14 
VAKIATION 
Chap. I. 
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 peculi¬ 
arity, 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- 
liorned 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 con¬ 
clude 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 intercrossing, that only a 
single variety should be turned loose in its new home. 
Nevertheless, as our varieties certainly do occasionally 
