324 WILLIAM LAWRENCE TOWER. 



ters are in the germ-plasm is something we do not know, and to 

 say that together they direct the development of one part after 

 another in orderly succession puts upon these determiners a 

 burden of great responsib'lity, almost of intelligence, and makes 

 necessary some coordinating mechanism behind it all. 



What we do know is that in some way, as yet unknown, in the 

 germ-plasm there is conditioned the basis upon which the attri- 

 butes of the future organism are to be built. It is a fact of 

 experience that the germ cells give, on being combined at fertili- 

 zation, results which suggest that the germ cells are of unlike 

 potentiality or constitution with respect to a given character, 

 and this is further strengthened by experiments wherein the 

 results are exactly predicable when the germinal constitution is 

 known. What this difference in the gametes is, we do not know, 

 but observed behaviors are interpreted as being, most probably, 

 due to the mechanical separation into different germ cells of 

 whatever is it that produces the contrasting attributes — segre- 

 gation during gametogenesis. 



There is, furthermore, a very considerable amount of evidence, 

 aside from that obtained by Neo-Mendelians, to substantiate 

 this factorial point of view. The Mendelian behavior is most 

 commonly found in color and color characters, and in color 

 characters two definite general processes are involved; first, the 

 production of the pigment to produce color; and second, the 

 localization of this pigment in definite positions in the organism. 

 The color itself is known very definitely to be, in a large number 

 of cases, if not generally, the result of the interaction of two 

 chemical substances. Riddle (1909) brings together from di- 

 verse sources the knowledge available concerning the formation 

 of the melanin pigments, and, as far as known, the production 

 of these pigments is uniformly due to the existence of an oxi- 

 dizing agent — ^tyrosinase — and a substance which is oxidizable 

 — tyrosin or allied compounds. In the test-tube of the physio- 

 logical chemist, the tyrosin will remain tyrosin indefinitely unless 

 it be oxidized by the tyrosinase into some simpler substances, 

 when the color-forming compound appears, and exactly the same 

 is true in the living organism. Also the tyrosinase will indefi- 

 nitely remain tyrosinase, without the formation of color, unless 



