214 THE LAW OF JOHANNSEN. 



netically pure groups, we think of as caused by the fact that 

 absent genes are not spontaneously acquired, and that reversely, 

 the loss of a gene, for which an organism is pure, is a 

 rather uncommon process. But apart from such permanent 

 differences as due to presence and absence of genes, can we not 

 also conceive of more transient differences due to quantitative 

 inequalities between the amount of a gene present in two organ- 

 isms which both have? At the outset, we must try to make 

 it understood, that quantitative differences in the amount of a 

 gene cooperating to the development are fundamentally dis- 

 tinct from qualitative changes in the genes themselves. 



Two bronzes, both composed of copper, zinc and nickel may 

 be very different in very many qualities according to the rel- 

 ative quantity of each of the composing metals, but there is a 

 fundamental difference between this relative proportion as a 

 cause for the diverse quality of the bronzes and a hypothetical 

 change in the nature of the copper or of the zinc. In fact, such 

 a qualitative change of the copper is not warranted by the 

 facts. 



The view, that there is a possibility of differences between 

 cells caused by a more or less of certain genes, is bound up with 

 the conception of genes as essentially chemical rather than 

 morphological. There is no other logical way to reconcile the 

 patent difference between cells of one individual with the geno- 

 typic sameness inferred from diverse observations. The only 

 alternative is Weismann's idea of a gradual simplification of 

 the genotype in generations of cells throughout development. 

 The facts of regeneration, of vegetative reproduction from one 

 epidermis-cell of certain plants, facts which have come to light 

 in a study of graft-hybrids, all contradict such a gradual drop- 

 ping out of "determinants". 



If we conceive of protoplasm as of an emulsion in which 

 genes play an integral part, it becomes clear that, without a 

 change in the personnel of genes present, great differences be- 

 tween cells and tissues made up of cells can be due to relative 

 preponderance of one or several genes, just as great differences 



