INTRODUCTION. 17 



they seem to have l>een lost sight of in the course of 

 subsequent events, and by a few were even held in the 

 light of productions from an unbalanced mind. For 

 example, as late as 1820 we find Ozanam expressing 

 himself on the subject as follows : " Many authors have 

 written concerning the animal nature of the contagion 

 of infectious diseases ; many have indeed assumed it to 

 be developed from animal substances and that it is itself 

 animal, and possesses the property of life. I shall not 

 waste time in efforts to refute these absurb hypotheses." 



Similar expressions of opinion were heard from many 

 other medical men of the time, all tending in the same 

 direction, all doubting the possibility of these micro- 

 scopic creatures belonging to the world of living things. 



It was not until between the fourth and fifth decade 

 of the present century that by the fortunate coincidence 

 of a number of important discoveries the true relation 

 of the lower organisms to infectious diseases was scien- 

 tifically pointed out. "With the investigations of Pasteur 

 upon the cause of putrefaction in beer and the souring 

 of wine ; with the discovery by Pollender and Davaine 

 of the presence of rod-shaped organisms in the blood of 

 all animals dead of splenic fever, and the progress of 

 knowledge upon the parasitic nature of certain diseases 

 of plants, the old question of " contagium animatum" 

 again began to receive attention. It was taken up by 

 Henle, and it was he who first logically taught this doc- 

 trine of infection. 



The main point, however, which had occupied the 

 attention of scientific men from time to time for a 

 period of about two hundred years subsequent to 

 Leeuwenhoek's discoveries, was the origin of these 

 bodies. Do they generate spontaneously, or are they 



