134 Heredity and Eugenics 



were simply high extremes belonging to "blood lines" that 

 were low in their average sugar content. These individuals 

 crossed with those from better "blood lines" and progress 

 was made very slow indeed. 



In this short discussion on selection the writer has 

 endeavored to make clear two points that may be sum- 

 marized as follows. Plants are exceedingly variable but 

 the majority of these variations are simply accelerations 

 or retardations of the development of the whole or of cer- 

 tain parts of the plant due to good or bad environment at 

 critical stages of the plant's growth. These variations are 

 not inherited because the reproductive or germ cells are not 

 affected. Other variations, however, are being constantly 

 produced by nature — though much more rarely — which 

 do effect the reproductive cells and are transmitted to the 

 plant's progeny. These variations are the basis of selec- 

 tion. They are constant from the beginning — although 

 their possible presence in a heterozygous condition may 

 make it seem otherwise — and remain so unless changed by 

 a second variation affecting the same constituent in the 

 reproductive cells that is due to develop the character in 

 question. The second point to be emphasized is that the 

 whole aim and action of selection is to detect the desired 

 heritable variants among the useful commercial plants 

 and through them to isolate a race with the desired char- 

 acters. When such a homozygous race is produced, selec- 

 tion can then do nothing until nature steps in and produces 

 another desirable variation. The progeny test is the way 

 to accomplish this end. It does this by showing us to which 

 strains each mother plant belongs. It is a sure test whether 

 the heterozygous condition is simple or complex. If it is a 

 question of which seeds of a maize ear are homozygous and 



