INTRODUCTION 25 



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. He demonstrated that some of 

 the rod-shaped organisms possess the power of passing into 

 a resting- or spore-stage in the course of their life-cycle, 

 analogous to the seeding stage of higher plants, 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 inconsistencies in the results of former investigations, 

 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 of 

 pre-existing similar forms. In other words, the law of 

 Harvey, Omne mvuin ex mo, or its modification, Omne 

 vivum ex vivo, was shown to apply not only to the more 

 highly organized members of the animal and vegetable 

 kingdoms, but to the most microscopic, unicellular creatures 

 as well. 



The establishment of this point gave an impetus to 

 further investigations, and as the all-important question 

 was that concerning the relation of the microscopic organ- 

 isms to disease, attention naturally turned into this chaimel 

 of study. Even before the hjqpothesis of spontaneous 

 generation had received its final refutation a number of 

 observations of a most important nature had been made 

 by investigators who had long since ceased to consider 

 spontaneous generation as a tenable explanation of the 

 origin of the microscopic living particles. 



