VARIATION UNDER DOMESTICATION 13 



ancestor ; why a peculiarity is often transmitted from 

 one sex to both sexes, or to one sex alone, more com- 

 monly but not exclusively to the like sex. It is a fact 

 of some little importance to us, that peculiarities appear- 

 ing- in the males of our domestic breeds are often trans- 

 mitted 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 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 



