JANUARY 25, 1901. ] 
To recapitulate.— While, in the first half 
of our century, the anatomy and physiology 
of the macrocosm and the microcosm, the 
heavens, the earth and man, were fairly 
well understood, there yet remained certain 
important natural phenomena either gener- 
ally not understood at all or else, as we 
know, almost completely misinterpreted, 
namely, as follows : 
1. Fermentation of every kind. 
. Putrefactions. 
Organic decomposition and decay. 
. Nitrification. 
. The origin of microscopic life. 
. Zymotic diseases. 
The end of the third decade of our cen- 
tury saw the beginning of a profound change 
in the accepted theories concerning all or 
nearly all of these phenomena. In 1836, 
Schulze proved that the oxygen theory of 
fermentation could not be correct. The 
next year Theodor Schwann confirmed his 
results by a different experiment, and by 
the improved achromatic microscope not 
only discovered, or rediscovered, the yeast 
plant, but also boldly asserted that the life 
and growth of this plant was the cause, and 
not the consequence, of the alcoholic fer- 
mentation. Thus, based on solid grounds, 
began the biological or germ theory of fer- 
mentation of which Schwann is entitled to 
be regarded as the founder, precisely as 
Pasteur some twenty years later became 
the founder of its direct descendant, the 
germ theory of disease. Confirmed or even 
slightly anticipated by Latour in his dis- 
eovery of the living and vegetable char- 
acter of yeast ; supported by Helmholtz and 
Mitscherlich in his assertion that no alco- 
holic fermentation of sugar ordinarily oc- 
curs except as caused by yeast; and by 
Schroeder and Von Dusch in his claim that 
the cause of fermentation and putrefaction 
in boiled liquids is not air, but floating mat- 
ters in the air; Schwann is entitled to be 
regarded as the immediate precursor of the 
Do to 
SCIENCE. 
125 
bacteriologists and as having effectually 
paved the way for the first and greatest of 
all bacteriologists, Louis Pasteur. 
Of Pasteur, the founder of our science, I 
shall only recall very briefly the principal 
facts of history : 
Pasteur brought to the study of all the 
problems that I have enumerated—and I 
hardly need remind you that among them 
are some of the most elusive, some of the 
most profound, and some of the most in- 
tensely practical, problems in all the field 
of natural knowledge—a thorough work- 
ing familiarity with physics and chemistry. 
Though not exactly a chemist, he was able 
to meet chemists upon their own ground. 
Though not exactly a microscopist, he was 
highly trained in physics and mineralogy, 
and thus quickly became a master of the 
microscope. Piqued by the fact that living 
ferments could do with his salts what he 
himself could not do, he began a careful 
study of ferments and fermentation, with 
the result that in a few short years he had 
completely confirmed and established the 
biological or germ theory of fermentation 
propounded by Schwann ; had extended it 
so as to make it include putrefactions; had 
shown that organic decomposition and de- 
cay jin nature are simply slow fermenta- 
tions or putrefactions ; had nearly, if not 
quite, overthrown the world-old theory of 
spontaneous generation; had studied the 
floating matter of the air, and actually 
found in it organized corpuscles, or germs, 
of molds and of infusorial or fermentation 
animalcules ; had invented and introduced 
for the first time methods for the cultiva- 
tion, and even for the pure cultivation, of 
living ferments which so much facilitated 
the investigation of microscopic life that we , 
now rightly regard them as constituting 
the very basis and essence of bacteriology, — 
which thus becomes a kind of microscopie 
gardening or horticulture of the micro- 
scopic world ; had also, by the use of these 
