INTRODUCTION. 21 



subject, suggested, in reference to the results obtained 

 by Needham, the possibility of the existence of " germs, 

 or their eggs," which have 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 made 

 this purely speculative suggestion it became the happy 

 privilege of Ferdinand Cohn, of Breslau, to demon- 

 strate 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 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-blo\r. It was no 

 longer difficult to explain the irregularities in the fore- 

 going 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 bac- 

 teria were the offspring from pre-existing similar forms. 

 In other words, the law of Harvey, Omne vitmm 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. 



The establishment of this point serv^ed as an impetus 

 2* 



