80 THE DISEASES OF DOMESTIC ANIMALS. 



which prevailed at that time to the action of some sort of grass- 

 hopper or locust, and recommended that the inhabitants make large 

 fires to drive away or kill them, in order to prevent the diseases. 



Later, we find various absurd ideas being supported, and cholera, 

 variola, syphilitic and other animals described by authors as being 

 the specific cause of such diseases. 



Such absurdities led to the ridicule of the idea of living infec- 

 tious elements, which found its counterpart in the idea of infectious 

 gases, a theory which has been of late abandoned, owing to the 

 great advances in microscopy and the technicalities of scientific re- 

 search during the last ten years. 



The Classification of Bactekia. 



As early as 1773 O. F. Miiller made an attempt to classify these 

 objects ; others followed, among them the noted German natural- 

 ist Ehrenberg in 1838, and the Frenchmen Dujardin (18'11) and 

 Davaine. 



Ehrenberg speaks of — 



1. Bacteria, as filaments linear and inflexible, and gives three 

 species. 



2. Yibrios : filaments linear, flexible ; nine species. 



3. Spirillum : filaments spiral, inflexible ; three species. 



4. Spirochaete : filaments spiral, flexible ; one species. 

 Dujardin (1841) classed them as — 



1. Bacterium : filaments rigid, with vacillating movement. 



2. Yibrio : filaments flexible, with undulatory movement. 



3. Spirillum ; filaments spiral, with rotary movement. 



Up to this time the bacteria had been looked upon as animals, 

 and placed at the foot of that kingdom. Later, the idea that they be- 

 longed to the vegetable kingdom has been gradually gaining ground, 

 and is at present almost universally accepted ; although the renowned 

 German naturalist Haeckel places them in his intermediate class or 

 kingdom of protista. Davaine (1859) was the first to clearly demon- 

 strate the vegetable nature of the vibrios, and their near relation to 

 the algte. 



Davaine's classification was as follows: Filaments straight or 

 bent, but not spiral, moving spontaneously. "Wlien rigid, bacterium ; 

 when flexible, vibrio ; when motionless, bacteridium ; when the fila- 

 ments were spiral, spirillum. 



From this time on we find the study of these objects assuming a 

 new and more exact character, thanks to the work of Pasteur, Da- 

 vaine, Haller, Cohn, Koch, and others. 



