TYPES OF FACTOR INTERACTIONS 171 



sion in hybridization depends, and other cases differ from this one only 

 in the number of factor differences involved. 



Among the most notable cases of reversion are those which Darwin 

 describes in pigeons and fowls. Darwin regarded these throw-backs to 

 wild types which he obtained by crossing various breeds of pigeons as 

 important evidence of phyletic origin, and largely on the basis of this 

 evidence concluded that the many varied modern breeds of pigeons are 

 monophyletic in origin, that they are all derived from a single wild species. 

 This species is the Wild Rock Pigeon, Columba livia, and in the wild it 

 has an extended range over Europe, Abyssinia, India, and Japan. Even 

 in the wild state it is variable, but under domestication breeds have 

 been developed which show truly remarkable differences, and Darwin 

 has described and illustrated these with great care. 



The hybridization experiments which Darwin conducted with domes- 

 ticated breeds of pigeons were undertaken for the purpose of establishing 

 relationship to the Wild Rock Pigeon. The phenomenon of throwing 

 blue in pigeons is an exceedingly common one, but Darwin conducted 

 experiments with breeds which had been bred for many generations and 

 rarely, if ever, gave blue birds. Cole has summarized the results of one 

 of his experiments about as in Fig. 81 : 



Barb d 1 X Spot 9 Barb cf X Fantail 9 



(Self black) 



(White with red (Self black) 



spot on forehead 

 and red tail). 



(Self white) 



F i Barb-spot FI Barb-fantail 



(Black or brown with some (Black with some white 



white splashes). flights and tail feathers). 



1 I 



Mongrel Barb-spot d" X Mongrel Barb-fantail 9 



(Bird without a trace of 



(Bird without a trace of 



blue, generation not stated). blue, generation not stated). 



Reversionary Blue 

 (One bird, head tinted with red) 



FIG. 81. Pigeon breeding experiment resulting in reversion. (After Cole.) 



Evidently on a factorial basis this case involves a complicated recom- 

 bination of factors, and it can only be said that the Barb, Spot, and Fan- 

 tail breeds which Darwin used differed from the wild pigeon in different 

 factors and that in this experiment the original set of factors which is 

 responsible for the blue color of the Wild Rock Pigeon was reconstituted. 

 Darwin points out that this bird differed only in a few unimportant de- 



