424 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
4. Cumulative factors.—These will be considered under the next 
heading, “Inheritance of quantitative characters.” 
In addition to the four types of factors given, the Jiterature of 
genetics also contains discussions on intensifying factors, diluting 
factors, distribution factors, etc. These, however, do not introduce 
any new mechanisms. 
5. Inheritance of quantitative characters.—This phase of the 
factor hypothesis, if true, is of fundamental importance not only to 
genetics but to general biology. It is based upon the conception of 
cumulative factors, and as it is presented it will be realized that it 
throws light not only upon numerous breeding experiments but also 
upon variation in general, which means evolution also. A cumulative 
factor was defined as one which, when added to another similar factor, 
affects the degree of development of the character. 
It will be recalled that Correns crossed red and white strains of 
Mirabilis and obtained pink hybrids. The suggested explanation of 
this result was that a single dose of the red determiner gives pink while 
a double dose gives red. When Correns inbred these pink hybrids, 
he obtained the result presented in Fig. 78, that is, 1 red, 2 pink, 
I white. This result is obvious and the mechanism is plain. 
With this diagram in mind we shall consider some of the experi- 
ments of Nilsson-Ehle at the Swedish Experiment Station. He 
crossed two strains of wheat with red and white kernels. The F,; 
individuals had light red kernels, which of course suggests a repetition 
of the situation shown by Mirabilis in the experiment of Correns. 
The F, generation, however, showed a very different result. The reds 
and whites appeared in the ratio of 15:1; but in addition to this, 
among the 15 reds there could be distinguished varying degrees of 
redness. Nilsson-Ehle suspected that 15:1 meant a dihybrid ratio, 
16 individuals being necessary to give the ratio, so that he constructed 
the tentative scheme shown in Fig. 84. 
This shows a regular dihybrid ratio, except that the two factors 
involved are similar. Applying the single dose and double dose con- 
ception, as used in the case of Corren’s pink Mirabilis, we reach the 
following conclusions: No. 1 only has four doses and therefore it only 
is deep red; Nos. 2, 3,5, 9 have three doses and are somewhat lighter 
red; Nos. 4, 6, 7, 10, 11, 13 have two doses and are still lighter red; 
Nos. 8, 12, 14,15 have one dose and are very light red; while No. 16 
alone has no dose and is the only pure white. This accounts for 
the 15:1 ratio, and the different shades of red. This is entirely in 
