562 STRUCTURE AND LIFE HISTORIES 



of thrifty parents, talented offspring of mediocre parents, 

 blue-eyed cliildren of brown-eyed parents,^ and so on. 



485. Increased Vigor from Crossing. — Experiments 

 with pedigreed cultures have disclosed a principle of the 

 utmost practical importance for the plant breeder. A 

 careful analysis of a field of Indian corn {Zea Mays) has 

 disclosed the fact that any given variety is very complex, 

 being heterozygous for many characters; in other words 

 any horticultural variety is a composite of numerous 

 elementary species, and is therefore heterozygous for 

 most of its characters. When pollination is allowed to 

 take place in the corn field without interference by man, 

 both crossing and selfing occur. As a result the yield, 

 in bushels per acre, remains about stationary, or gradu- 

 ally becomes less and the variety changes and deterior- 

 ates by the segregation and recombination of the numerous 

 elementary species that compose it. 



By artificial self-pollination for several generations {e.g., 

 five or more) less complex strains result, which are homo- 



^ If both parents have blue eyes the children can never have brown eyes; 

 if one parent has brown eyes and one blue, the children may be both blue- 

 and brown-eyed, or all brown-eyed, for brown eye-color in man is dominant 

 over blue color. "When both parents have brown eyes, part of the children 

 may have blue eyes and part of them brown, or they may all be brown- 

 eyed. As used here, the term "brown eyes" means all eyes having brown 

 pigment, whether in small spots (gray eyes), or traces (hazel eyes), or 

 generally distributed (brown, or sometimes black, eyes). The term "blue 

 eyes" designates only those cases in which brown pigment is entirely lacking. 



Fig. 409. — Zea Mays. In the experiment, the results of which are here 

 illustrated, nine strains of Indian corn were selected according to the 

 number of rows of kernels on the cob, varying from 8 to 24 rows. These 

 were pollinated by hand each year, with mixed pollen, in such manner that 

 self-pollination was entirely prevented. An average ear of each strain is 

 shown in the first row above. In the second row is shown an average 

 ear of each strain after self-fertilization for five generations. Note the 

 resulting decrease in the number of rows, lack of filling out of the ears, 

 and other marks of inferiority. The last row shows the remarkable and 

 immediate increase of vigor resulting in the Fi generation of hybrids be- 

 tween various pairs of the selfed strains. (Photo supplied by G. H. Shull.) 



