68 MR. R. STAPLES-BROWNE ON [Feb. 18, 



that " the head was tinted Avith a shade of i-ed, evidently derived 

 from the Spot, and was of a paler blue than in the rock-pigeon, 

 as was the stomach." (v. Animals & Plants under Domestication, 

 2nd edition, vol. i. p. 209.) 



In the experiments here described, as a Spot pigeon was not 

 readily obtainable, a black and white Nun pigeon was substituted. 

 When the Barb-Fantail mongrels were mated to the Barb-Nun 

 mongrels, however, no reversionary types appeared {v. Exps. 1, 

 34, 37, 38, and 39). It was, however, found that, when the 

 Barb-Fantail crossbreds were mated together, some birds were 

 produced having certain blue feathers. Yarious experiments with 

 birds produced from the cross between the Barb and Fantail 

 were carried out at some length, but, owing to the limitations of 

 space, the Barb-Nun crosses were soon discontinued. 



The blue colour, when it appeared, was found chiefly in the tail 

 and neighbouring parts, as had already been observed by Darwin. 

 Associated with this the black tail-bar was invariably present. 

 The wing-coverts and backs of the reversionary types obtained 

 were generally of a smoky-black colour, thus obscuring the two 

 wing-bars found in the rock-pigeon. 



The reversionary type in pigeons may in some cases be obtained 

 in the first cross-bi-ed generation (F. 1). Such a result Darwin 

 observed when he crossed a Nun with a red Tumbler. 



The fact that in the Barb-Fantail cross the reversionary blue 

 does not appear until F. 2, is interesting. The F. 1 generation 

 contains all the elements introduced by the parental types ; never- 

 theless it is not reversionary in colour, but resembles the black 

 Barb except for the addition of some white. From the fact 

 therefore that the blue reversionary form can be produced by such 

 F. 1 birds, it is clear that they contain some element which 

 prevents the ajopearance of the blue. This element is evidently 

 the factor for black self- colour ; and the experiment shows that 

 this element is dominant, or, more strictly, epistatic, to the blue. 

 The black factor must thus be regarded as an element not derived 

 from the wild pigeon, but added to it by some subsequent 

 variation. When, by recombination of the various elements, the 

 F. 2 forms are produced, those combina,tions which contain the 

 blue in the absence of the black factor exhibit the blue, while 

 those which contain the black in addition cannot exhibit it. 



In some other cases of reversion on crossing [e. g., Sweet Peas 

 and Stocks), the reversion can be proved to be due to the meeting 

 of complementary factors. In the case of the Bai-b- Fantail cross 

 the evidence is not yet sufficient to show whether the factors 

 needed to produce the atavistic condition are all present in the 

 Bai'b, and their effect merely hidden by the presence of the black 

 factor, or whether a necessary factor is introduced by the 

 Fantail ; but the fact that no blues came in the F. 2 made from 

 F 1 (Barb x Fantail) x F. 1 (Barb x Nun) distinctly suggests 

 that some factor of the blue did come from the Fantail. 



