WRIGHT 



in the absence of all pigment. There 

 are two distinct series by which black 

 may be reduced toward white. There 

 is first the type of dilution found in 

 brown and tow-colored human hair, 

 or in dilute black guinea-pigs, which 

 reveals the sepia color of the pigment. 

 A very different kind of dilution of 

 black is to be seen in the so-called 

 blue or maltese mammals— blue mice 

 and rabbits, maltese cats, etc. The 

 effect is somewhat similar to that in 

 blue roans among horses and cattle 

 and seems to be due to a similar cause 

 on a finer scale. Blue roans have an 

 intermingling of jet black hairs and 

 white hairs while the maltese mam- 

 mals have dense black pigment masses 

 alternating with colorless spaces 

 within the hair. 



The colors which do not enter into 

 either of these dilution series or their 

 combinations are those which have a 

 distinct orange-yellow tinge such as is 

 seen conspicuously in red human hair, 

 red and yellow cattle, bay, chestnut 

 and dun horses, tan dogs, etc. The 

 most highly pigmented colors of this 

 kind are the so-called reds. The pig- 

 ment granules appear orange-yellow 

 in such hair, but it does not seem to be 

 settled whether there is an essential 

 chemical difference from sepia-brown 

 granules or merely some structural 

 difference. The appearance of the in- 

 tense reds varies somewhat in differ- 

 ent mammals but there seems little 

 reason for doubting their essential 

 similarity. Red undergoes different 

 modes of dilution comparable to those 

 described for black. Reduction to yel- 

 low or cream is comparable to the 

 sepia type of dilution of black, while 

 a more coarsely granular type of dilu- 

 tion comparable to maltese is found in 

 light reds. These light reds are slightly 

 redder in hue than the yellows of sim- 

 ilar intensity. The two series may, of 

 course, be combined. 



81 



Finally there are intergrades of vari- 

 ous sorts between the different sepia 

 and yellow series. A coarse-grained 

 mixture gives the effect of bay, dun, 

 or sooty yellow depending on the in- 

 tensity^ of the colors. A finer t\'pe of 

 intergrade seems to be present in the 

 chocolate color of brown mice, rab- 

 bits, guinea-pigs and liver-colored 

 dogs. These browns, however, are 

 much closer to the sepias than to the 

 reds and yellows and are not always 

 distinguishable. Genetic evidence 

 sharply distinguishes browns which 

 are due to reduction of black toward 

 white, and browns which are reduc- 

 tions toward yellow. 



By combining the different kinds of 

 dilution with the different kinds of 

 intergrades between sepia and yellow 

 an almost infinite variety of colors is 

 produced, while the complex patterns 

 in which these colors may be dis- 

 tributed make possible still further 

 diversity in color effect. 



Skin color in general corresponds 

 roughly with hair color. The pig- 

 ments in the eyes are like those in the 

 skin and fur, but the appearance is 

 generally much modified by struc- 

 tural effects. In dilute human eyes, for 

 example, the appearance is blue, al- 

 though the pigment is sepia brown. 

 In the rodents the reflection from the 

 back of the retina gives a red color 

 when the pigment is reduced, which 

 in the complete absence of pigment 

 becomes pink as in albinos. 



CHEMISTRY OF MELANIN 



A large amount of work in the last 

 twenty years has firmly established 

 the hypothesis that melanin is an 

 oxidation product of tyrosin or re- 

 lated products of protein metabolism. 

 Enzymes have been extracted in a 

 great many animals and plants, which 

 have the power of oxidizing tyrosin 

 and related substances to dark brown 



