52 APPLICATION OF PKINCIPLES OF HEREDITY TO BREEDING. 



them by selection, but selection may be valuable as a means of holding 

 them up to a high standard. 



Reverting again to the hybrid produced by crossing long-headed 

 winter wheat with a club-headed spring wheat we shall now con- 

 sider the effects produced, first, by mass selection and, second, by 

 individual selection in the progeny of such a hybrid. Figure 5 

 shows the result of mass selection for the winter club type. Since 

 both the winter character and the club character are dominant in 

 this cross, four of the nine types occurring in the second generation 

 will appear to be winter club wheats. These are the upper four 

 types of figure 5. The other five can be distinguished at once, 

 because they will either show the long-head character or the spring 



character. To deter- 

 mine whether or not 

 a wheat is winter or 

 spring in character it 

 should be planted in 

 the spring. If it makes 

 a crop the same season 

 it is a spring wheat. 

 If it waits until the next 

 season before it heads 

 out it is a winter wheat. 

 The four types in the 

 second generation 

 which appear to be win- 

 ter club wheats are 

 those having the con- 

 stitution WWCC, 

 WWCc, ^YwCC, and 

 WwCc, in which W 

 stands for the winter 

 character, w for the spring character, C for the club character, and c 

 for the long-head character. The first of these four types is already 

 fixed and will reproduce itself faithfully. The others are heterozygote 

 with reference to one or both character pairs, and will consequently 

 the next year produce some progeny which will be either spring or 

 long-headed wheat, or both. Suppose, now, that in the second gen- 

 eration we discard everything except these four types. Figure 5 shows 

 what the result will be. The diagram of figure 5 is easily interpreted 

 if we understand that the space between the top horizontal line and 

 the uppermost curved line represents type WWCC, the space between 

 the two uppermost curves represents type WWCc, and so on. It is 

 seen that in the third generation, type WWCC has increased greatly in 



Types. 



jmcc 



WWCc 

 WwCC 



WwCc 



[WWcc 

 I Wwcc 

 wwCC 

 wwCc 

 wwcc 



2 3 4 5 



fenerations. 



Fig. 5.— Graphic illustration of ten generations of a hybrid in a self- 

 fertilized species selected for type WWCC. The letters and spaces 

 between curves liave the same significance as in figure 4. 



