20 
PRINCIPLES OF BACTERIOLOGY 
teenth and the beginning of the nineteenth century did 
not bring forth any startling discoveries, although some 
interesting observations were made by Plenciz, who ad¬ 
vanced the opinion that each infectious disease was due 
to a specific microorganism, by Otto Muller and Ehren- 
berg, the latter having contributed the classification which 
in its main points, is tenable even today. 
The next real advance came in 1838 when Cagniard- 
Latour, a physicist, showed the living nature of yeasts, 
found in fermenting substances, a fact corroborated by 
Schwann; this observation gave rise to much discussion 
and, argument as the general opinion of the nature of 
fermentation at that time was that it was due to the 
decomposition of the protein matter, a process held to be 
essentially a chemical one. This new conception of fer¬ 
mentation was not, however, definitely accepted until 
Pasteur, the great French scientist, made known his class¬ 
ical studies on the fermentation in wine and beer. 
At this time the mooted and extremely important 
question was that of “spontaneous generation”—were 
the “animalcules” seen by Kircher and Leeuwenhoek and 
other microorganisms produced by others of their own 
kind or could they be produced from different “things,” 
as, for example, mice were produced, according to Pliny, 
from dirty linen ? 
Supported by Buffon, Needham, in 1850, claimed that 
the theory of spontaneous generation was correct, as he 
could demonstrate that pieces of putrefying material, 
sealed in flasks, were found, a few days later, to contain 
enormous numbers of microorganisms; his experiments 
were repeated by an Italian investigator, Spallanagani, 
who having subjected the flasks to considerable heat, could 
not verify Needham’s results. Then came the experi¬ 
ments of Schwann, Schultze, Schroeder and Dresch, who 
