20 THE BACTERIA. 
ganisms and especially to the bacteria. Their 
origin, their evolution, the physiological peculi- 
arities of their nutrition and reproduction, are 
the object of numerous labors, and give rise to 
passionate discussions relating to the subject of 
spontaneous generation, polymorphism of fungi, 
theories of fermentation, and the pathology of 
virulent and infectious maladies. For this reason 
an exposition of these researches, often contradic- 
tory, is extremely difficult. We will make it sub- 
cinctly, insisting especially upon the labors relating 
to the classification of the bacteria, and reserving 
to ourselves the privilege of returning to the his- 
tory of several points, when we approach their 
study in the special chapters of this thesis. 
The first important memoir published after 
that of M. Davaine upon the bacteria is that of 
M. Hoffmann, in 1869. He demonstrates: First, 
that the bacteria are plants, having a very distinct 
cellular organization; second, that they can only 
be classified in accordance with their form and 
size, at first into monads and linear bacteria, and 
the latter into microbacteria, mesobacteria, and 
megabacteria; (M. Hoffmann includes with the 
linear bacteria, Vibrio, Bacterium, and Leptothriz, 
which are bacteria united in a chaplet;) third, 
that mobility or immobility is not a specific char- 
acter, but may present itself in the same species 
under the influence of changes of temperature, of 
density of medium, etc. M. Hoffmann studies 
also the origin of the bacteria, and rejects the 
- hypothesis of a spontaneous generation. As. to 
