6o MENDELISM chap. 



bringing together of two complementary factors, we 

 must regard it here as due to the association of two 

 complementary absences. To this question, how- 

 ever, we shall revert later in discussing the origin of 

 domesticated varieties. 



There is one other instance of reversion to which 



Black Barb x White Fantail Black Barb x Spot ^ 



I I 



Dark x Dark 



.1 

 Among the offspring one very similar 



to the wild blue rock. 



we must allude. This is Darwin's famous case of 

 the occasional appearance of pigeons reverting to 

 the wild blue rock {Columba livid) when certain 

 domesticated races are crossed together. As is 

 well known, Darwin made use of this as an 



Black y White 

 Barb ^ Fantail 



-i 



Black V Black 



(White Splashed) 



(White Splashed) 



r \ 1 T 



Black Black Blue Blue White 



(White Splashed) (White Splashed) 



(9) S) (4) 



argument for regarding all the domesticated varieties 

 as having arisen from the same wild species. The 

 original experiment is somewhat complicated, and 

 is shown in the accompanying scheme. Essentially 



1 This is an almost white bird, the colour being confined to the 

 tail and the characteristic spot on the head. 



