No. 518] SHORTER ARTICLES AND DISCUSSION 117 



Mr. Riddle must not forget that we have a long list of incon- 

 trovertible facts that are not in disagreement with the facts of 

 melanin production cited by him, and which are not dependent 

 on any theory involving discontinuous variation, hut which may 

 be interpreted in such a way as to take cognizance of the ascer- 

 tained facts of physiological chemistry. Riddle throws all 

 ' 'factor" hypotheses overboard, apparently under the impres- 

 sion that they all depend on the Weismannian doctrine of deter- 

 minants. Fortunately, such is not the case. These hereditary 

 factors are established facts, while the Weismannian conception 

 of them is, in the writer's view, probably wholly wrong. 



Riddle points out that the chromogens in animals are tyrosin 

 and related aromatic compounds. Tyrosinase, an oxidizing 

 enzyme, converts tyrosin into melanins. Fuerth and Schneider 

 concluded that "tyrosinase-like ferments are widely distributed 

 in the animal organism and probably always appear wherever 

 and whenever a physiological or pathological formation of 

 melanin occurs." Gessard shown! that the presence of acids, 

 alkalis and salts has a marked effect on the colors produced 

 by the action of tyrosinase on tyrosin. Bertrand determined 

 the type of substance, of which there are many representatives, 

 which tyrosinase can oxidize to melanins. In the oxidizing 

 process each of these substances are converted step by step 

 through a series of colors before reaching the final stage. They 

 vary somewhat as to initial and final colors, the early stages 

 being lighter than the later. The series of colors usually runs 

 from yellow to orange, proceeding onward to brown or black. 

 Any benzine nucleus with a hydroxyl attached can be converted 

 to a melanin by tyrosinase. Some substances have red or 

 mahogany as the final stage reached in the oxidation process. 



Riddle further points out that the facts of the pathological 

 development of melanins show the dependence of tyrosin oxida- 

 tion upon somatic conditions which may be of temporary, in- 

 termittent, or reversible character, but he assumes, without 

 sufficient basis, that these facts preclude the possibility of ac- 

 counting for observed phenomena of color inheritance on a basis 

 of specific transmission and segregation in the germ cells. He 

 seems to think that any kind of segregation in the germ cells 

 necessarily implies pangenes, such as those proposed by de Vries, 

 or determinants, such as those proposed by Weismann. It is 

 easily shown that this is not the case. The facts of segregation 



