Riddle, Experiments on Melanin Color Formation etc. ^ t =; 



rests not on rigorous chemical test, but upon breeding behavior, 

 and like many other interesting conclusions of her valuable paper, 

 it must be regarded as an Interpretation, an expression of opinion, 

 and not as something proved. I think it can be confidently stated 

 that Wheldale's work leaves us quite without any valid 

 chemical evidence whatever, that specific chromogens form 

 the basis of the different color varieties of organisms. 



We see then that some definitely directed search has been 

 made, both for specific color-producing enzymes, and for specific 

 chromogens. As a result it can be said that the search for such 

 specific chromogens has not been successful; and that, as 

 regards the crucial chemical evidence for the existence of specific 

 color-forming enzymes, Durham's work Stands as the 

 definite substantiation of C u e n o t 's theory. 



I have now repeated and extended that work, using diffe- 

 rently colored varieties of fowls, and of pigeons, and have obtained 

 different and contrary results. In a word, in my experiments 

 the extracts of black skins, added to tyrosin and ferrous sulphate, 

 have yielded black color; those of yellow or red skins have 

 yielded black color; those of white skins have yielded black 

 color. This however only in cases where much bacterial de- 

 composition occurred in the Juices. In perfectly fresh and germ- 

 free material n o color was formed, and no evidence of any 

 tyrosinase activity was obtained. In marked contrast 

 to D u r h a m 's results, and making all the difference to Cue- 

 n o t ' s hypothesis — the skins do not show a specific 

 enzymecapableofoxidizing tyrosin to that color. 

 Nevertheless I have repeatedly followed the methods described 

 by D u r h a m without getting equivalent or like results. Her 

 methods have, I think, also been improved at one or two points, 

 but by no means have we been able to get any indication what- 

 ever that we were at all within ränge of anything resembling 

 tyrosinase specificity. 



These results harmonize well wkh the account of color forma- 

 tion as I (1909) have previously described it in its relation to 

 inheritance. And, in working with these extracts one will find 

 it specially profitable to remember a point the theoretical impor- 

 tance of which was there insisted upon, namely that a long series 

 of oxidations occur in the resolution of tyrosin — or other chro- 

 mogens — to black pigment under the influence of a true tyrosinase. 

 In such cases several distinct colors appear merely as s t e p s or 

 s t a g e s in this single oxidation process. The growing Mendelian 

 conception that e a c h variety of color is represented in the 

 germ by a g r o u p of separate, wholly segregable representative 

 particles is there fore an impossible picture of the facts. Several 

 very distinct colors appear as but so many steps in the oxidation 

 of a single chromogen acted upon by a single enzyme; and these, 



