Control of Heredity: Eugenics 261 



and 8 phenotypes, one of each of the latter being homozygous and 

 breeding true, as is shown in Fig. 32 for trihybrid peas. 

 These 8 phenotypes of this cross are STR, STW, SSmR, SSmW, 

 LTR, LTW, LSmR, LSmW. In Fig. 95, C and D, and Fig. 96, 

 A, B, C, are shown 5 of these 8 phenotypes which were ob- 

 tained by Castle from this cross. These figures well illustrate the 

 new combinations of Mendelian characters which may be obtained 

 by cross breeding. 



Hybridization. This is the chief method employed by Bur- 

 bank in producing his really wonderful "new creations in plant 

 life." By extensive hybridization he brings about many new com- 

 binations of old characters, a few of which may be commercially 

 valuable, and sometimes actually new characters or mutations 

 appear, possibly as a result of the interaction of old characters, 

 or rather of their factors. Lotsy, for example, maintains that the 

 sole source of variation is crossing, and Bateson says that the 

 new breeds of domestic animals made in recent times are the 

 carefully selected products of the recombination of pre-existing 

 breeds, and that most of the new varieties of cultivated plants are 

 the outcome of deliberate crossing. 



One of the striking results of modern work in plant-breeding 

 has been the discovery of the greatly increased vigor of certain 

 hybrids as compared with either pure-bred parent. In general it 

 is not possible to tell without previous experience what the char- 

 acter of the hybrid of two races or "lines" will be; sometimes it 

 is more and sometimes less vigorous than either parent, but not 

 infrequently it is more vigorous. East and Shull have shown 

 that hybrids between two races of corn may be very much larger 

 and more fertile than either parent. In some instances the yield 

 of corn per acre has been increased from 20-30 bushels to 80-90 

 bushels, and in one case to more than 250 bushels per acre (Figs. 

 97, 98). Unfortunately such hybrid races of corn do not con- 

 tinue to breed true and the crossing must be made anew in each 

 generation if maximum results are to be had. Nevertheless this 

 method of hybridization or "heterozygosis," as it has been called, 



