40 THE ORIGIN OF SPECIES 



extension, and tliat, 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 ex- 

 plaining the laws of em1)ryology. 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 nearl}'' the same 

 manner as the increased length of the horns in the off- 

 spring 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, grad- 

 ually but invariably revert in character to their aborigi- 

 nal 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 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 perfect 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. Never- 

 theless, as our varieties certainly do occasionally revert 

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



