298 FLIES AND DISEASE 



bacilli and their allies, and so on are distinguishable. 

 The species within these subgroups can be further 

 differentiated by observing the reactions of the 

 bacilli with blood, and finally by their actions on 

 living animals. 



This short account of a single bacterial group 

 illustrates the difficulties encountered in deter- 

 mining with certainty the identity of a bacterial 

 species. While the elaboration of new distinguish- 

 ing methods facilitates the recognition of certain 

 types, advances in knowledge in other directions 

 tend to increase the difficulties. 



The bacteriologist has not only to study the 

 distribution, the powers of resistance and the life- 

 histories of bacteria outside the body so as to be 

 able to distinguish them, but he has to study a 

 much more intricate and important subject, their 

 actions and life-histories within the body, and their 

 relation to disease and infection. It is unnecessary 

 to attempt to explain the methods used in such 

 researches, but enough has been said to show that 

 such work can be undertaken with success only 

 by specialists, and that even with special knowledge 

 and care trustworthy and far reaching conclusions 

 can seldom be arrived at without years of careful 

 study. 



The work of the protozoologist is on similar 

 lines. He is aided by the fact that some species 

 of the protozoa can be identified under the micro- 

 scope, but he labours under the serious disadvantage 



