100 FERMENTS AND MICRO-PARASITES. 



cultivations through a long series; if he introduced a 

 trace of the last cultivation into an animal, the disease 

 in question appeared with all its characteristic symptoms 

 after the typical incuhatiou period ; death followed after 

 a definite time. The results of the autopsy were always 

 the same ; organisms of the same form, and with the 

 same characters as those introduced, were found in 

 enormous numbers in the hlood and tissues ; and traces 

 of the hlood containing the organisms when inoculated into 

 other animals, caused in them the same fatal disease. 



In the case of these diseases the causal connection of 

 the micro-organisms was therefore proved with complete 

 certainty ; and it was natural to draw similar conclusions 

 with regard to the other numerous infective diseases 

 which had the same characters. Nevertheless it is best, 

 and will contribute more to the development of knowledge 

 with regard to the micro-parasites, if we proceed with the 

 greatest caution, avoid generalisations, and only proclaim 

 a disease as parasitic when we succeed in finding micro- 

 organisms with well-marked morphological character- 

 istics, in demonstrating their presence in such numbers 

 and with such a distribution that all the phenomena of 

 the disease can thereby be explained, in transmitting them 

 to other higher animals, or if possible in cultivating them 

 for several generations on an artificial soil, and repro- 

 ducing the characteristic disease by inoculation of 

 animals with minute quantities of these cultivations. 



That these minute organisms act as parasitic exciting 

 agents of disease, is as much beyond question as is the 

 function of similar minute beings in exciting fermenta- 

 tion and putrefaction. And it is to this fact that the 

 great and many-sided importance of micro-organisms 

 in relation to hygiene and public health is due. It was 

 the processes of fermentation and putrefaction of organic 

 substances in our surroundings which first awakened 

 uneasiness and distrust, and called the efforts of modern 

 hygiene into being, and the most important, and also 

 the most difficult part of the hygienic investigation of soil, 

 water, air, and dwellings, lies in ascertaining those 

 conditions which favour the development and spread of 

 the exciting agents of disease. 



