30 GENETICS 



It was subsequently found that the two modes in this 

 polygon were due to the fact that the material in 

 question was a mixture of two closely related species, 

 Paramecium aurelia and Paramecium caudatum, the 



Number of 

 Individuals 



FIG. 6. The body width of a population of the protozoan Para- 

 mecium, showing a polygon with two modes. A, Paramecium 

 aurelia. B, Paramecium caudatum. After Jennings. 



individuals of which arranged themselves around their 

 own mean in each instance. 



Although such an explanation does not always turn 

 out to be the right one, the biometrician is led to suspect 

 when a two or more moded polygon appears that he is 

 dealing with a mixture of more than one kind of ma- 

 terial, each of which fluctuates around its own average. 



Heterogeneous material, it should be noted, does 



