CAUSE OF HYBRID VIGOR 167 



the atom, the principle of the pure line has been firmly 

 established by an ever-increasing body of evidence, and 

 is of the utmost importance in a proper understanding of 

 the facts involved in inbreeding and outbreeding. 



The first application of these principles to the problem 

 of inbreeding was made with the results from maize al- 

 ready described. It was shown that self-fertilization 

 automatically brings about homozygosity, and with it a re- 

 duction of a great deal of the variability commonly shown. 

 Along with this reduction in variability, certain charac- 

 ters manifest themselves which are more or less unfavor- 

 able to the plants' best development. Plants with sterile 

 tassels and sterile ears and plants which lack chorophyll 

 appear, cannot reproduce themselves, and are eliminated. 

 Other characters come to light which do not cause the 

 extinction of the plants directly, but which more or less 

 handicap them in their development ; for example, partial 

 chlorophyll deficiency, dwarfness, bifurcated organs, con- 

 torted stems and deficient root systems. 



All of these characters are shown in small numbers in 

 a cross-pollinated field of maize, but not in sufficient fre- 

 quency to reduce productiveness seriously. As the result 

 of self-fertilization some of the strains obtained possess 

 certain of these unfavorable characters as regular fea- 

 tures. Here, then, is an explanation of part of the in- 

 jurious effects of inbreeding. Unfavorable characters 

 are segregated out, which reduce the developmental effi- 

 ciency of the plants which possess them. But if unfavor- 

 able characters are concentrated in some lines, favorable 

 characters are concentrated in others. Some have more 

 of the favorable than of the unfavorable, hence some 

 strains resulting from inbreeding are better than others. 



