LAWS OF VARIATION 146 



variation appearing in the several breeds. We may, 

 I think, confidently come to this conclusion, because, 

 as we have seen, these coloured marks are eminently 

 liable to appear in the crossed offspring of two distinct 

 and differently coloured breeds ; and in this case there 

 is nothing in the external conditions of life to cause 

 the reappearance of the slaty-blue, with the several 

 marks, beyond the influence of the mere act of crossing 

 on the laws of inheritance. 



No doubt it is a very surprising fact that characters 

 should reappear after having been lost for many, per- 

 haps for hundreds of generations. But when a breed 

 has been crossed only once by some other breed, the 

 offspring occasionally show a tendency to revert in 

 character to the foreign breed for many generations — 

 some say, for a dozen or even a score of generations. 

 After twelve generations, the proportion of blood, to 

 use a common expression, of any one ancestor, is only 

 1 in 2048 ; and yet, as we see, it is generally believed 

 that a tendency to reversion is retained by this very 

 small proportion of foreign blood. In a breed which 

 has not been crossed, but in which both parents have 

 lost some character which their progenitor possessed, 

 the tendency, whether strong or weak, to reproduce 

 the lost character might be, as was formerly remarked, 

 for all that we can see to the contrary, transmitted for 

 almost any number of generations. When a character 

 which has been lost in a breed, reappears after a great 

 number of generations, the most probable hypothesis 

 is, not that the offspring suddenly takes after an ancestor 

 some hundred generations distant, but that in each 

 successive generation there has been a tendency to re- 

 produce the character in question, which at last, under 

 unknown favourable conditions, gains an ascendancy. 

 For instance, it is probable that in each generation of 

 the barb-pigeon, which produces most rarely a blue 

 and black-barred bird, there has been a tendency in 

 each generation in the plumage to assume this colour. 

 This view is hypothetical, but could be supported by 

 some facts ; and I can see no more abstract improba- 



