INTRODUCTION. 21 



lished his views in 1762, maintained that not only 

 were all infectious diseases caused by micro-organisms, 

 but that the infective material could be nothing else 

 than a living organism. On these grounds he en- 

 deavored to explain the variations in the period of 

 incubation of the different infectious diseases. He also 

 insisted that there were special germs for each infectious 

 disease by which the specific disease was produced. 

 Plenciz believed, moreover, that these organisms were 

 capable of multiplication in the body, and suggested 

 the possibility of their being conveyed from place to 

 place through the air. He also made original investi- 

 gations into the process of decomposition, and having 

 found " animalcube " in all decomposing matter, he 

 became so thoroughly convinced of the causative rela- 

 tion of these organisms to the process that he formu- 

 lated the law that decomposition takes place by means 

 of living organisms, and is possible only through their 

 increase. 



These views, it is true, were largely speculative, and 

 rested upon insufficient experiment; but they were so 

 plausible, and the arguments put forward in their sup- 

 port were so logical and convincing, that they continued 

 to gain ground, in spite of considerable opposition and 

 ridicule, and in many instances the conclusions reached 

 have since been proved to be correct. The fact that 

 infectious diseases were of sudden occurrence, breaking 

 out often in isolated places, and that they frequently 

 remained clinging for long periods to certain localities, 

 leaving others unaffected, was evidence that they were 

 not produced by a gaseous infective agent. Moreover, 

 the mode of infection, its unlimited development among 

 large numbers of individuals, and gradual spread over 



