22 B A CTERIOL OG Y. 



blood of animals dead of splenic fever, and with the 

 progress of knowledge upon the parasitic nature of cer- 

 tain diseases of plants, the old question of "contagium 

 aniraatum" again began to receive attention. It was 

 taken up 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 the "animalcules." Do 

 they generate spontaneously, or are they the descendants 

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

 important question. Among the earlier participants 

 in this discussion were many of the most distinguished 

 men of the day. 



In 1749 Needham, 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 his reasons for 

 holding this view. He maintained that the bacteria 

 which appeared alxjut a grain of barley germinating in 

 a carefully covered watch-crystal of water were the 

 result of changes going on in the barley-grain itself, 

 incidental to its germination. 



Spallanzani, in 1769, drew attention to the laxity of 

 Needham's experimental methods, and demonstrated that 

 if infusions of decomposable vegetable matter be placed 

 in flasks, which, after being hermetically sealed, were 

 heated for a time in boiling water, no living organisms 

 would be detected in them, nor would decomposition 

 appear in the infusions so treated. The objection 

 raised by Treviranus, viz., that the high temperature 



