58 University of California Publications in Zoology C V L - n 



localized secretion of chromogen." Gortner (1911d) after 

 further investigation of different melanins concludes that they 

 are formed by the interaction of an oxidase and an oxidizable 

 chromogen. He finds that there are at least two types of melanin 

 which may be differentiated by their appearance and by their 

 solubility in acids. He thinks it probable that the two types are 

 formed by the oxidation of different chromogens. 



Some of these later results, it seems to me, lead one to take a 

 little different view of the situation from the one taken by 

 Riddle. He (1909, p. 334) speaks of the different colors of the 

 tyrosin series as obtainable by different degrees of oxidation, for 

 instance he says: "At present the biological data are wanting to 

 quantitatively seriate all of the several colors ; but there is appar- 

 ently enough data to warrant the definite statement that yellow 

 mice are forms with the power to oxidize tyrosin compounds 

 to an intermediate point." If we adopt this view we are under 

 the necessity of postulating some chemical difference between 

 the oxidase in yellow mice and that in brown mice that causes 

 this difference in oxidation. 



The results of Bertrand (1908) and of Abderhalden and 

 Guggenheim (1907, 1908) show that the end result of the series 

 of colors shown by melanin pigment is different when the tyrosin 

 exists in different combinations, or when different chromogens are 

 used. Gortner showed that the spots on the elytra of the potato 

 beetle are due to a localized secretion of the chromogen rather 

 than to a difference in the oxidase which is apparently present 

 over the whole area. It would appear from these facts that 

 slight differences in color between two individuals are due to 

 slight differences in the combination in which the chromogen 

 exists (or existed when the pigment was produced) and that 

 variations in the marking of different individuals are due to 

 slight variations in the character or diffusion of the chromogen 

 in different localities. 



The difference in color in the case of the tadpoles here de- 

 scribed seems to be brought about by a reduction in the amount 

 of black pigment rather than by a change in the color of the 

 pigment. This reduction in the amount of black pigment allows 



