BACTERIAL CLASSIFICATION II 



number of individuals, group themselves on a curve 

 which follows the simple mathematical law of chance. 

 All natural phenomena, governed by a large number of 

 independent variable factors, follow this law; and form, 

 when plotted, a Curve of Frequency, with a high point 

 or Mode from which the curve falls off regularly, but 

 not always symmetrically, on both sides. The height 

 of a thousand men chosen at random forms, for example, 

 such a curve. A few very tall men are at one end, a 

 few very short men at the other, and from each end the 

 numbers increase to a maximum at some intermediate 

 point. The distances from the bull's-eye of a thousand 

 shots fired at a target, the weight of a thousand seeds 

 of the same plant, the altitude of a thousand hills, would 

 form a similar curve. 



If two large arrays of individuals from the same popu- 

 lation are measured by this method the curves obtained 

 will be nearly identical. But if arrays from diverse 

 origins are compared significant differences appear. The 

 position and height of the modal point, and the shape of 

 the curve, differ. These differences are due to constant 

 factors of heredity or environment operating on one 

 group and not on the other. They measure the individ- 

 ualities of the groups as a whole, and serve to discrimi- 

 nate their racial types, even though particular members 

 are indistinguishable. 



Where mutually infertile species only are to be dis- 

 tinguished refined methods of study are scarcely neces- 



