92 FERMENTS AND MICRO-PARASITES. 



belong to the group of diseases which I have termed typical, 

 whose sharply defined stages indicate a development of the 

 cause in accordance with definite laws, such as we only find 

 among living beings. 



" What was stated above with regard to the properties of 

 the cause of miasmatic diseases in general holds good as 

 regards the multiplication of contagia by assimilation. It 

 can, however, only be absolutely proved in the case of the 

 inoculable diseases where we are able to define accurately 

 both the point of entrance and the quantity of material 

 taken up, and the proof becomes the more insufficient the 

 more in any given epidemic the number of the cases produced 

 by miasma exceeds those arising by contagion. That the 

 cause of the disease has multiplied in the region affected 

 by the epidemic is probable whenever the latter spreads 

 gradually from small beginnings and attains large dimensions. 



"It is only when its development and reproduction in the 

 diseased body is demonstrated that we are justified in de- 

 signating the material which occasions epidemic diseases as 

 a contagium, and the analogy of these contagia with parasites, 

 the analogy of the miasmatic contagious diseases with the 

 results of the deposit of parasitic organisms in living bodies 

 previously referred to becomes at once evident. This analogy, 

 as I have indicated above, has led to the discovery of parasites 

 as the cause of many affections formerly termed contagious 

 diseases. There are, however, a number of diseases in the 

 contagium of which nothing has been found which recalls the 

 forms of known species of animals and plants. Nevertheless, 

 this negative result is not so certain that we can, therefore, 

 absolutely refuse to reckon the contagia among these micro- 

 scopic parasites. It is not necessary to assume that the 

 organisms which act as contagia are too small for our optical 

 means. But the smallest animals can only be distinguished 

 from the cells, nuclei, and granules which occur in so many 

 tissues and excreta^ especially in pus, by their movements, and 

 the smallest plants only in certain stages of their develop- 

 ment by the arrangement of their elementary constituents. 

 The granules of which the Botrytis bassiana consists behave 

 exactly like pigment granules or the molecules of pus. It is 

 possible, therefore, that bodies of very various and of great 

 significance may be concealed among the molecules which 

 occur in every microscopical object. It is scarcely necessary 

 to add that these speculations are as yet only hypothetical, 

 but they are not superfluous even in the cases where animal 

 or vegetable parasites have been, or will yet be discovered in 

 the contagium. The question will, however, still remain, 

 whether the parasite is an accidental inhabitant of the con- 



