206 



GENERAL BIOLOGY 



methods, for it shows that the material is not homo- 

 geneous. In reality we have two groups, with two 

 modes, but members of both groups are mingled 

 within the range of the classes that are common 

 to both, and in consequence we cannot determine 



the limits of the 

 curves that in- 

 tersect. 



An example of 

 this is figured be- 

 low. In rhinoc- 

 eros beetles there 

 are two long 

 horns which pro- 

 ject forward from 

 the head and the 

 thorax. Bateson 

 measured the 

 horn on the heads 

 of some 342 bee- 

 tles and found 

 that, with respect 

 to this character, the insects grouped themselves 

 in two classes, the curve of variation being a 

 " double-humped " one. With respect to the 

 length of the wing-covers, however, the same 342 

 beetles were shown to be homogeneous, the curve 

 for this measurement having but one mode. 

 Again, in a certain Chrysanthemum, De Vries 

 found that, although the number of ray-florets 

 (often miscalled petals) of the flower varied from 



FIG. 76. Polygons of variation based 

 upon the measurements of 342 rhinoceros 

 beetles. Two modes are evident, one about 

 4 mm., the other about 8j mm. 



