its
grandfather or grandmother or other much more remote ancestor; why a
peculiarity is often transmitted from one sex to both sexes or to one
sex alone, more commonly but not exclusively to the like sex. It is a
fact of some little importance to us, that peculiarities appearing
in the males of our domestic breeds are often transmitted either
exclusively, or in a much greater degree, to males alone. A much more
important rule, which I think may be trusted, is that, at whatever
period of life a peculiarity first appears, it tends to appear in the
offspring at a corresponding age, though sometimes earlier. In many
cases this could not be otherwise: thus the inherited peculiarities
in the horns of cattle could appear only in the offspring when nearly
mature; peculiarities in the silkworm are known to appear at the
corresponding caterpillar or cocoon stage. But hereditary diseases and
some other facts make me believe that the rule has a wider extension,
and that when there is no apparent reason why a peculiarity should
appear at any particular age, yet that it does tend to appear in the
offspring at the same period at which it first appeared in the parent. I
believe this rule to be of the highest importance in explaining the
laws of embryology. These remarks are of course confined to the first
APPEARANCE of the peculiarity, and not to its primary cause, which may
have acted on the ovules or male element; in nearly the same manner as
in the crossed offspring from a short-horned cow by a long-horned bull,
the greater length of horn, though appearing late in life, is clearly
due to the male element.

Having alluded to the subject of reversion, I may here refer to
a statement often made by naturalists--namely, that our domestic
varieties, when run wild, gradually but certainly revert in character to
their aboriginal stocks. Hence it has been argued that no deductions can
be drawn from domestic races to species in a state of nature. I have in
vain endeavoured to discover on what decisive facts the above statement
has so often and so boldly been made. There would be great difficulty
in proving its truth: we may safely conclude that very many of the most
strongly-marked domestic varieties could not possibly live in a wild
state. In many cases we do not know what the aboriginal stock was, and
so could not tell whether or not nearly perfect reversion had ensued.
It would be quite necessary, in order to prevent the effects of
inter