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.