500 



THE AMERICAN NATURALIST [Vol. XLIII 



component of the barring factor is called the "marking factor." 

 If the marking factor is present and not the barring, the mouse 

 would be black. A black mouse therefore contains a yellow 

 band but this can not be seen because the black and chocolate 

 pigments obscure it. If this is correct one would expect to 

 extract from a black mouse the yellow pigment by a solvent for 

 yellow. I shall give below the evidence that negatives this 



Hagerdoorn also thinks that yellow may be due in some 

 cases to the absence of all pigments but yellow. In such a case 

 either the marking or the inhibiting factor may also be present 

 without its presence being noticeable. The following six classes 

 of yellow mice are recognized: 



(o) Present black, chocolate and yellow plus inhibiting factor. 



(6) Present chocolate and yellow plus inhibiting factor. 



(c) Present yellow plus inhibiting factor plus marking. 



(d) Present yellow plus inhibiting factor. 



(e) Present yellow plus marking factor. 

 (/) Present yellow. 



Hagerdoorn states that he has "proof" of the existence of 

 five of these six groups. Group (c) alone has not yet been 

 recognized. Moreover he obtained homozygous individuals of 

 each of these groups. Homozygous individuals of the same group 

 bred to each other produce pure yellow strains, that is, strains 

 that never produce any other color than yellow. The reason 

 that Cuenot and others never obtained pure yellow is due to the 

 fact that they crossed yellows of different strains and under 

 such circumstances mice of other colors will appear. In passing, 

 however, it should be noted that in Hagerdoorn 's classification 

 of yellows if individuals of strains (a), (b), (d) and (f) were 

 crossed only yellows should appear in the first and in all subse- 

 quent generations. Thus the chance of getting pure yellows 

 is as four to two, and it does not seem probable with so much 

 in favor of hitting upon these combinations that such strains 

 would not have been obtained by Cuenot or Durham, who have 

 studied the problem extensively. 



Hagerdoorn 's evidence in favor of his classification of yellow 

 is obtained by crossing yellows, not with each other, but with 

 mice of other colors or with albinos of known (?) composition. 

 The pairings are too complicated to discuss here in detail and in 

 the absence of numerical data, to show how often the results 



