212 DOMESTIC pigeons: Chap. VL 



blue birds witli the several characteristic marks. I have said 

 that it must be assumed that each race has been crossed with 

 C. livia within a dozen, or, at the utmost, within a score of 

 generations ; for there is no reason to believe that crossed 

 offspring ever revert to one of their ancestors when removed 

 by a greater number of generations. In a breed which has 

 been crossed only once, the tendency to reversion will 

 naturally become less and less in the succeeding generations, 

 as in each there will be less and less of the blood of the 

 foreign breed ; but when there has been no cross with a 

 distinct breed, and there is a tendency in both parents to 

 revert to some long-lost character, this tendency, for all that 

 we can see to the contrary, may be ti-ansmitted undiminished 

 for an indefinite number of generations. These two distinct 

 cases of reversion are often confounded together by those 

 who have written on inheritance. 



Considering, on the one hand, the improbability of the 

 three assumptions which have just been discussed, and, on 

 the other hand, how simply the facts are explained on the 

 principle of reversion, we may conclude that the occasional 

 appearance in all the races, both when purely bred and more 

 especially when crossed, of blue birds, sometimes chequered, 

 with double wing-bars, with white or blue croups, with a 

 bar at the end of the tail, and with the outer tail-feathers 

 edged with white, affords an argument of the greatest weight 

 in favour of the view that all are descended from Columba livia, 

 including under this name the thi'ee or four wild varieties or 

 sub-species before enumerated. 



To sum up the six foregoing arguments, which are opposed 

 to the belief that the chief domestic races are the descendants 

 of at least eight or nine or perhaps a dozen species ; for the 

 crossing of any less number would not yield the characteristic 

 differences between the several races.' Firstly, the improba- 

 bility that so many species should still exist somewhere, but 

 be unknown to ornithologists, or that they should have 

 become witliin the historical period extinct, although man 

 has had so little influence in exterminating the wild C. livia. 

 Secondly, the improbability of man in former times having 

 thoroughly domesticated and rendered fertile under confine- 



