4 INTRODUCTION 



by the experiments of Spallanzani (1777) and of Schulze (1836), 

 it still continued to be accepted by many scientific men until it 

 was combated by Pasteur, 1860 to 1872. After the accurate 

 observations of Pasteur upon fermentation and putrefaction and 

 his successful defense of them through a long period of contro- 

 versy, the assumption of spontaneous generation as applied to 

 bacteria was discredited and has been very generally given up. 

 Only a very few observers 1 still claim the existence of evidence 

 in support of its application here. The more prominent advo- 

 cates 2 of the assumption of spontaneous generation or abio- 

 genesis seem inclined now to apply it to some group of living 

 beings still beyond the limits of actual observation. 



Closely related to the assumption of abiogenesis has been the 

 assumption of heterogenesis among the bacteria, the notion that 

 various kinds of microbes could readily be produced from 

 one species. Although very successfully combated by Pasteur, 

 this idea still persisted for many years in the early bacteriological 

 literature, the observed new species of microbes actually resulting 

 from faulty technic by which new germs had gained entrance to a 

 former pure culture. These observations are often repeated 

 unwittingly by beginners in bacteriology. The validity of bac- 

 terial species is now unquestioned. On the other hand, the vari- 

 ability in the descendants of a single cell through a greater or 

 less range, and the possibility of producing morphologically and 

 physiologically different strains of the same species by appro- 

 priate environmental conditions are now well known, resulting 

 again very largely from the fundamental work of Pasteur in the 

 production of attenuated cultures of the germs of chicken cholera, 

 and of anthrax. 



The systematic relationships and the classification of bacteria 

 were first studied by O. F. Mueller (1786). Ehrenberg (1838) 

 made the first serious attempt at a comprehensive classification 



1 Bastian, The Evolution of Life, London, 1907. The Origin of Life, London, 



1913- 



2 Schafer, Nature, Origin and Maintenance of Life, Science, 1912, Vol. XXXVI, 

 pp. 289-312. 



