42 RELATIONSHIPS OF THE COCCACE^E 



of the five genera to which reference has been made above. 

 The organisms belonging to the genus Diplococcus do not 

 easily lend themselves to comparative study on account 

 of the difficulty with which they may be cultivated; and 

 representatives of the genus Ascococcus occur only in 

 certain peculiar habitats. We have limited our study to 

 forms which can be found in ordinary environments and 

 which may be cultivated on ordinary laboratory media; 

 that is, to the genera Streptococcus, Micrococcus, and 

 Sarcina, as ordinarily understood. 



We procured our cultures, in approximately equal pro- 

 portions, from five different sources: from the internal 

 tissues of the diseased human body, from the outer sur- 

 faces of the normal human body, from water, from earth, 

 and from air. Cultures classed under Habitat I, the 

 tissues of the diseased body, were obtained chiefly from 

 the Boston City Hospital, and the Massachusetts General 

 Hospital, k of Boston and the Johns Hopkins Hospital of 

 Baltimore. The cultures classed under Habitat II, sur- 

 faces of the normal body, were obtained from three 

 sources. A considerable number were isolated from 

 serum tubes which had been received by the Boston 

 Board of Health for diphtheria diagnosis but which had 

 shown negative results. Another series of cocci was iso- 

 lated from the hands of students in the Massachusetts 

 Institute of Technology. Finally a small number of 

 cultures were obtained from excreta of man and animals. 

 Under Habitat III, cultures were obtained from a wide 



