148 THE AMERICAN NATURALIST [Vol. XLIX 



pigs, and not at all the slate blue of dilute rabbits, mice 

 and cats. Thus, while I have not been able to find any 

 critical evidence, the suggestion seems worth making that 

 a series of allelomorphs of albinism may be in part re- 

 sponsible for differences in intensity of human pigmen- 

 tation. 



Summing up : the results in guinea-pigs and rabbits sug- 

 gest that there is a hereditary factor in mammals, which 

 may exist with stability at different stages of divergence 

 from the normal ; that divergence from the normal in the 

 factor tends to produce in the animal a corresponding re- 

 duction in the quantity of melanin pigment throughout 

 the body, conspicuously in fur, skin and eyes, of which re- 

 duction the limit is complete albinism ; that in this reduc- 

 tion the qualitative differentiation of the pigment is a 

 factor, in that yellow pigment is affected more strongly 

 than black and its threshold of complete inhibition is 

 reached with less divergence of the factor ; that in the re- 

 duction the location of the pigment is a factor, in that there 

 is less tendency toward reduction at the extremities— feet, 

 ears and nose— than elsewhere in skin and fur; that, 

 finally, any stage of divergence is dominant to any stage 

 more remote from the normal. 



In conclusion, I wish to thank Professor Castle for the 

 opportunity to carry on this work and for numerous 

 suggestions during its progress. 



December 24, 1914 



LITERATURE CITED. 

 Castle, W. E. 1905. Heredity of Coat Characters in Guinea-Pigs and 

 Rabbits. Carnegie Institution of Washington. Publication 

 No. 23. 



1914. Some New Varieties of Rats and Guinea-Pigs and their Rela- 

 tion to Problems of Color Inheritance. Amer. Nat., Vol. 48, 

 p. 65. 



Castle, W. E. and G. M. Allen. 1903. The Heredity of Albinism. Proc. 



Pearson, K., E. Nettleship, and C. H. Usher. 1913. A Monograph on 

 Albinism in Man. Parts I and IV. Dulau & Co., London. 



Dwarf Form. Reports to the E vol. Com. of the Roy. Soc, 



