150 DISTINCT SPECIES PRESENT 



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 ioth parents have lost some 

 character which their progenitor possessed, the tendency, 

 whether strong or weak, to reproduce the lost character 

 might, as was formerly 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 hyj^othesis is, not that one individual suddenly 

 takes after an ancestor removed by some hundred genera- 

 tions, but that in each successive generation the character 

 :n 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 to produce blue plumage. The abstract 

 improbability of such a tendency being transmitted through 

 a vast number of generations, is not greater than that of 

 quite useless or rudimentary organs being similarly trans- 

 mitted. A mere tendency to produce a rudiment is indeed 

 sometimes thus inherited. 



As all the species of the same genus are supposed to be 

 descended from a common progenitor, it might be expected 

 that they would occasionally vary in an analogous manner; 

 so that the varieties of two or more species would resemble 

 each other, or that a variety of one species would resemble 

 in certain characters another and distinct species, this 

 other species being, according to our view, only a well- 

 marked and permanent variety. But characters exclusively 



