THE LAW OF JOHANNSEN. 209 



pure in respect to its set of genes, did not produce any herit- 

 able change in this material. 



From these and later observations, we formulated what we 

 called the law of Johannsen : "The nature of the genes does not 

 admit of qualitative variation" (1915). 



We will have to examine this law in the light of some recent 

 papers which show the effect of selection in pure clones of uni- 

 cellular organisms. 



Johannsen's law of the qualitative stability of the genes is 

 derived from repeated observations as to the purity and sta- 

 bility of all those groups of organisms, which we have good 

 reason to believe to be identical in respect to the set of genes 

 carried, a purity and a stability which is not permanently af- 

 fected by a change in the environment or by selection. 



Whenever we want to discuss this law and its universal va- 

 lidity, we must remember that in its simplest form, as we have 

 formulated it, it only states that the genes themselves are 

 qualitatively stable. The usual ineffectiveness of selection in 

 pure lines and pure clones for instance, we explain by assuming 

 that in such material the individuals really have all the same 

 set of genes. But at the same time we know, that the ineffect- 

 iveness of selection and of change of environment to change 

 such material, is only the result of this nature of the genes, and 

 will hold good only so long as within this material there do not 

 occur processes, which change the set of genes carried. And we 

 must not confound a temporary change of the quantitative 

 increase or decrease of genes in the cytoplasing a number of 

 cell-generations with the qualitative change in the genes them- 

 selves. 



We may certainly not turn round the statement that genes 

 are qualitatively stable, and that therefore, the germ-plasm of 

 material pure for its set of genes is not amenable to change by 

 selection, and make it read so, that the test of the effectiveness 

 of selection in a pure line or a pure clone is a test of the validity 

 or otherwise, of Johannsen's law. 



A pure line is a group of individuals which have arisen by 



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