40 A MANUAL OF MENDELISM 



published in 1909 by the Carnegie Institute of Washing- 

 ton, Professor Castle reports the finding and identifica- 

 tion of the two remaining colours, which turned out to 

 be cream and dilute tortoise. Cream seems to have 

 been produced for some time without being noticed, for, 

 in reference to the large number of yellows resulting 

 from certain hybrid crossings, it is stated that " the 

 category yellow is probably too large because of a 

 failure to discriminate between yellow and cream, a 

 difference which at first we failed to record." How 

 cream was produced is not recorded ; but, if it be 

 noticed that grey, blue-grey, and yellow are the first 

 three groups in the two-pair set 



Grey XYZ 



Blue-grey XYz 



Yellow xYZ 



xYz 



it is obvious that, if blue-grey and yellow were mated, 

 the smallest group in their hybrids' progeny should 

 carry the characters xYz. Similarly, by mating either 

 of the two groups already found carrying two dominants 

 with the group carrying the third dominant belonging 

 to the set, that is blue-grey, XYz, with tortoise-shell, 

 xyZ, or yellow, xYZ, with blue, Xyz, the whole eight 

 groups, including xYz and xyz should appear in their 

 hybrids' progeny. 



Blue and yellow were mated, and, if it be allowed 

 that some of the yellows were really creams, it will be 

 seen that the expected eight groups are produced and 

 that the two new colours, cream and pale tortoise, are 

 the sixth and eighth in the set, thus ; 



