344 



EVOLUTION AND GENETICS 



Yields of Maize Crosses Compared with Parental Yields 

 (Modified from Babcock and Clausen, after Collins) 



Name of Hybrid 



Maryland dent by Hopi 

 Tuscarora by Cinquantino 

 Kansas dent by Chinese 

 Chinese by Chihuahua 

 Hopi by Chinese 

 Chinese by Xupha 

 Brownsville by Chinese 

 Brownsville by Guatemala Red 

 Huamamantla by Hairy Mexican 

 Hairy Mexican by Chinese 



fc, « f-' 



O El. « ■< Z 



w " > O 



< S o « 



t; -< Q ., w 



o o w ^^ - 



K Z X « £ 



Ph ^ m 



29 

 95 

 58 

 76 



126 

 6 



100 



-9 

 7 



114 



Many other plants have been improved in the same way. 

 Tomatoes, cucumbers and strawberries not only give vigorous 

 plants in the Fi generation Ixit produce heavier yields than the 

 most prolific parents. Hybrid varieties of some of these plants 

 are commonly used, but difficulties arise if the plant must be 

 raised from seed because of the segregation of different combina- 

 tions of characters in the F2 generation. This has limited the 

 commercial utilization of heterosis, although plant breeders have 

 shown that in many cases the additional difficulty of securing 

 seed is more than compensated b.y the productivity of the 

 hybrid. The simplest method of producing seed is to maintain 

 the two parent strains and cross them whenever seed is required. 

 Since the fertility of most seeds lasts several years it is not neces- 

 sary to hybridize every year. 



No example of hybridization for character combinations is 

 more striking than that of the Concord grape. "Ephraim Wales 

 Bull produced the Concord grape as a result of eleven years of 

 patient work in crossing the native species, Vitis lahrusca, with 

 European varieties, raising the seedlings and testing selections. 

 'From over 22,000 seedlings there are 21 which I consider valu- 

 able,' he writes. Although the hybrid nature of the Concord and 

 other derivatives of Vitis labrusca has been questioned, the evi- 

 dence from extensive tests of selfed seedlings of this and several 

 other standard American varieties as reported by Hedrick and 



