It is commonly assumed, perhaps from monstrosities affecting the embryo at
a very early period, that slight variations or individual differences
necessarily appear at an equally early period. We have little evidence on
this head, but what we have certainly points the other way; for it is
notorious that breeders of cattle, horses and various fancy animals, cannot
positively tell, until some time after birth, what will be the merits and
demerits of their young animals. We see this plainly in our own children;
we cannot tell whether a child will be tall or short, or what its precise
features will be. The question is not, at what period of life any
variation may have been caused, but at what period the effects are
displayed. The cause may have acted, and I believe often has acted, on one
or both parents before the act of generation. It deserves notice that it
is of no importance to a very young animal, as long as it is nourished and
protected by its parent, whether most of its characters are acquired a
little earlier or later in life. It would not signify, for instance, to a
bird which obtained its food by having a much-curved beak whether or not
while young it possessed a beak of this shape, as long as it was fed by its
parents.
I have stated in the first chapter, that at whatever age any variation
first appears in the parent, it tends to reappear at a corresponding age in
the offspring.
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