14 VAKIATION Chap. I. 



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 pecu- 

 liarity 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 con- 

 fined 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 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 



