PREFACE vii 



families are eight minor groups which seem to merit 

 generic rank. Each is marked by the correlation of 

 several apparently independent characters, and the eight 

 form, in general, a more or less linear series, connecting 

 such purely parasitic forms as the meningococcus with 

 the saprophytic cocci, so common in the air. Within 

 each genus are included three or four distinct specific 

 types, each marked by a single peculiarity but sharing 

 the common characters of the genus as a whole. 

 Species and genera alike are connected by numerous 

 intergrading varieties ; but the central types in each case 

 are defined by the preponderating frequency of their 

 occurrence. 



Extensions of this work by the examination of other 

 cultures of cocci, by more extended tests, are much to 

 be desired. Such investigations will no doubt add new 

 genera and species and may show that some of those 

 now apparently warranted are not really valid. It 

 seems to us, however, that the results here presented 

 offer a working basis for classifying the Coccaceae which 

 corresponds in the main with their natural relationships. 

 The results so far attained have convinced us that the 

 study of the numerical frequency of individual characters 

 and of their mutual correlations offers a sound basis for 

 bacterial classification; and we trust that other workers 

 may be persuaded to adopt similar biometrical methods 

 in studying the relationships of simple and variable 

 forms of life. 



