42 GEORGE HARRISON SHULL 



averaged in an undivided population. When treated in this way, we find that 

 the Fo's have an average grain-row number of 13.1 and average yields of 49.5 

 bushels per acre, while the F3 had an average of 13.4 grain-rows and pro- 

 duced an average of 30.4 bushels per acre. If we associate the average yield 

 of the Fi families, 79.0 with these values for Fo and F3, we see the beginning 

 of the characteristic curve in which the loss of yield from one generation to 

 the next is about twice as great as the loss for the next following generation . 

 It remains to consider the last three sections of Table 2.7, in which are 



Fig. 2.6 — Total yields of ear corn of two selfed strains, Strain 16 and Strain 20, in the fore- 

 ground (exaggerated, of course, by foreshortening), and their Fi, F2, and F3 hybrids, left to 

 right, successively, in the background. As may be seen in Table 2.7, these yields, calculated 

 in terms of bushels per acre, are 12.76 and 21.82 for the two inbreds, and 83.21, 50.81, 



and 36.43 for the three hybrid families. 



included the results of more complicated crossing which had become possible 

 through the accumulation of simpler crossing in preceding years. In section I 

 are given two "three-way" crosses and four iterative crosses involving Fi 

 combinations and one iterative cross involving an F2 combination, each repre- 

 senting a cross between a hybrid and an inbred. As might be e.xpected, these 

 seven families although similar in form show no special consistency, since 

 they involve various combinations of five different inbreds and five different 

 hybrids. 



In Table 2.7, section K, are presented what I believe to be the first "four- 

 way" or so-called "double crosses" ever made among inbreds. The elements 

 of one of these double crosses are shown in Figure 2.7. These double crosses 

 were made some five or six years before Dr. D. F. Jones pointed out the 

 potentialities of such crosses in producing hybridized seed corn at a price 



