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. 
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