryo is formed; and the variation may be due to the male
and female sexual elements having been affected by the conditions to which
either parent, or their ancestors, have been exposed. Nevertheless an
effect thus caused at a very early period, even before the formation of the
embryo, may appear late in life; as when an hereditary disease, which
appears in old age alone, has been communicated to the offspring from the
reproductive element of one parent. Or again, as when the horns of
cross-bred cattle have been affected by the shape of the horns of either
parent. For the welfare of a very young animal, as long as it remains in
its mother's womb, or in the egg, or as long as it is nourished and
protected by its parent, it must be quite unimportant whether most of its
characters are fully acquired a little earlier or later in life. It would
not signify, for instance, to a bird which obtained its food best by having
a long beak, whether or not it assumed a beak of this particular length, as
long as it was fed by its parents. Hence, I conclude, that it is quite
possible, that each of the many successive modifications, by which each
species has acquired its present structure, may have supervened at a not
very early period of life; and some direct evidence from our domestic
animals supports this view. But in other cases it is quite possible that
each successive modification, or {444} most of them, may have appeared at
an extremely early period.

I have stated in the first chapter, that there is some evidence to render
it probable, that at whatever age any variation first appears in the
parent, it tends to reappear at a corresponding age in the offspring.
Certain variations can only appear at corresponding ages, for instance,
peculiarities in the caterpillar, cocoon, or imago states of the silk-moth;
or, again, in the horns of almost full-grown cattle. But further than this,
variations which, for all that we can see, might have appeared earlier or
later