22 BACTERIOLOGY. 
wide areas—the incubation, course of, and resulting 
immunity in recovery from infectious diseases—all 
pointed to the probable cause being a living organism. 
Among other distinguished men of the day whose 
observations exerted a most powerful influence upon 
the doctrine of infection may be mentioned Henle. 
His writings (Pathological Investigations, 1840, and 
Teat-book of Rational Pathology, 1853), in which he 
described the relation of micro-organisms to infectious 
diseases, and defined the character and action of bac- 
teria upon certain phases and symptoms of these affec- 
tions, are remarkable for their clearness and precision. 
But, meanwhile, the question which most interested 
these investigators into the cause of infectious diseases 
was, Whence are these micro-organisms derived which 
were supposed to produce them? Were they the result 
of spontaneous generation due to vegetative changes in 
the substances in which the organisms were found, or 
were they reproduced from similar pre-existing organ- 
isms—the so-called vitalistic theory? This question is 
intimately connected with the investigations into the 
origin and nature of fermentation and putrefaction. 
Among those who advocated the theory of spon- 
taneous generation was Neidham, who, in 1749, at- 
tempted to prove by experiment the truth of his opin- 
ions. He placed a grain of barley in a watch-glass 
containing water, covered it carefully, and allowed it 
to germinate. On later examination he found bacteria 
present, which he maintained were the result of changes 
in the grain itself due to its germination. 
In 1769, Spallanzani showed by another experiment 
that the theory of spontaneous generation was incor- 
rect. He demonstrated that if putrescible infusions 
