Constancy of Strains 65 



individuals were not inherited, and long continued selection 

 was without effect. This sort of study has come to be 

 known as the "pure line'* work, and the general result of 

 it all has been that "selection has no effect within a pure 

 line." 



Such results have profoundly modified the theories of 

 heredity and of evolution. The Danish investigator Jo- 

 hannsen (1913) has based on them and similar results a 

 general system or doctrine of heredity. According to Jo- 

 hannsen's views, the hereditary constitution of a given organ- 

 ism is a perfectly definite thing, not subject to gradual or 

 indefinite fluctuation. This constant hereditary constitution 

 he calls the genotype. The genotype is comparable to a 

 definite chemical compound. It may become altered some- 

 times, as a definite chemical compound may by certain reac- 

 tions be transformed into another and diverse compound; 

 such complete transformations are called mutations; they 

 are extremely rare. The genotype is not subject to slight 

 and gradual alterations, any more than is the nature of the 

 chemical compound NaCl. When diverse genotypes are 

 mingled, as in reproduction from two parents, the results 

 give us what is called Mendelian inheritance. We cannot 

 take up the details of this here, but the same principles hold. 

 Selection in such cases, according to this view, merely brings 

 about varied combinations of things already existing; the 

 nature of its effects is therefore the same as when it is ap- 

 plied to organisms descending from but a single parent. 

 This view of the matter may perhaps be said to have dom- 

 inated recent work in heredity. 



This constancy of races in organisms descending from a 

 single parent, and the application, which appears unavoid- 

 able, of the same point of view to the cases where there are 

 two parents, presents very great difficulties for the theory 



