12 VARIATION UNDER DOMESTICATION. 



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 impor- 

 tant 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 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 em- 

 bryology. These remarks are of course confined to the firsfe 

 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 stock. 

 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 endeavored 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 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 per- 

 fect reversion had ensued. It would be necessary, in order 

 to prevent the effects of intercrossing, that only a single 

 variety should have been turned loose in its new home. 

 Nevertheless, as our varieties certainly do occasionally revert 

 in some of their characters to ancestral forms, it seems to 

 me not improbable that if we could succeed in naturalizing, 

 or were to cultivate, during many generations, the several 



