426 READINGS IN 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, hybrids of intermediate intensity of red as 
before. The F, generation, however, showed a different situation. 
Reds and whites were obtained in the proportion of 63:1; 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 
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Pure Red Grades of Pak White 
Fic. 85.—Another method of visualizing Nilsson-Ehle’s 15:1 ratio (see 
Fig. 84). (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, generation may be represented by Fig. 86. 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 
