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 literature 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, i 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 Fi 

 individuals had light red kernels, which of course suggests a repetition 

 of the situation shown by Mirabilis in the experiment of Correns. 

 The F2 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. i 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 



