No. 520] MEN DELI AN PHENOMENA 



call the germ plasm, and I am inclined to believe that this 

 is by far the most important typo of evolutionary change. 

 Whether changes of this character are continuous or dis- 

 continuous is not a very important question. It is more 

 important to ascertain their cause and nature. I imagine 

 they are largely chemical changes in the composition of, 

 or changes in the relative amounts of sul (stances present 

 in, the germ plasm. If these changes are continuous the 

 work of Johnnnson, Xilsscn, Jennings ami East, above 

 referred to, indicates that they are very slow. Such 

 changes are certainly hereditary, and are thus a factor, 

 and I believe the most important factor, in evolution. 



One point regarding fundamental changes in the germ 

 plasm it is important to remember: a gradual change 

 might go on for a long time in the chemical activity of the 

 cell before reaching a point where any outward manifesta- 

 tion of this change would appear. For instance; in some 

 purple flowers there is reason to believe that the color is 

 due to some such action as that of acids and alkalies upon 

 the color of litmus, the flower being red or purple accord- 

 ing to the amount of some substance present. Xow a 

 gradual change might go on in the relations between the 

 substances concerned without producing any visible effect 

 until a certain point is reached, when a marked effect is 

 produced, giving what appears to be a discontinuous 

 change, but what may be in reality a critical point in a 

 slow and gradual change. 



Eiddle, in his recent very interesting paper on melanin 

 color formation 8 has shown that in all probability melanin 

 colors develop as the result of the action of an enzyme 

 upon a chromogen, and that this action is a complex one. 

 The chromogen is first converted into a new substance, 

 the same enzyme then converts this substance into a third, 

 and so on, producing a long series of substances. The 

 earlv. numbers in this series give no color, but finally a 

 stage is reached where a number of steps in the series give 

 color. Riddle seems to think that all the melanin colors 



