194 GENETICS AND EUGENICS 



example, the matter of size and skeletal proportions in rab- 

 bits. It is perfectly clear from the experiments described 

 that in such cases no dominance occurs, and also that no 

 segregation of a simple Mendelian character takes place, but 

 it is possible to explain the observed facts by the combined 

 action of several similar but independent factors, the new 

 principle which Nilsson-Ehle has brought forward. This is 

 known as the principle of multiple factors. Let us apply such 

 an hypothesis to the case in hand. 



Suppose a cross be made involving ear-lengths of approxi- 

 mately four and eight inches respectively, as in one of the 

 crosses made. The Fi young are found to have ears about 

 six inches long, the mean of the parental conditions, and the 

 F2 young vary about the same mean condition. If a single 

 Mendelian unit-character made the difference between a 

 four-inch and an eight-inch ear, the F2 young should be of 

 three classes as follows: 



Classes 4 in. 6 in. 8 in. 



Frequencies .1 2 1 



(Compare Fig. 131, bottom left, and Table 28.) The grand- 

 parental conditions should in this case reappear in half the 

 young. This clearly does not occur in the rabbit experiment. 

 But if two unit-characters were involved, Fi would be un- 

 changed, all six inches, yet the F2 classes would be more 

 numerous, viz., four, five, six, seven, and eight inches, and 

 their relative frequencies as shown by the height of the 

 columns in Fig. 131, middle left, one, four, six, four, one. 

 The grandparental states would now reappear in one-eighth 

 of the F2 young, while three-eights would be intermediate. 

 It is certain, however, that in rabbits the grandparental con- 

 ditions, if they reappear at all, do not reappear with any such 

 frequency as this. 



If three independent size-factors were involved in the cross, 

 the Fi individuals should all fall in the same middle group, 

 as before, viz., six inches, but the F2 classes should number 

 seven, and their relative frequencies would be as showTi in 



