154 THE BEHAVIOR OF UNIT 



five independently variable characters are in- 

 volved ; the proportions in which the several sorts 

 of young are produced by each class of gray 

 parent confirms this conclusion. If the number 

 of independent unit characters concerned were 

 one greater, as it is in guinea-pigs, the total num- 

 ber of classes of parents would be doubled to 

 sixty-four; if it were one less, the number of 

 classes of parents would be reduced one-half, to 

 sixteen. 



What now are the five variable unit characters 

 concerned in producing the gray .coat of a rab- 

 bit and what are their relations one to another? 

 In answering this question it will be necessary 

 to mention a sixth unit character which contrib- 

 utes to the result, though not itself variable. It 

 will be convenient also to designate each sep- 

 arate unit character by a letter or symbol. The 

 six unit characters to which reference has been 

 made are : — 



1. C, a general color factor, something necessary to 

 the production of all pigment, wanting only in albinos. 



2. B, a factor for black, some substance, which acting 

 upon C, produces black pigment; this is in rabbits an 

 unvarying factor, though in other mammals it is often 

 variable. 



The four remaining factors modify the action of 

 one or the other of these two ; they are : — 



3. A, a pattern-factor governing the distribution of 

 black on the individual hair, so that it converts black 

 into gray, blue into blue-gray, and sooty into yellow. 



