82 Plant Genetics 



on the basis of external appearance, and external appear- 

 ance is often a very poor index of hereditary capacity. 

 Intelligent selection is based on germinal constitu- 

 tion only, and therefore its results are quicker and 

 surer. 



Another phase of the subject should be considered. 

 When a plant breeder is trying to improve his crops by 

 selection for quantitative characters, although he uses the 

 old method of selection, he is likely to be making some 

 gain, as the experience of hundreds of years has shown. 

 The germinal constitution of his crop plants is masked 

 by fluctuations of course, but this mask is not complete. 

 Most of the plants he selects are bound to possess high 

 numbers of factors of the right kind, and he probably 

 rejects most of the plants with few factors. In any 

 event, he has generally succeeded in the long run in 

 getting a somewhat improved race. 



A summarized statement of this situation may be 

 helpful. Our recently developed knowledge of the in- 

 heritance of quantitative characters seems to justify 

 artificial selection, but it does not justify the old blind 

 method of selection. It emphasizes the need of intelli- 

 gent, trained selection and shows how such selection can 

 be made. In order to do this one must understand 

 the mechanism of the inheritance involved and must 

 understand the make-up (race or pedigree) of the plants 

 dealt with, being sure that they are of pure race or 

 strain, for selection from the ordinary mixed races of crop 

 plants is not only tedious at best but often leads to 

 chaotic results. 



The situations just considered enable one to under- 

 stand two phenomena which have been baffling scientific 



