l8 THEORIES OF SPONTANEOUS GENERATION 



a series of his experiments which show that the white 

 maggots in meat are simply the larvae of flies. He kept meat 

 or fish in a large vessel, covered with the finest Neapolitan 

 muslin, and, for still more complete protection, covered the 

 vessel with a frame on which muslin was stretched. Al- 

 though plenty of flies alighted on the muslin, no maggots 

 appeared in the meat. Redi pointed out that he had suc- 

 ceeded in observing how the flies laid their eggs on the 

 muslin, but that only when these eggs fell on to the meat did 

 they develop into meat maggots. From this he concluded 

 that decaying substances are only a place or a nest for the 

 development of insects, but that the laying of eggs is an 

 essential preliminary to their development ; without eggs 

 the maggots never appear.^* 



It should not be thought, however, that Redi had suc- 

 ceeded in completely ridding himself of the notion of 

 spontaneous generation. In spite of his brilliant experiments, 

 which he had interpreted correctly, this learned man freely 

 admitted the possibility that spontaneous generation might 

 occur in other cases. Thus he states that worms in the intes- 

 tines or in timber arise on their own from rotting materials. 

 Moreover, in his opinion, the maggots which are found in 

 oak galls are formed from the juices of the plant. Only later 

 was this opinion refuted by the investigations of the scientific 

 physician Vallisneri (1661-1730). 



This example makes it clear that what has been repeated 

 for centuries (though often wrongly) is not easily confuted. 



Throughout the eighteenth century, and even in the 

 beginning of the nineteenth century, many scientists and 

 philosophers of different tendencies and schools, and even 

 more writers and poets, often described in their works various 

 fantastic instances of the spontaneous generation of beasts, 

 fishes, insects and worms, or made it clear that they con- 

 sidered that such a phenomenon was quite possible. 



As observations of nature became more refined and, in 

 particular, knowledge of the structure of living things became 

 more detailed, so it was admitted, though only very giadually, 

 that the spontaneous generation of such complicated things 

 from structureless filth and decaying matter was impossible. 

 In this way the belief in the spontaneous generation of all 



