26 BACTERIOLOGY. 



Robin, in 1853, had observed these highly refractile 

 bodies ; but it was not until 1876 that the development 

 of spores was carefully investigated and explained by 

 Cohn and later by Koch. These observers showed that 

 certain rod-shaped organisms possess the power of pass- 

 ing into a resting or spore-stage under peculiar conditions 

 of growth, and when in this stage they are much less 

 susceptible to the injurious action of higher tempera- 

 tures than when in their normal vegelative condition. 



With this discovery the controversy of spontaneous 

 generation was finally settled. If these micro-organ- 

 isms, some of them being capable of producing the 

 more resistant spores, were present in the air, dust, 

 soil, water, etc., it was easy enough to explain the 

 irregularities in the foregoing experiments ; nor was it 

 any longer to be doubted that these bacteria, through 

 their products, were the cause, not the effect, of fer- 

 mentation and putrefaction, and that when organic sub- 

 stances were completely sterlized and protected against 

 the entrance of living germs from without, no develop- 

 ment of micro-organisms occurred in them. 



Stimulated by the establishment of the fact that fer- 

 mentation and putrefaction were due to the action of 

 living organisms reproduced from similar pre-existing 

 forms, the study of the causal relation of these micro- 

 organisms to disease was taken up with renewed vigor. 

 Reference has already been made to the opinions and 

 hypothesis of the earlier observers as to the microbic 

 origin of infectious diseases. The first positive grounds, 

 however, for this doctrine, founded upon actual ex- 

 periment, were the investigations into the cause of cer- 

 tain infectious diseases in insects and plants. Thus 

 Bassi, in 1837, demonstrated that a fatal infectious 



