78 HABCKEL 



that science had empirically disproved spontaneous 

 generation. An old popular belief held that fleas 

 and lice were born every day from non-living dirt 

 and dust, but that had been refuted long before. 

 No ^gg) ^0 animal : every living thing develops 

 from an egg. This had been laid down as a fixed 

 rule. When the microscope revealed an endless 

 number of tiny creatures in every drop of stagnant 

 water, in the air and the dust and the soil, 

 it was a question whether the rule was not 

 wrong. Surely these simplest of all living things, 

 apparently, were born by spontaneous generation ? 

 However, the question was believed to have been 

 settled in two ways. Schwann, the co-discoverer 

 of the cell-theory, had made certain experiments 

 which seem to prove directly that even these tiny 

 beings, the infusoria and bacteria, were never 

 formed in a vessel containing water and dead 

 matter, if it had been carefully assured beforehand 

 that the minute living germs of these animals that 

 floated in the air could not penetrate into the vessel. 

 At the same time Ehrenberg and others stoutly 

 denied that the infusoria were the "simplest" 

 organisms, or that they could conceivably be bom 

 in that way. They declared that the infusoria 

 were ''perfect organisms" in spite of their small- 

 ness. The belief that these tiny creatures consisted 

 of '' one cell," and so formed, as it were, the 

 ultimate elements of the plant and animal worlds 

 on the lines of the cell-theory, was seriously 

 menaced, and apparently on the way to be destroyed. 

 Finally, the tapeworm and similar parasites had 



