vn] Yellow Rabbits 117 



which seemed to him to be chocolate. No examination of 

 the pigments of these rabbits has been made. 



Agouti, as we know from the experiments of Hurst and 

 others, is epistatic to black. Next, Castle (53) has shown and 

 Hurst independently discovered^ the fact that the yellows 

 are of two distinct types. A. Yellows with white bellies and 

 tails, which always bear the agouti factor, G. B. Yellows 

 with blue bellies and tails, which do not contain G. The 

 latter have been spoken of as sooty yellows, and to English 

 fanciers they are known as tortoise-shells. Respecting the 

 pigments of these types full information is wanting. Miss 

 Durham found that agouti certainly contains black, choco- 

 late and yellow ; that black contains black and chocolate ; 

 but hitherto the yellows have not been studied on a com- 

 prehensive scale, and all that we know is that yellows may 

 contain both black and chocolate granules in addition to 

 yellow. None have yet been seen to contain yellow only. 



Following Castle, the genetic composition and behaviour 

 of the several types may be represented thus : 



Agouti (wild grey) GBY. 



Black gBY. 



Yellow A, white belly GbY. 



Yellow^, blue belly gbY. 



This scheme is so far satisfactory, but two difficulties 

 arise when an attempt is made to compare the phenomena 

 seen in the rabbit with those observed in mice and guinea- 

 pigs ; for first the scheme takes no account of chocolate ; 

 and secondly, in the mouse yellow is dominant to black, 

 not recessive as in the rabbit. There is however no doubt 

 as to the essential facts, that yellow A crossed with black 

 gives agouti, while yellow B crossed with black gives 

 simply black dominant. A further confirmation of this 

 symbolic representation is found in the fact observed by 

 Castle that agouti x yellow B gives blacks in F z as the 

 scheme demands f. The nature of the factor G is obscure. 

 Castle regards it as a pattern-factor governing the distribu- 

 tion of the colours ; but a grey rabbit differs from a black 



* Communicated to the Internat. Congress Zool. 1907 : as yet un- 

 published. 



t Compare the similar phenomena in mice, p. 78. 



