eritance are for the most part unknown; no one
can say why the same peculiarity in different individuals of the same
species, or in different species, is sometimes inherited and sometimes
not so; why the child often reverts in certain characteristics to its
grandfather or grandmother or 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 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 the 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 reappear 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
silk-worm 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 the primary cause which may have acted on the ovules or on the male
element; in nearly the same manner as the increased length of the horns
in the offspring from a short-horned cow by a long-horned bull, 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 invariably 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 varieti