INTRODUCTION. 21 



perature to which some of the infusions employed in 

 Needham's experiments had been subjected. 



More than a hundred years after Bonnet had made 

 this purely speculative suggestion it became the task of 

 Ferdinand Cohn, of Breslau, to demonstrate its accuracy. 



Cohn repeated the foregoing experiments with like 

 results. He concluded that the irregularities could only 

 be due to either the existence of more resistant species 

 of bacteria or to more resistant stages into which certain 

 bacteria have the property of passing. After much 

 work he demonstrated that certain of the rod-shaped 

 organisms possess the power of passing into a resting 

 or spore stage in the course of their life cycle, and 

 when in this stage they are much less susceptible to the 

 deleterious action of high temperatures than when they 

 are growing as normal vegetative forms. With the 

 discovery of these more resistant spores the doctrine of 

 spontaneous generation received its death-blow. It was 

 no longer difficult to explain the irregularities in the 

 foregoing experiments, nor was it any longer to be 

 doubted that putrefaction and fermentation were the 

 result of bacterial life and not the cause of it, and that 

 these bacteria were the offspring from pre-existing 

 similar forms. In other words, the law of Harvey, 

 Omne vivum ex ovo, or its modification, Omne vivum ex 

 vivo, was shown to apply not only to the more highly 

 organized members of the animal and vegetable king- 

 doms, but to the most microscopic, unicellular creatures 

 as well. 



The establishment of this point served as an impetus 

 to further investigations, and as the all-important ques- 

 tion was that concerning the relation of these micro- 

 scopic organisms to disease, attention naturally turned 



2* 



