14(5 



THE AMERICAN NATURALIST 



[Vol. XLIX 



the first hypothesis would be expected to be heterozygous 

 both for albinism and the modifying factor, have been 

 crossed with albinos. None of them have given both 

 albinos and dilutes. It is of course expected under the 

 hypothesis of allelomorphism that no intense animal 

 should transmit both albinism and dilution. The view 

 that dilution is red-eye plus a modifying factor has also 

 been eliminated. Red-eye has never appeared in our 

 stock guinea-pigs, which must therefore be pure for any 

 modifying factor which might change a red-eye to a dilute. 

 But red-eyes of any generation crossed with stock albinos 

 have never given dilutes in 105 young. Therefore dilu- 

 tion can not be red-eye plus a modifying factor. The only 

 remaining hypothesis is that dilution is an allelomorph of 

 red-eye and albinism. It is dominant to both since dilute 

 by dilute has often given red-eyes and albinos, while the 

 latter varieties crossed inter se have never given dilutes. 



Thus we have four allelomorphs corresponding to four 

 grades of pigmentation. The existence of such series has 

 a bearing on the nature of unit factors. The results could 

 be explained by perfect coupling, but such an explanation 

 seems highly arbitrary where the characters fall into a 

 natural physiological series. The series seems to suggest 

 that we have four variations in some one entity. Fur- 

 thermore, while we have only four such variations at the 

 Bussey Institution, it may well be that others exist else- 

 where, forming perhaps a continuous series. Such varia- 

 tions in this factor probably do not occur frequently. 

 When they do occur they probably take place by distinct 

 steps. The rather frequent occurrence of albinos in wild 

 species, without intermediates, indicates that variation 

 from one extreme to the other in the condition of the 

 factor may take place. 



A point which has a bearing on the physiology of pig- 

 ment is the fact that members of the albino series of allelo- 

 morphs do not cause diminution in quantity of pigment, 

 merely as pigment, but affect yellow pigment differently 

 from black. This is seen most clearly in the red-eyes, in 



