INTRODUCTION. 25 



subject, suggested, in reference to the results obtained 

 by Needham, the possibility of the existence of " germs 

 or their eggs," which had the power to resist the tem- 

 perature to which some of the infusions employed in 

 Needham's experiments had been subjected. 



More than a hundred years after Bonnet had in- 

 dulged in this pure speculation it became the happy 

 privilege of Ferdinand Cohn, of Breslau, to demonstrate 

 its accuracy and importance. 



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. 

 He demonstrated that some of the rod-shaped organ- 

 isms 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 dele- 

 terious action of high temperatures than when they are 

 growing as normal vegetative forms. With the discov- 

 ery of these more resistant spores the doctrine of spon- 

 taneous generation received its death-blow. It was no 

 longer difficult to explain the inconsistencies in the re- 

 sults of former investigations, nor was it any longer to be 

 doubted that putrefaction and fermentation were the re- 

 sult 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 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 kingdoms, but to the most 

 microscopic, unicellular creatures as well. 



