VARIATION 29 



ATLANTIC AND PACIFIC SHELLS COMPARED 



particular character measured, namely, the relative size 

 of the mouth aperture compared with the height of the 

 entire shell. 1 



A further analysis of the data in this particular case 

 shows that this conclusion is probably biologically in- 

 correct, a discovery which does not invalidate it, how- 

 ever, as an illustration of a method of determining 

 relative variability. 



b. Bimodal Polygons 



Sometimes two conspicuous modes make their ap- 

 pearance in a frequency polygon, as Jennings found, 

 for example, in measuring the body width of a popula- 

 tion of the protozoan Paramecium (Fig. 6). 



^'Variation in Urosalpinx." Walter. Amer. Nat. 1910, Vol. 

 XLIV, pp. 577-594. 



