oxygen and other nutritious elements, by the excretion 

 of substances injurious to animal cells, or in several of 

 these ways combined. The formation of substances in- 

 compatible with the life of the animal cells seems to play 

 a prominent role in the production of injurious effects by 

 at least some varieties. 



The simplicity of organization and vital requirements 

 explains their extensive distribution in nature : every 

 moist substance of organic origin and all water contain- 

 ing even a trace of organic matter is favorable soil for 

 one or more varieties ; the upper layers of the earth, 

 containing these essential ingredients, and remaining 

 comparatively warm, constitute a continual breeding- 

 place for these organisms. The minuteness and light- 

 ness of bacteria explain their presence in the atmos- 

 phere ; they are swept by currents of air from dry or 

 moist surfaces ; they float in clouds of dust ; they are 

 carried by insects ; the persistence of their vitality, the 

 rapidity of their propagation, result in practical ubiquity. 

 Direct microscopic observation of atmospheric dust, and 

 the experiments of Tyndall with the electric beam in a 

 dark chamber, have shown that wherever we find dust, at 

 moderate temperature and altitude, we may expect to 

 find bacteria. Yet the atmospheric bacteria are probably 

 not so numerous as has been pictured. The observations 

 of Miquel and of Koch show that even in a laboratory 

 many litres of air contain no organisms. Whether or not 

 bacteria are swept from surfaces of liquids ; whether after 

 once drying upon a given surface they can be removed 

 by air-currents, are as yet undecided questions which may 

 have practical bearings in the future. At present we know 

 no laws of atmospheric distribution wherein bacteria ex- 

 hibit other behavior than particles of dust in general. 



The champions of spontaneous generation, compelled 

 to surrender their maggots in decaying meat to the sim- 

 ple demonstration that covering the meat with fine gauze, 

 which prevented the access of flies, prevented also the 

 development of maggots; forced to abandon intestinal 

 worms by the successive demonstrations of numerous 



