THE LAW OF JOHANNSEN. 



The way in which an organism develops, determines its 

 qualities, and in the end the behaviour of an organism under 

 the opportunities given by its environment depends upon the 

 reaction of its cells upon their immediate environment. This 

 reaction of cells, in so far that it is different from that of cells of 

 other organisms, is dependent upon their constitution, chemi- 

 cal and physical. And ultimately, we think of this constitution 

 of the cells, as given in the set of genes inherited in the original 

 cell or cell-complex from which the organism grew up. 



It is evident, that the difference in reaction, which we ob- 

 serve between groups of cells in one and the same individual, 

 cannot be expressed in terms of presence and absence of genes, 

 for no such fundamental difference between the cells of one 

 organism is observable in those cases in which we can make a 

 somatic cell reproduce a whole individual. 



Nevertheless the difference in reaction upon the immediate 

 environment, which we observe between cells and cell-complexes 

 within one organism must be due to a difference in the con- 

 stitution of these cells. 



The constitution of a cell cannot be determined by the mere 

 presence of a definite set of genes, heritable substances, for if 

 it were, the difference in quality between cells of one individual, 

 or of one pure clone of uni-cellular creatures would be ac- 

 companied either by a change in the set of genes, or by a change 

 in the quality of the genes themselves. 



The conception of genes as direct determinants for qualities 

 necessitates the assumption of a qualitative instability of the 

 genes themselves. And the fact that we have been trying to 

 account for the facts of variability without assuming a quali- 



