xv.] SPONTANEOUS GENERATION. 357 



the experiments and the conclusions of Needham to a 

 searching criticism. It might be true that Needham's 



experiments yielded results such as he had described, but 

 did they bear out his arguments ? Was it not possible, 

 in the first place, that he had not completely excluded 

 the air by his corks and mastic ? And was it not pos- 

 sible, in the second place, that he had not sufficiently 

 heated his infusions and the superjacent air? Spal- 

 lanzani joined issue with the English naturalist on 

 both these pleas, and he showed that if, in the first 

 place, the glass vessels in which the . infusions were 

 contained were hermetically sealed by fusing their 

 necks, and if, in the second place, they were exposed 

 to the temperature of boiling water for three-quarters 

 of an hour, 1 no animalcules ever made their appearance 

 within them. It must be admitted that the experi- 

 ments and arguments of Spallanzani furnish a complete 

 and a crushing reply to those of Needham. But we 

 all too often forget that it is one thing to refute a prop- 

 osition, and another to prove the truth of a doctrine 

 which, implicitly or explicitly, contradicts that prop- 

 osition, and the advance of science soon showed that, 

 though Needham might be quite wrong, it did not 

 follow that Spallanzani w^as quite right. 



Modern chemistry, the birth of the latter half of the 

 eighteenth century, grew apace, and soon found herself 

 face to face with the great problems which biology had 

 vainly tried to attack without her help. The discovery 

 of oxygen led to the laying of the foundations of a 

 scientific theory of respiration, and to an examination 

 of the marvellous interactions of organic substances 

 with oxygen. The presence of free oxygen appeared 

 to be one of the conditions of the existence of life, and 



1 Sec Spallanzani, " Operc," vi., pp. 42 and 51. 



