SPONTANEOUS GENERATION 6 



larvae of flies, and that their appearance could be very simply prevented by pro- 

 tecting the meat with gauze, through which the flies could not pass to deposit 

 their eggs. The discovery by Leeuwenhoek of the world of microbial organisms 

 gave a powerful stimulus to the somewhat decadent theory. Here, at all events, 

 were living things which obeyed no known law of reproduction, and whose exist- 

 ence seemed to lend support to a belief which had long been accepted by eminent 

 authorities, and which had thereby acquired a natural prestige. 



From the start of his inquiry, Pasteur leaned towards the opposing school of 

 those who believed that spontaneous generation was a myth, that these micro- 

 scopic organisms, like other living things, were reproduced in some way from 

 similar pre-existing cells. He had already convinced himself that these organized 

 cells were the active agents of fermentation. Clearly then they could not arise 

 de novo during the changes for which they were themselves responsible, but must 

 have been introduced from without. Their marked specificity, maintained through 

 repeated transferences from one specimen of fermentable fluid to another of the 

 same kind, was strong evidence in favour of their autonomous reproduction. 

 Here again Pasteur had tentatively adopted the correct solution before starting 

 his experimental inquiry, but the main interest of his part in the controversy lies 

 in the consummate skill with which he developed methods which enabled him to 

 give clear demonstrations where others had left doubt and confusion, and which 

 determined the main rules of a technique which has made possible the cultivation 

 and study of bacteria. 



Neglecting for the moment the vaguer conceptions of the pre-experimental era, 

 the position in 1859 was as follows. Needham, an Irish priest, had published in 

 1745 a memoir describing the spontaneous generation of microbial organisms in 

 closed flasks of putrescible fluids, which had been heated to destroy pre-existing 

 life. These views were strongly supported by the celebrated naturalist Buff on in 

 1749. An Italian abbot, Spallanzani, countered in 1769 with the publication of 

 a series of admirable experiments in which he criticized Needham's results, and 

 showed that, with longer heating, the fluid in such flasks remained clear and 

 sterile. This controversy narrowed into a dispute as to the nature of the prin- 

 ciple which survived short periods of heating, but was destroyed by long heating 

 in flasks hermetically sealed. For Spallanzani the principle was a living germ, 

 for Needham it was a " vegetative force," resident in the air, or perhaps in the 

 putrescible fluid. In any case such argument was sterile, and although it was 

 generally admitted that the honours remained with Spallanzani, no final judgment 

 was pronounced. 



At this time oxygen was regarded as an element of quite peculiar power 

 and significance, and the experiments of Appert (1810) on the preservation of 

 food-stuffs, by heating and hermetical closure of the containing vessels, followed 

 by a weighty expression of opinion by Gay-Lussac, had led to a general belief 

 that the exclusion of this gas was the essential factor in ensuring the absence of 

 fermentation. Schwann (1837) showed that the air in a flask containing a putres- 

 cible fluid, which had been sterilized by boiling, could be renewed by drawing in 

 air which had passed through a glass tube immersed in a bath of fusible alloy 

 kept at high temperature, and by this means he demonstrated that the presence 

 of oxygen alone would not cause the appearance of micro-organisms in the fluid. 

 Unfortunately, in the same memoir, Schwann reported other experiments, in which 

 he introduced heated and unheated air into flasks, containing a sterilized solution 



