328 OSCAR RIDDLE. 



is quite out of keeping with any " once for all determination " by 

 the shuffling of color "factors" through the germs. 



In this connection attention should be called to the fact that 

 Spiegler ('03) has reported the finding of a white melanin in the 

 white hair of horses, and in sheep's wool. It would seem that 

 the melanin isolated by him represents a more advanced stage 

 of oxidation than does black melanin. If this is true it is obvi- 

 ously an important fact. The isolation of a similar pigment 

 from the white hair of albinos seems, however, neither to have been 

 sought for, nor found ; but it is quite possible that such a pig- 

 ment also exists in mammalian albinos. There are nevertheless 

 some reasons --chiefly biological rather than chemical for be- 

 lieving that among birds, at least, a white color exists which rep- 

 resents a less advanced stage of oxidation than that in any other 

 of the melanin color series; in fact, here the "white" seems to 

 be a purely "physical " color. The chromatophores do not de- 

 velop, and no white pigment is histologically discoverable. Ap- 

 parently, therefore, the oxidation of tyrosin, etc., is here not 

 carried far enough to produce any color whatever. 



The actual facts regarding the "white" pigment or color of 

 birds and mammals are not yet clear. " White " forms at present 

 a most awkward, and at the same time a most interesting gap in 

 our knowledge of the melanin series. The elementary formulas 

 for the black and white pigments given in the table are Spiegler's 

 findings for the white and black hair of the horse. Certain 

 phases of Spiegler's research have been confirmed by Wolff ('04), 

 but the particular facts which we have referred to above have not 

 been reviewed or confirmed. 



We are fortunate in having, from physiological experiments 

 with melanin pigments in living animals, some facts which con- 

 firm the data from chemistry and pathology regarding the 

 mechanism of melanin production. On this point special attention 

 may be called to the recent work of Gustav Tornier. His work 

 furnishes a splendid view of the control of the color of the in- 

 tegument. The colors of Amphibia, from larval stages to old 

 age, were determined at will by controlling the physiological 

 state, particularly the nutrition, of the animals. When the color 



