PART IV. — COLOR. 



COLOR VARIATION IN RELATION TO COLOR FACTORS. 



A preliminary discussion of color variation in the rabbit was made by 

 Castle (:07a). Since that paper was written several obscure points have 

 been cleared up. In the light of our present knowledge an attempt will 

 be made to describe, in terms as simple as possible, the color varieties 

 of rabbits and the mode of their production. 



The gray pigmentation, common to wild rabbits, is comj)lex in its nature, 

 and all other color varieties are relatively simjjler. The gray coat results 

 from the joint action of several independent color factors; all other types 

 of pigmentation result from a weakening or entire loss of one or other of 

 the several factors concerned in producing a gray coat. In other words, 

 color variation in the rabbit is wholly retrogressive. We are able to recog- 

 nize the existence in the gray coat of the rabbit of 8 independent factors. 

 To assume the existence of so many factors will probably seem to some 

 absurd; at first it seemed so to us; but we have been forced step by step 

 to the assumption that they exist as the simplest way of explaining the 

 observed facts. 



The factor hypothesis was first introduced by Cudnot (:o3) to explain 

 the latent transmission of pigment characters through albinos; it was devel- 

 oped independently by Tschermak (:o3) to explain similar phenomena 

 (kryptomerism, the existence of hidden factors) in beans; and has been 

 further extended by Bateson (:o6) and his associates. 



The 8 color factors which are recognizable in the case of tlie gray rab- 

 bit, and the symbols which we shall use to designate them, are as follows: 



Symbol XT. A common color factor necessary to the production of all 

 pigment, wanting only in alljinos. 

 B. A factor for black, some substance which acting upon C 

 produces black pigmentation. 

 ~Br. A factor for brown, some substance which acting upon C 



produces a chocolate-brown pigmentation. 

 — Y. A factor for yellow, some substance which acting upon C 

 produces yellow pigmentation. 

 I. An intensity factor, which determines whether the pigmenta- 

 tion shall be intense (as in black and in yellow), dilute 

 (as in blue and in cream), or of some inttrmcdiate degree 

 of intensity. 

 A. A pattern factor which causes the black or brown pigments 

 to be excluded from certain portions of the individual 



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