HEREDITY 



conform with a simple Mendelian system. Con- 

 sider, for example, the matter of size and 

 skeletal proportions in rabbits. 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 not certain that the ob- 

 served facts may not be explained by the com- 

 bined action of several similar but independent 

 factors, the new principle which Nilsson-Ehle 

 has brought to our attention. Let us apply 

 such a hypothesis to the case in hand. 



Suppose a cross be made involving ear- 

 lengths of approximately 4 and 8 inches respec- 

 tively, as in one of the crosses made. The F t 

 young are found to have ears about 6 inches 

 long, the mean of the parental conditions, and 

 the F 2 young vary about the same mean con- 

 dition. If a single Mendelian unit-character 

 made the difference between a 4 inch and an 8 

 inch ear, the F 2 young should be of three 

 classes as follows: 



Classes 4 in. 6 in. 8 in. 



Frequencies 121 



134 



