PASTEUR, KOCH, AND OTHERS 277 



tagioiis diseases. The three greatest nanies connected with 

 the rise of bacteriology are those of Pasteur, Koch, and Lister, 

 the resuhs 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 kboratory without contact 

 or close association with living substance? This is a ques- 

 tion of biogenesis — life from previous life — or of ahiogenesis 

 — life without preexisting 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, ex- 

 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 c^uestion of spontaneous origin of life was in 

 a crude and grotesque condition. It was thought that frogs 



