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 vegetative 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 
