CHARACTERS IN HEREDITY 155 



4. E, a factor governing the extension of black over 

 the body generally; in its most extended distribution, 

 black occurs on all hairs of the body, in its most 

 restricted distribution (R) it scarcely extends beyond 

 the eye, and the skin of the extremities, the hair being 

 practically devoid of black pigment and appearing yel- 

 low ; that this factor is distinct from B, is shown by the 

 fact that it can, in guinea-pigs, be dissociated from B 

 and become associated with brown pigmentation. 



5. U, a factor governing the distribution of C over 

 the body; if C covers the whole body (condition U), the 

 whole body is pigmented; if C covers part of the coat 

 only (condition S), the rest is occupied by spots of 

 white. That this unit is distinct from C is shown by 

 the fact that it is transmissible through animals which 

 lack C, that is, through albinos. 



6. I, a factor governing the intensity of the pig- 

 mentation. It is a modifier of C, for it affects all pig- 

 ments alike, yellow and brown as well as black, all of 

 which pigments have their common basis in C; but I is 

 distinct from C, for it is transmissible through albinos, 

 which lack C. When I is present all the pigments are 

 intense ; when I is absent, or rather weakened to the 

 condition D, the pigmentation is dilute, as in blue and 

 cream, the dilute conditions of black and j^ellow 

 respectively. 



It is clear from what has just been said that 

 these various factors, though separately variable, 

 are not entirely independent of each other. Some 

 produce no visible effects unless others are pres- 

 ent. Thus if C alone is wanting, none of the 

 others is visible. To aid in expressing the inter- 

 relationship of the factors I think it useful to 

 imitate the organic chemist and employ dia- 

 grams. Thus a diagram might be constructed as 

 follows to express the relations of the six factors 



