266 



EVOLUTION, GENETICS, AND EUGENICS 



Continuing these investigations, Nilsson-Ehle next discovered a 

 new strain of red-grained wheat, which, when crossed with the pure 

 white strain, yielded F x hybrids of intermediate intensity of red as 

 before. The F 2 generation, however, showed a different situation. 

 Reds and whites were obtained in the proportion of 63 : i ; the 63 reds 

 as before falling naturally into different groups on the basis of degree 

 of redness. Applying the same conception as before Nilsson-Ehle 



Pure Red 



Grades of Pink 



White 



Fig. 67.— Another method of visualizing Nilsson-Ehle's 15: 1 ratio (see Fig. 66). 

 (From Coulter and Coulter.) 



discovered that in this case he was dealing with a trihybrid situation. 

 Without constructing the usual Mendelian diagram, which would have 

 to be extensive enough for 64 individuals, the situation as it appeared 

 in the F 2 generation may be represented by Figure 68. If the graph is 

 surmounted by a curve we recognize the regular "probability curve," 

 exactly the kind of curve biometricians use to represent the fluctuating 

 individuals about a specific type. 



This conception of cumulative factors, therefore, has far-reaching 

 significance. For a long time biologists have recognized individual 



