ANALOGOUS VARIATIONS. 141 



ability by Mr. Walsh, who has grouped them under his law 

 of equable variability. 



With pigeons, however, we have another case, namely, 

 the occasional appearance in all the breeds, of slaty-blue 

 birds with two black bars on the wings, white loins, a bar - 

 at the end of the tail, with the outer feathers externally 

 edged near their basis with white. As all these marks are ; 

 characteristic of the parent rock-pigeon, I presume that no 

 one will doubt that this is a case of reversion, and not of 

 a new yet analogous variation appearing in the several 

 breeds. We may, I think, confidently come to this con- 

 clusion, because, as we have seen, these colored marks are 

 eminently liable to appear in the crossed offspring of two 

 distinct and differently colored 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, probably 

 for hundreds of generations. But when a breed has been 

 crossed only once by some other breed, the offspring occa- 

 sionally show for many generations a tendency to revert in 

 character to the foreign breed — some say, for a dozen or even 

 a score of generations. After twelve generations, the pro- 

 portion of blood, to use a common expression, from one 

 ancestor, is only one in 2048 ; and yet, as we see, it is gen- 

 erally believed that a tendency to reversion is retained by 

 this remnant 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, as was formerl}' remarked, for all that we can see to 

 the contrary, be transmitted for almost any number of gen- 

 erations. When a character which has been lost in a breed, 

 reappears after a great number of generations, the most 

 probable hypothesis is, not that one individual suddenly 

 takes after an ancestor removed by some hundred genera- 

 tions, but that in each successive generation the character in 

 question has been lying latent, and at last, under unknown 

 favorable conditions, is developed. With the barb-pigeon, 

 for instance, which very rarely produces a blue bird, it is 

 probable that there is a latent tendency in each generation. 

 £0 po4uce blue plumage. The abstract improbability of 



