80 



deduced from a study of variation in 

 characters at the other end of the de- 

 velopmental history. The wonder is 

 that with such a method it has been 

 possible at all to demonstrate unit 

 variations in the germ cell. One of the 

 most remarkable results of recent 

 genetic work has been the detailed 

 correlation of these hypothetical char- 

 acters of the germ cell with structures 

 actually observed there. 



It remains for genetics to assist em- 

 bryology and biochemistry in filling 

 in the links in the chain between germ 

 cell and adult in specific cases. Varia- 

 tions of adult characters must be 

 traced back through the contributing 

 causes at each stage of development 

 until, if possible, something is learned 

 of the nature of the ultimate germinal 

 factors involved and, on the other 

 hand, the ramifying influences of unit 

 variations in the germ cell must be 

 traced forward through development. 

 Probably the most favorable point of 

 attack for such work is in color in- 

 heritance in animals and plants. A 

 larger number of unit factors have 

 been isolated than in other kinds of 

 characters and are available for com- 

 parative study. Much progress has 

 been made in the chemistry and mode 

 of formation of many of the pigments, 

 notably the melanins, with which we 

 are chiefly concerned in the higher 

 animals. The very fact that it has been 

 relatively easy to isolate unit factors 

 in work on color inheritance suggests 

 that in this case the chain of processes 

 between germ cell and adult may be 

 relatively simple. Observations which 

 indicate that melanin pigment is 

 formed in the cytoplasm of cells by 

 the secretion of oxidizing enzymes 

 from the nucleus suggest that the 

 \ chain may be very short indeed when 

 it is remembered that genetic factors 

 are probably characters of the chro- 

 mosomes and that these seem to be 



WRIGHT 



distributed unchanged from the germ 

 cell to all other cells. 



The present paper is an attempt to 

 make as simple a classification of color 

 factors in mammals, based on their 

 efi^ects, as possible, and to suggest a 

 working hypothesis which will relate 

 the biochemical knowledge concern- 

 ing melanism with this classification 

 and with certain peculiar relations be- 

 tween the colors. The writer wishes 

 to emphasize, however, that for the 

 present the chemical terms are used 

 rather for the sake of giving a definite 

 scheme to which genetic facts may be 

 referred than for their own sake. Since 

 the earliest work on color inheritance, 

 many geneticists, notably Cuenot,^ 

 Castle,- and Little,^ have tried to give 

 a physiological interpretation to their 

 results. The hypothesis advanced here 

 is based to some extent on their con- 

 clusions with modifications intended 

 to bring under one point of view cer- 

 tain curious new facts. 



COLOR IN MAMMALS 



Melanin pigment is found in the 

 skin, fur and eyes of mammals. The 

 present paper will deal largely with 

 the gross effects as our knowledge of 

 the ultimate differences of the colors 

 is still very unsatisfactory. Only mam- 

 mals are dealt with, as in other classes 

 on which genetic work has been done 

 the pigment colors are largely masked 

 by structural effects. 



The most highly pigmented condi- 

 tion is found in the color black. The 

 pigment granules in this case are not 

 really black but a very dark sepia 

 brown. White in mammals seems al- 

 ways to be a structural color found 



1 Cuenot, L., 1903. Arch. Zoo/. Exp. et 

 Gen. (4), vol. i. Notes et Revue, p. 33. 



2 Castle, W. E., H. E. Walter, R. C. Mul- 

 lenix and S. Cobb, 1909. Cam. Inst. Wash. 

 Pub!., 114. 



3 Little, C. C, 1913. Cam. Inst. Wash. 

 Piibl, 179. 



