THE PAEAMECIUM AND ITS ALLIES 223 



held that many of the larger animals, such as eels, bees, 

 and flies, were generated without parents. This was the 

 theory of "spontaneous generation." In time this theory 

 became much more restricted. It was found that the 

 maggots in putrid meat are not generated "spontaneously" 

 out of the meat, but are derived from flies' eggs, and, in 

 their turn, develop into fertile flies. But the idea that 

 Infusoria are formed out of inorganic material continued 

 to be held until much more recently, until Pasteur, Tyn- 

 dall, and others demonstrated that fluids heated to a 

 sufficiently high temperature for a sufficient time, and 

 then, while hot, sealed from contact with air, do not 

 develop Infusoria, no matter how long they may be kept. 

 This method of excluding Infusoria and other minute 

 organisms, especially bacteria, is employed to-day in can- 

 ning meat, vegetables, and fruit. The experiments re- 

 ferred to gave a death-blow to the theory of spontaneous 

 generation, and led to the conclusion that all Infusoria are 

 derived from living germs. 



Whence the living germs come which enter the water 

 it is not difficult to determine. Many Infusoria can pass 

 into a quiescent " spore " stage in which they may be dried 

 and blown about without loss of life. Dry grass, straw, 

 and other substances contain some of the germs, and others 

 float in the air and fall as dust into the water. Even 

 drinking water may contain here and there an infusorian 

 or its germ. When, therefore, one fills a clean vessel 

 with pure water, and puts hay or dry leaves in it, and lets 

 it stand open to the air in a warm place, the result is 

 pretty sure to ba that germs develop in the mixture. 

 The heat and the organic infusion merely facilitate this 

 development. 



