SP ONTANEO US GENERA TION. 



85 



The views of Redi prevailed for a century, and the hypothesis of 

 spontaneous generation had become completely discredited. But 

 meantime the microscope had been improved, and a new world of life 

 revealed. When animal or vegetable substances are infused for a time 

 in water, swarms of creatures are produced in it, called infusorial ani- 

 malcuhe, and which are so small that they can only be seen with a pow- 

 erful magnifier. This was a new aspect of the production of life, 

 and favored the view of its spontaneous origin. In the middle of the 

 eighteenth century, an Englishman named Needham took the ground 

 that, although putrefying meat may not engender insects, it may yet 

 give rise to animalculce. " If," said JSTeedham, " these animalcuke come 



Fig. 1. 



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W\ 



K <V%* 



Some op the Most Common Primordial Forms op Life. ( x 800.) 

 A. Plastide-particles ; B. Bacteria ; C. Torula ; D. Vibriones ; E. Spirilla ; F. Leptothrix. 



from germs, these germs must exist, either in the infused substance, 

 the water, or the adjacent air. All germs are killed by heat ; if, there- 

 fore, I boil the infusion, seal it up, and then heat the whole vessel, I 

 shall destroy the germs." He did this ; but, after waiting for a time, 

 the anirnalculse still appeared in the closed vessel. The experiment 

 seemed conclusive in favor of spontaneous generation, or life without 

 germs ; but, again, a learned Italian appeared and attacked the hypothe- 

 sis. Spallanzani repeated Needham's experiments with more vigilant 

 precautions. He closed the tubes more effectually, and exposed them 

 to a greater heat, after which the anirnalculse failed to appear. The 

 real issue in the case was thus fairly reached, and the question became 

 one of the existence of atmospheric germs, and of their power of resist- 

 ing heat. The results of Spallanzani were generally held conclusive 

 against the hypothesis of spontaneous generation ; but, toward the 

 middle of the present century, the question was again opened, and 

 it has been assiduously investigated and hotly discussed by men of 

 science for the last forty years. We cannot even mention the numer- 

 ous contributions to it that have been made by eminent scientists, but 



