MODERN SCIENTIFIC THOUGHT 95 



namely, of a chromo-gen or color base in the presence of an oxidizing 

 enzyme or oxidase. Tyrosin, for instance, a colorless chemical com- 

 pound and a product of the decomposition of tissue proteids, can be 

 oxidized, in the presence of the enzyme tyrosinase, through a series of 

 colors : pink, red, deep brown to black, the color depending, other things 

 equal, on the concentration of the enzyme and the duration of its activ- 

 ity. Tyrosinase has been isolated from many organisms, and has been 

 definitely connected with pigment formation in many cases. We are 

 dealing here with known substances, not hypothetical vital units; with 

 chemical processes that can be followed in the laboratory test tube. 

 That an organism may develop a color characteristic of its parents, in 

 the light of these facts which are representative of a considerable num- 

 ber, it is only necessary that in the course of its development tyrosinase 

 be formed under conditions that make a reaction with the tyrosin in 

 the tissues possible. Local production of tyrosinase would lead to local 

 coloration, to spotting or characteristic marking. The amount of tyro- 

 sinase — that is, its concentration — in connection with local conditions 

 that might favor or inhibit the reaction in varying degrees, would deter- 

 mine the characteristic shade of color. 



It is impossible in the brief time at my disposal to consider the 

 various complications of this type of problem. The difficulties are very 

 great in the way of investigations which as yet have hardly begun. 

 Enough may have been said, however, to indicate the direction of some 

 of the most recent and most promising work. If color characters are 

 dependent upon chemical reactions, other characters probably are also. 

 In fact, recent work upon the old problem of the heritability of acquired 

 characters has brought to light interesting chemical possibilities in 

 inheritance, and lifted the incubus of presumption laid by Weismann 

 upon the whole subject in the shape of the determinant hypothesis 

 almost twenty years ago. 



Modern epigenesis recognizes an organized germ, more or less dif- 

 ferentiated, but vastly simple in comparison with the preformed germ. 

 That color may be produced at a given stage in the development of an 

 organism, it is not necessary that the tyrosinase, upon which the forma- 

 tion of the color may depend, should be present as such in the fertilized 

 ovum. It is only necessary that the conditions for its ultimate produc- 

 tion be present — relatively simple conditions, that bring about a series 

 of reactions of the type known in physiological chemistry as autocatal- 

 yses, in which one phase in the reaction determines the succeeding 

 phase. Not only is this sort of conception more simple than the deter- 

 minant hypothesis, but it is stimulating. It is workable. It leads to 

 results that are sympathetic with the most advanced scientific work of 

 the day. It is not a final explanation. It is an implement of research. 



