18 BAGTEBIOLOGY. 



all animals dead of splenic fever, and with the progress 

 of knowledge upon the parasitic nature of certain dis- 

 eases of plants, the old question of " contagium ani- 

 matum" again began to receive attention. It was taken 

 np by Henle, and it was he who first logically taught 

 this doctrine of infection. 



The main point, however, that had occupied the atten- 

 tion of scientific men from time to time for a period of 

 about two hundred years subsequent to Leeuwenhoek's 

 discoveries was the origin of these bodies. Do they 

 generate spontaneously, or are they the descendants of 

 pre-existing creatures of the same kind ? was the all- 

 important question. Among the participants in this 

 discussion were many of the most distinguished men of 

 the day. 



In 1749 TSTeedham, who held firmly to the opinion 

 that the bodies which were attracting such general atten- 

 tion developed spontaneously, as the result of vegetative 

 changes in the substances in which they were found, 

 attempted to demonstrate by experiment the grounds 

 upon which he held this view. He maintained that 

 the bacteria which were seen to appear around a grain 

 of barley which was allowed to germinate in a watch- 

 crystal of water, which had been carefully covered, were 

 the result of changes in the barley-grain itself inci- 

 dental to its germination. 



Spallanzani, in 1769, drew attention to the laxity of 

 the methods employed by Needham, and demonstrated 

 that if infusions of decomposable vegetable matter were 

 placed in flasks, which were then hermetically sealed, 

 and the flasks and their contents allowed to remain 

 for a time in a vessel of boiling water, neither living 

 organisms'"_^could be detected nor would decomposition 



