VARIATIONS AND VARIETIES 199 



VARIATIONS AND VARIETIES. 



As in the case of all other living things, bacteria are modi- 

 fied by their environment. Such modifications manifest 

 themselves in various ways. In some instances they are 

 degenerative, involving alterations in form and function 

 that are easily detected by appropriate methods of exami- 

 nation. Often such changes are but transitory and are 

 referable to the influences of well-known causes, the removal 

 of which permits the bacteria to resume their normal state. 

 (See Involution Forms.) In other instances more or less 

 prolonged environmental influences, of which we know but 

 little, appear to have brought about alterations in function 

 with no appreciable changes in form. Sometimes the one 

 or the other of such modifications may be brought about 

 at will by appropriate experimental methods. 



From the early days of modern bacteriology confusion 

 has arisen at times in connection with the establishment of 

 definite species. 



It was frequently found that among the species, as deter- 

 mined by methods then available, individual members of 

 a species would exhibit variations in particular functions 

 that differentiated them from the accepted type. Some- 

 times these differences were morphological, more often they 

 were physiological. Occasionally they could be detected by 

 crude culture methods then in vogue more often as the 

 studies progressed, they were demonstrable only by more 

 refined special methods. To recall this confusion one need 

 but mention the marked functional variations of Bacillus 

 coli communis and the striking morphological differences 

 seen in Bacillus diphtherise. 



By the discovery of methods better suited to bring out 



