380 HEREDITY AND ENVIRONMENT 



races from poor ones, and this is the whole 

 rationale of the artificial selection practiced by 

 breeders. 



The elimination of certain races by natural 

 selection may be an important factor in evo- 

 lution though it has nothing to do with the 

 formation of new characters or new races but 

 serves merely as a sieve, as deVries has ex- 

 pressed it, to sort the individuals which are 

 supplied to it. Selection has no power to 

 make or change characters, but it may pre- 

 serve certain lines and eliminate others and 

 thus fix the type of a species. Finally the 

 elimination of the unfit by natural selection is 

 still the only natural explanation of fitness, or 

 adaptation, in organisms. 



III. METHODS or MODERN GENETICS 



1. MendeUan Association and Dissociation 

 of Characters. Breeders have long known 

 that it is possible to get certain desirable char- 

 acters of an organism from one race and other 

 desirable characters from another race. But 

 since the discovery of the Mendelian principles 

 of heredity such new combinations of old char- 



