THE DETERMINATION OF DOMINANCE. 329 



tinuous in results, in the interaction between the genetic state 

 and the dynamic forces only continuity of process is probable 

 on the basis of present physical knowledge. 



This genetico-dynamic-hypothesis of germ-plasm constitution 

 and modifiability, forms a convenient basis of departure for a 

 host of evolution problems, and it is from such a theoretical 

 basis that I have sought to study certain phases of the Mendelian 

 phenomena. 



Dominance. 



The subject of dominance and recessiveness is at present 

 decidedly an open question. What do we mean by domi- 

 nance, and what is it that determines dominance? In the 

 simpler cases, as illustrated by Mendel's peas and many others, 

 there is no difficulty in stating positively which one of the two 

 characters entering into a cross is the dominant member. Such 

 examples are fairly abundant; they behave with considerable 

 uniformity and represent a type of behavior for which there is 

 some definite cause. What is it that makes one character re- 

 cessive to the other? The Neo-Mendelian explanation is that 

 one character is recessive and the other dominant because the 

 recessive character is the absence of that which goes to make 

 the dominant character. This explanation, however, while it 

 may fit some characters, melanin pigment, for example, could 

 not by any means fit all cases. 



Thus, for example, in the experiments given in this paper, in 

 crossing black stripes and no stripes, we may well conceive of 

 the black stripes as being due to the presence of the two factors 

 which produce black, and of no stripes as being due to the fact that 

 one or both of the producing factors is absent. As far as the 

 causation of color is concerned, this is actually the case, because 

 I showed in 1903 that the black and brown color in these beetles 

 is due to a substance in the cuticula which must be oxidized 

 by an enzyme to produce the dark pigments. This is a clear 

 example of the dominance of the presence over the absence, in 

 some of the experiments and not in others. But the case is differ- 

 ent in such crosses from that between tall and short stature in pea 

 plants, tailed and rumpless condition in poultry, etc. ; these must 

 in one way or another be due to some sort of activity within the 



