Variation and Heredity 489 



requires intelligent care. Hybridization is primarily 

 useful in order to effect the combination of desirable 

 characters and the elimination of undesirable ones. Never- 

 theless, it is generally agreed that in some cases of alterna- 

 tive inheritance there may be left an influence of the cross, 

 so that the characters may not reappear in their original 

 purity. Moreover, as a result of hybridization latent or 

 reversionary characters may appear, and crossing has a 

 tendency to intensify variability. 



A knowledge of Mendelian behavior has necessitated a 

 change in the methods of selecting hybrid offspring. In 

 alternative inheritance there can be no selection in the F a 

 generation. In the F 2 generation selection for the pure 

 recessives may be made. However, since homozygous 

 dominants usually distinguish themselves from hetero- 

 zygous dominants only in the absence of segregation in 

 subsequent generations, it is necessary to isolate individ- 

 uals and to test these in breeding plots. When several 

 character pairs are involved, all of which require considera- 

 tion, selection with respect to dominant characters may 

 become very complex and tedious. 



LABORATORY WORK 



Variation. Utilizing convenient plants in the greenhouse or 

 in the field make a careful study of variation with respect to 

 single characters readily enumerated, or measured. The number 

 of ray flowers, or of bracts in certain composites, the number of 

 flowers in a cluster, the number of eyes upon potatoes, the num- 

 ber of leaflets in compound leaves, the difference in weight of 

 grains or corn, and many other similar characters may be em- 

 ployed. In such cases the counts or measurements should be 

 made within the single variety. Determine the classes of 



