PASTEUR, KOCH, AND OTHERS 277 
tagious diseases. The three greatest names connected with 
the rise of bacteriology are those of Pasteur, Koch, and Lister, 
the results of whose labors will be considered later. 
Among the general topics which have been clustered 
around the study of bacteria we take up, first, the question 
of the spontaneous origin of life. 
THE SPONTANEOUS ORIGIN OF LIFE 
It will be readily understood that the question of the spon- 
taneous generation of life is a fundamental one for the biol- 
ogist. Does life always arise from previously existing life, 
or under certain conditions is it developed spontaneously ? 
Is there, in the inorganic world, a happy concourse of atoms 
that become chained together through the action of the sun’s 
rays and other natural forces, so that a molecule of living 
matter is constructed in nature’s leboratory without contact 
or close association with living substance? This is a ques- 
tion of biogenesis—life from previous life—or of abiogenesis 
—life without preéxisting life or from inorganic matter alone. 
It is a question with a long history. Its earliest phases do 
not involve any consideration of microscopic forms, since they 
were unknown, but its middle and its modern aspect are con- 
cerned especially with bacteria and other microscopic organ- 
isms. The historical development of the problem may be 
conveniently considered under three divisions: I. The period 
from Aristotle, 325 B.c., to the experiments of Redi, in 1668; 
II. From the experiments of Redi to those of Schulze and 
Schwann in 1836 and 1837; III. The modern phase, ae 
tending from Pouchet’s observations in 1859 to the present. 
I. From Aristotle to Redi.—During the first period, the 
notion of spontaneous generation was universally accepted, 
and the whole question of spontaneous origin of life was in 
a crude and grotesque condition. It was thought that frogs 
