Studies on variation and selection. 



149 



cases become genetic developmental factors, because they, by their 

 cooperation to the development, influence this in a marked way, 

 thus helping to determine an organisms qualities. We can study 

 these genetic factors in their influence on the qualities and we must 

 remember, that by limiting ourselves to a study of the inheritance 

 of "characters" for their own sake, we do not go down to the prin- 

 cipal things. In some special cases, where we are dealing with only 

 a few genetic factors, which all happen to be mutually independent 

 in transmission as well as in influence, the inheritance of the several 

 "dominant characters" is fairly parallel with that of the correspond- 

 ing genes. But as soon as we get complications, this simple relation 

 does not exist, and a study of the "partial dominance" or "blending 

 inheritance" of certain characters loses all fundamental interest. In 

 complicated cases, it does not really interest us how certain 

 "characters" behave in crosses, but only how many, and what 

 genetic factors produce the differences we observe, what influence 

 they have, each considered alone, and how they modify each others 

 influence on the development. In other words, what really interests 

 us, Geneticians, is the study of one definite cathegory of develop- 

 mental factors. Genetics has its exclusive scientific significance as 

 a well-defined subdivision of Bio-mechanics (Entwicklungsmechanik) 

 and anybody, who cares for the scientific side of Genetics, will do 

 well to restrict, as far as possible, the insignificant morphological 

 side of it. Genetics must not be the study of the behaviour of 

 certain qualities of organisms in crosses, but only the study of the 

 nature of those developmental factors which are transmitted through 

 the germ, leaving experimental Botany and Zoology the study of the 

 developmental factors not so transmitted. And we are only in so far 

 interested in crosses, and in the behaviour of the qualities of the 

 organisms in crosses, as they help us in our study of everything 

 concerning the genes, the genetic factors, and not in the least for 

 the sake of these qualities themselves, or the relations of these 

 qualities. 



The tendency to classify the results of crosses with an eye to 

 the relation in which certain qualities envisaged ultimately find them- 

 selves is hopelessly turning our attention away from the fundamental 

 problems with which Genetics as a science is concerned. 



We hear, that if we cross a certain kind of white fowl to a certain 

 sort of black one, the black colour of this last one is completely 

 dominant over the white colour of the first parent. In an other case 



