THE PHENOMENA OF REVERSION 327 



mongrel female barb-fantail and a mongrel male barb-spot were 

 paired. Neither of these birds 'had the least blue about them, 

 nevertheless the offspring ' (the number is not stated) ' from 

 the above two mongrels was of exactly the same blue tint as 

 that of the wild rock-pigeon from the Shetland Islands over 

 the whole back and wings ; the double black wing-bars were 

 equally conspicuous.' * In this case the ancestral determinants 

 evidently prevailed all the more strongly because they were 

 opposed by racial determinants of three or four different kinds, 

 the controlling forces of which could not simply be cumulative, 

 like those of the ancestral determinants, but could only partially 

 weaken and neutralise one another. 



The fact which may be deduced from Darwin's observations, 

 viz. that the offspring of simple crosses display practically a 

 very similar tendency to reversion, can also easily be explained 

 theoretically. For the germ-plasm of a well-established race 

 will contain a certain percentage of ancestral determinants, and 

 the o^erm-cells of an individual will be liable to but few fluct- 

 uations in this respect. In such simple cases of crossing, an 

 almost similar number of ancestral determinants, as well as of 

 racial determinants, must therefore come together at each fer- 

 tilisation ; and the struggle between these different kinds of 

 determinants must always produce approximately the same 

 result. A perfect uniformity in the offspring of the same cross 

 cannot be expected ; but if reversion does occur in some cases, 

 it will be absent in others, and if it occur partially in some, 

 certain parts in others will revert. An instance of the former 

 kind is seen in the above-mentioned cross between a black barb 

 and a white fantail, and of the latter in the case described by Dar- 

 win on p. 207 of his 'Animals and Plants under Domestication/ 

 Vol. I. He crossed a white nun with a red tumbler, and reared 

 five young, all of which presented traces of reversion. One 

 possessed a blue tail ; the second and third ' presented a trace of 

 the bar at the end ' of the blue tail : the fourtli • was brownish, 

 and the wings showed a trace of the double bar" ; and the fifth 

 ' was pale blue over the whole breast, back, croup, and tail, but 

 the neck and primary wing-feathers were reddish, and the wing- 

 bars presented two distinct bars of a red colour.' Thus all the 

 five young had reverted — some more and some less markedly — 



* Loc. cit., Vol. I., p. 209. 



