230 BIOLOGY OF SPECIALIZED GROUPS 



as nearly as possible, the pabulum on which these micro- 

 organisms grow naturally. 



It is supposed that at one period of time all pathogenic 

 bacteria were saprophytes, but, having their biological charac- 



FIG. 47. Film preparation of discharge from a wound in a case of tetanus, 

 showing B. tetani in " drumstick " spore formation. After Muir and Ritchie. 



ters modified, they have become parasitic on account of natu- 

 ral selection and environment. 



Experiments have been made with various species of bac- 

 teria, and it has been shown that it is possible to modify bac- 

 teria to such an exterf that many of the strict parasites can 

 be made to become saprophytes, and conversely. Some sap- 



