VARIATION 



49 



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 cauda- 

 tum, the individuals of which arranged themselves 

 around their own mean in each instance. 



Number ol J 3 

 ribs 



15 16 17 16 19 



El 



FIG. 27. The ribs of leaves from two beech trees. When put together 

 they form a polygon which does not reveal its double origin. From 

 data by Pearson. 



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 material, each of which fluctuates 

 around its own average. 



Heterogeneous material, it should be noted, does 

 not always give a bimodal curve. For example, 

 if Pearson's two lots of beech leaves mentioned 



