HISTORICAL INTRODUCTION. A 



that the plague in Toulon and Marseilles in 1721 arose from a 

 similar cause. In fact, by some, all diseases were attributed to 

 vermicules, and this led to the theory being ridiculed arid discredited. 



In spite of adverse criticism, the theory of contagium vivum 

 survived, and Linnseus acknowledged it by placing the micro- 

 organisms discovered by Leeuwenhoek, the contagia of specific 

 fevers, and the causes of putrefaction and fermentation, into one 

 class — " chaos." The theory was further supported by the writings 

 of Plenciz, who, in 1762, very ably discussed the nature of contagium, 

 as well as the relation of animalcules to putrefaction and disease. 

 However, no proofs in Support of these theories were forthcoming,, 

 and gradually the idea of contagium vivum fell into obscurity, and 

 indeed came to be regarded by some as an absurd hypothesis. 



Though a causal relation of animalcules to diseases was for a 

 time discredited, the natural history of these micro-organisms was 

 studied with increasing interest. In 1778 Baron Gleichen described 

 and figured a great number of micro-organisms which he had 

 discovered in various vegetable infusions. Joblot, Lesser, Reaumur, 

 HUl, and many others worked at the same subject. Hill remarked 

 that there was hardly the least portion of matter or the least drop 

 of fluid of any kind naturally found in the earth, which was not 

 inhabited by multitudes of animalcules. But these observers incUned 

 rather to searching for new forms than to studying more thoroughly 

 those which had been already discovered ; and, as a result, but little 

 scientific progress \yas made until the time of Miiller, of Copen- 

 hagen. Miiller, in 17&6, criticised the work of previous writers, 

 and pointed out that they had been too much occupied witli merely 

 finding new micro-organisms. Miiller took into account the form 

 of the micro-organism, its mode of progression, and other biological 

 characters, and on such data based a classification. Thus the 

 scientific knowledge of these minute beings was considerably advanced 

 by his writings and illustrations. 



The subject which now eclipsed all others in interest was the 

 origin of these micro-organisms. Two rival theories were widely 

 discussed — spontaneous generation, and development from pre-exist- 

 ing germs; and the researches that were made in the course of 

 this discussion, and the discoveries which resulted, indirectly yet 

 materially advanced the germ theory of disease, and explain many 

 of the phenomena in the life-history of the pathogenic microbes 

 which have been brought to light in recent years. 



Spontaneous development of micro-organisms in putrescible 

 infusions was believed in by many, but was supported by no one 



