108 THE AMERICAN NATURALIST [Vol. LII 



ure to secure a dihybrid ratio from the mating of blues, 

 one is driven to assume linkage, and apparently a quite 

 close linkage, of the dominant of one allelomorphic pair 

 to the recessive of the other. As I hope to make clear, 

 however, the linkage may not be complete, since it would 

 easily be possible for crossing-over to occur occasionally 

 with very slight likelihood of detection. 



Goodale (1917, p. 213) has very recently shown that 

 crossing-over occurs in the sex chromosome of the male 

 fowl, though he has not as yet presented his evidence in 

 detail. The universality of the laws of heredity through- 

 out the plant and animal kingdoms is such that it would 

 be a matter of surprise if crossing-over in fowls did not 

 also occur in chromosomes other than the sex chromosome. 



There is at present no certain criterion by which to pre- 

 dict whether, having assumed crossing-over in the auto- 

 somes of fowls, it occurs in one sex only or in both ; and 

 if in but one sex, which it may be, unless one chooses to 

 suppose that it occurs only in the sex homozygous for the 

 sex chromosome, as in Drosophila. If it occurs in both 

 sexes, it is apparently so rare an event in Andalusians 

 that the probability of securing two cross-over individuals 

 in a mating made for purposes of analysis is so small as 

 to be almost negligible. 



After a somewhat extended microscopic study of blue- 

 gray feathers from blues, blue-splashed and certain 

 crosses, it seems more in accordance with their apparent 

 action to refer to the factor responsible for changing black 

 into bluish-gray as a restrictor, designated as R rather 

 than M (Mosaikfactor) as was done by Goldschmidt. 

 Similarly in place of Q (Quantitat) I would suggest E, as 

 responsible for the extension of pigment to all the feathers 

 of the body. 



Using this terminology and assuming for the moment 

 complete linkage, a cross between individuals of the black 

 and blue-splashed races, respectively, would appear as 

 follows : 



Er Er = black XeR eR = blue-splashed ; 

 Fj EreR = blue; 



