COLOR 49 



this pattern black is largely excluded from the lower surfaces and from 

 a spot over each eye, where yellow then shows. The correctness of this 

 hypothesis is shown by the existence of this same pattern among brown- 

 pigmented dogs. The brown-and-tan has chocolate-brown pigment above 

 and tan (yellow) below, as well as a spot over each eye. It bears the same 

 relation to self-brown that black-and-tan does to self-black. On this 

 interpretation brown-and-tan is brown plus pattern, and black-and-tan 

 is black plus pattern. If, then, brown-and-tan is crossed with self-black, 

 black-and-tan offspring should result in F M and in F 2 there should be 

 obtained black-and-tan, brown-and-tan, self-black, and self-brown, in 

 the proportions 9:3:3:1. The experiment is commended to dog breeders. 



INTERRELATIONS OF FACTORS B, Br, AND Y. 



Returning, after this digression, to a consideration of the interrelations 

 of the three pigments, black, brown, and yellow, the fact seems clearly 

 established that black and brown are closely related but alternative con- 

 ditions dependent for their distribution upon two factors, which we may 

 designate E and S, whereas yellow is dependent for its distribution solely 

 upon one of these two factors, S. It would seem probable, therefore, 

 that in the genesis of the hair pigments, yellow is a first product of the 

 interaction of C and Y, which may or may not be further modified to pro- 

 duce brown or black, depending upon whether certain other factors (B and 

 Br) are or are not present. The amount and distribution of the yellow 

 pigment produced is conditioned by a factor which may assume phases 

 U, S, S', etc. The amount of the yellow pigment which is converted into 

 black or brown and its distribution is conditioned by another factor which 

 may assume phases E, E', etc., to R. 



GAMETIC STRUCTURE AND VARIATION. 



A diagram like those employed by the organic chemist may help to show 

 the relationships to each other of these 8 assumed pigment factors. 



C, the general color factor, is indispensable y B 



to the manifestation of any of the others. All / \ 



the others may be represented as linked A <p - - Y E 



directly or indirectly with it. E, however, is Br 



a modifier of B and Br alone, and is there- 

 fore joined with them alone in the diagram; and since B and Br are 

 assumed to act only after Y has acted, they are represented as joined 

 with it. 



Homozygous gray rabbits, wild ones for example, possess and transmit 

 all these 8 factors in each of their gametes. The diagram, therefore, 

 expresses their gametic composition. A homozygous black rabbit lacks, 

 of all these 8 factors, A alone. A yellow rabbit has R (restricted) in place 

 of E (extended black or brown), but otherwise is like the gray, or else the 



