namely, of a chromo-gen or color base in the presence of an oxidizing enzyme or oxidase. Tyrosin, for instance, a colorless chemical compound 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 activity. 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 number, 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 tyrosinase—that is, its concentration—in connection with local conditions that might favor or inhibit the reaction in varying degrees, would determine 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 differentiated, 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 formation of the color may depend, should be present as such in the fertilized ovum. It is only necessary that the conditions for its ultimate production be present—relatively simple conditions, that bring about a series of reactions of the type known in physiological chemistry as autocatalyses, in which one phase in the reaction determines the succeeding phase. Not only is this sort of conception more simple than the determinant 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.
