262 GENETICS IN RELATION TO AGRICULTURE 



pairs as parents for each generation. While the selection of similarly 

 pigmented individuals would tend gradually toward a homozygous 

 condition,with respect to the specific factors conditioning pigmentation, 

 yet it is altogether likely that under the conditions of the experiment 

 a considerable degree of heterozygosity was maintained. In other words 

 the selection practised did not isolate pure lines, the plus and minus 

 strains did not become homozygous. Much of the work done in the 

 past in ameliorating animals and plants has been by this method of 

 selecting phenotypes but not genotypes, which accounts in part for the 

 frequent necessity of continuous selection in maintaining improved strains 

 or breeds. In reviewing the development of plant breeding we shall note 

 certain cases of early recognition of the effects of genotypic selection, 

 a principle which is now accepted as fundamental in all breeding 

 operations. 



