OLD TYPES AND NEW 155 



which is largely, if not entirely, independent of environ- 

 mental factors. 



Accordingly, the work of the successful })reeder 

 consists in simply taking what nature spontaneously 

 furnishes to him rather than in attempting to force 

 nature into producing something new. These muta- 

 tions, when isolated, may become the progenitors of 

 desirable new lines. 



d. The Method of Vilmorin 



This is an isolation method which has been success- 

 fully applied to the sugar-beet industry. The seeds 

 from each plant to be tested are sown in separate beds 

 from which upon maturity samples are taken and 

 tested for sugar content. The plants from the bed 

 furnishing the sample which contains the highest per- 

 centage of sugar are then used as the seed producers 

 for the next generation. In this way by continual 

 selection an improved strain may be maintained. 



e. The Method of Johanssen 



The method of isolating pure lines or homozygotes 

 out of a mixed population has been considered in 

 Chapter VT. As in the method of de Vries of isolating 

 mutations so, too, in the pure line method it is recog- 

 nized that the germplasm is the source of initiatory 

 changes and that the technique of establishing new 

 types consists in sorting out homozygous strains of 

 this germplasm. 



The method of Johanssen is quite different from 

 those of Hallet and Rimpau in that the ideal organi- 



