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 x Spot @ Barb &@ X Fantail @ 
(Self eae hite with red (Self black) | (Self white) 
spot on forehead 
' and red tail). 
F, Barb-spot F, Se 
(Black or brown with some (Black with some white 
white splashes). flights and tail feathers). 
| 
| 
Mongrel Barb-spot @ x Mongrel Barb-fantail Q 
(Bird without a trace of | (Bird without a trace of 
blue, generation not stated). blue, generation not stated). 
l 
Reversionary Blue 
(One bird, head tinted with red) 
Fic. 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- 
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