PREFACE. 



This book isrthe outgrowth of an attempt to classify 

 certain bacteria of sanitary importance belonging to 

 the family of the Coccaceae. The task was a difficult 

 one on account of the marked variability of the organ- 

 isms in question. As in other groups of bacteria, an 

 almost infinite number of minutely differing varieties 

 were apparent. Biometrical methods, which have yielded 

 such fruitful results in anthropology, and in various 

 researches in heredity, seemed to offer the most hopeful 

 method of attacking the problem. The attempt was 

 therefore made to discover natural types among the 

 Coccaceae by a study of the numerical frequency with 

 which various characters occur. Those characters, or 

 combinations of characters, which are exhibited by a 

 large number of races, as they are found under natural 

 conditions, may be taken to mark true centers of vari- 

 ation, about which the rarer varieties should properly 

 be grouped. 



Five hundred different strains of Coccaceae were iso- 

 lated from various sources, — from the human body, in 

 health and disease, from air, from water, and from 



