INTRODUCTION 21 



logical relation to the process that he formulated the law: 

 that decomposition can only take place when the decompos- 

 able material becomes coated with a layer of the organisms, 

 and can proceed only when they increase and multiply. 



However convincing the arguments of Plenciz may appear, 

 they seem to have been lost sight of in the course of subse- 

 quent events, and by a few were even regarded as the pro- 

 ductions of 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 disease; 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 effort to refute these absurd 

 hypotheses." 



Similar expressions of opinion were heard from many 

 other investigators of the time, all tending in the same 

 direction, all doubting the possibility of these microscopic 

 creatures belonging to the world of living things. 



It was not until between the fourth and fifth decades of 

 the nineteenth century that by the fortunate coincidence 

 of a number of important discoveries the true relation of 

 the lower organisms to infectious diseases was scientifically 

 pointed out. With the fundamental investigations of 

 Pasteur upon the souring and putrefaction of beer and wine; 

 with the discovery by Pollender and Davaine of the presence 

 of rod-shaped organisms in the blood of animals dead of 

 splenic fever, and with 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 doctrine of infection. 



