MICROPHYTOLOGY 133 



Does it produce spores, or does it multiply by cell-division 

 only? 



A. The bacillus of tubercle was at first thought to be 

 capable of a parasitic existence only, because it did not 

 thrive except at temperatures at or near blood-heat. It 

 has'now been found that, although it does not grow vigor- 

 ously unless at a favourable temperature, it can maintain a 

 torpid existence under more trying conditions than was 

 thought possible at first. On the other hand, the bacillus of 

 lock-jaw (tetanus) is, in some localities, a common inhabi- 

 tant of the soil. Since this terrible pest but rarely finds its 

 way into the animal body, a most interesting problem 

 presents itself. What does the bacillus feed upon in the 

 soil? What relation do its occasional visits to the animal 

 body bear to its habitual residence in the soil ? Do myriads 

 of generations pass their lives in the soil in order that, from 

 time to time, a few may be bred in the body. We say gen- 

 erations, although it should be borne in mind that these 

 unicellular organisms are not generated, neither do they die. 

 They merely divide. Those which are not destroyed by 

 outside agencies are immortal. 



As we have already pointed out, the germs which produce 

 disease are few in number compared with those which never 

 affect animals or Man. Water may teem with microbes and 

 yet be perfectly wholesome to drink. Indeed, in the struggle 

 for existence among these minute organisms the more deli- 

 cate "pathogenic" microbes usually go to the wall, so that 

 the presence of innocent microbes in large numbers may 

 under certain circumstances be a guarantee that none 

 which are noxious have had a chance of survival. Mic- 

 robes are not man's enemies only, but among the best of 

 his friends. 



The bacteria which habitually live in the soil produce 

 results compared with which the effects of pathogenic germs 

 are trifling, if living things be looked at as a whole. The 

 story of the Kentish farmer who boiled the rags which he 



