BACTERIAL CLASSIFICATION 13 



greater value in the systematic analysis of types among 

 the bacteria, where so many factors combine to pre- 

 serve varietal differences on so wide a scale. If indi- 

 vidual strains only are considered, an infinite series of 

 differences appear. If the same strains are considered 

 statistically, that is if the frequency of a given character be 

 taken into account, it is apparent that certain combina- 

 tions of characters are much more common than others. 

 Measurement of almost any character by quantitative 

 methods shows that the bacteria examined group them- 

 selves on a simple or complex curve of frequency. The 

 modes of this curve indicate centers of variation about 

 which the individuals fluctuate; and these centers of 

 variation are the real systematic units of the group. 

 The recognition of such centers, as specific types, offers 

 the natural and satisfactory compromise between sys- 

 tematic multiplicity and vague generalization. 



The grouping of specific types is an even more impor- 

 tant problem than the definition of the types themselves; 

 and here the correlation data obtained by biometrical 

 study are of assistance. A true natural classification 

 is tree-like and includes branches and twigs of 

 varying grades of importance. Genera of bacteria 

 should be aggregates of those specific types which are 

 most nearly related; and the basis of the relationship 

 will differ in each individual case. As Robinson has 

 pointed out in an admirable paper on generic classi- 

 fication (Robinson, 1906), " a difference having great 



