lOO Reversion in Pigeons [ch. 



a black is crossed with an albino bearing the factor G, 

 greyness. Here the reversion is due to the meeting of 

 this factor with the colour-element present in the black 

 animal. The case of the Sweet Pea or the Stock, when 

 two colourless types crossed together produce coloured /^, 

 is of exactly the same nature, except in so far as both of 

 the complementary factors are in this case imperceptible 

 until they meet each other. The occurrence of the ratio 9 

 coloured viz. reversionary : 7 uncoloured is a proof of the 

 correctness of this mode of representing the facts. 



When reversion is correctly represented we are led on 

 to form some mental picture of the essential nature of a 

 variation. It must occur by the omission or by the intro- 

 duction of a factor. 



Reversion in Pigeons. 



In connection with the reversion produced by crossing 

 two white individuals the famous case of pigeons may be 

 considered, though in that example one parent was coloured. 



Darwin was the first to draw attention prominently to 

 such phenomena, and the experiments which he made 

 consequent on a hint given by Boitard and Corbie have 

 long been classical. In these experiments the blue colour 

 with black bars resembling the colour of the wild rock 

 pigeon {Columba livia) was reproduced as a result of cross- 

 ing birds quite free from such characteristics. Darwin's 

 experiments were very complicated"^. He was concerned 

 only with the question of origin, and to provide a qualitative 

 demonstration that the peculiarities of the blue rock pigeon 

 could be reproduced from modern varieties which had 

 ostensibly lost them. 



Mr Staples- Browne (255) has recently investigated the 

 matter afresh in a simpler case. He crossed a Black Barb 

 with a White Fantail, producing F^ black with some white 

 feathers. From such F^ birds /% was produced, consisting 

 of 



Blacks. Blacks Blues. Blues Whites, 



with white with white 



feathers. feathers. 



* Ajwnals ajid Plants, ed. 2, 1885, i. p. 207. 



