EXPERIMENTAL STUDY OF HEREDITY 69 



of thrifty parents, talented offspring of mediocre parents, 

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



61. 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 elemen- 

 tary 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 selling occur. As a result the yield, in bushels per 

 acre, remains about stationary, or gradually becomes less 

 and the variety changes and deteriorates 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- 

 zygous for one or more characters, and the yield per acre 

 may thus become greatly reduced.^ If now, two of these 

 simplified strains, homozygous for many characters, and 



1 If both parents have blue eyes the children with rare exceptions have 

 blue 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 tends 

 to be 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 pig- 

 ment is entirely lacking. 



^If a high-yielding strain was separated out by selection, the yield, 

 would of course be increased above the average of the mixed field. 



