158 PROBLEMS OF GENETICS 



Pinus Chrysoptera 



i. Mantle and lower parts yellow (Y 1 ). X. Mantle and lower parts grey (y l ). 



2. Wing-bars white (y 2 ). 2. Wing-bars yellow (Y 2 ). 



3. Cheek and throat not black (b). 3. Cheek and throat black (B). 



The grey pigment of the mantle is common to both, but is 

 masked by the yellow in pinus, the net result being an olive- 

 green. 10 



I am much indebted to Dr. F. M. Chapman for the loan of 

 the coloured plate in which these distinctions are shown. It first 

 appeared in his book, North American Warblers. 



We cannot tell whether yellow or not-yellow is due to the 

 presence of a factor, but we may suppose that one or other 

 gives the special colour to the parts. The black of character 3 

 is no doubt a dominant. Thus pinus becomes Y^b and chry- 

 soptera in y x Y 2 B. The Lawrencei which has the underparts 

 yellow, wing-bars white, and black patches is Y x y 2 B and leuco- 

 bronchialis which has mantle and underparts not-yellow, wing- 

 bars yellow and no black patches is y^b. This representation, 

 it should be clearly understood, is tentative and approximate 

 only. The characters are not really sharp, for there is much 

 grading ; but allowing for the effects of heterozygosis and for some 

 actual breaking-up of factors I believe it gives a fairly correct 

 view of the case. In particular we can see how it meets the dif- 

 ficulty which Chapman felt in accepting leucobronchialis as in 

 any sense derived from pinus which has a yellow breast, and 

 chrysoptera which has a black throat, seeing that leucobronchialis 

 has neither. We now recognize at once that this form could be 

 produced by ordinary re-combination of the absence of Y 1 with 

 the absence of B. 



I note also with great interest that the modern observers 

 agree that the so-called hybrids may have the song either of 

 the one species, or of the other, or a song intermediate between 

 the two. It may also be added that these two types have several 



10 It would aid greatly in factorial analysis if the descriptive term "green" 

 could be avoided in application to cases where the green effect is due only to a mix- 

 ture of black and yellow pigments. The absence of yellow is the sole difference 

 between the mantle and underparts of pinus and chrysoptera. 



