HYBRIDIZATION AND SELECTION. 



53 



proportion. Type WWCc about holds its own until the fourth gen- 

 eration, after which it decreases. The same is true of type WwCC, 

 but type WwCc decreases rapidly from the second generation on- 

 ward. At the end of ten generations practically the whole popula- 

 tion is of type WWCO, which is pure winter club wheat, with a 

 very small admixture of other types. This shows the effect of mass 

 selection after hybridization in the case of self -fertilized crops. The 

 result is a much more rapid approach to the one type selected than 

 occurs in the corre- 

 sponding case with 

 cross-fertilized crops as 

 shown in figure 3. 



The effect of indi- 

 vidual selection, i. e., 

 selection in which the 

 seed of the individuals 

 selected is kept sepa- 

 rate, is shown for self- 

 fertilized crops in fig- 

 ure 6. In this figure, 

 as in the preceding, the 

 space beginning oppo- 

 site each type symbol 

 shows the proportion 

 of that type from gen- 

 eration to generation. 

 In this method of selec- 

 tion we save each sec- 

 ond-generation individual which appears to be of type WWCC. This 

 includes all plants of types WWCC, WWCc, WwCC, Siud WwCc. The 

 seed of each plant is kept separate. In the next generation we save 

 seed only from those rows or plots in which there has been no splitting 

 up. This gives us at once the type WWCC in pure and fixed form. 

 This is shown in figure 6, where the space representing type WWCC 

 occupies the whole diagram beyond the third generation. 



CROSS-FERTILIZED SPECIES. 



The problem of utilizing hybridization in the production of new 

 forms in cross-fertilized species which are propagated from seed is 

 somewhat complicated by the fact that the individuals to be used in 

 crossing may themselves be heterozygote for many characters. Di- 

 versity in such species differs from that in self-fertilized species in 

 this respect: In the latter we usually have in homozygote form all 

 the combinations possible of the characters found in the group, 



165 



jmcc 



WwCc 

 [WWcc 



\wwCC 

 \wwCc 



^^^^ Z 3 4 5 6 7 8 9 .0 



Generations. 



Fig. 6.— Graphic illustration of the effect ol individual selection in 

 a self-fertilized species on progeny of the hybrid Ww Cc. After 

 the third generation the race is pure for the selected characters 

 (both dominant in this case). The letters and spaces between 

 curves have the same significance as in figure 4. 



