III. THE GROWTH OF ANIMALS AND THE 

 FUNCTIONS OF THEIR ORGANS 



Spontaneous generation. — A jar of water containing a 

 handful of hay wiU liberally swarm with microscopic ani- 

 mals at the end of a week if left standing in a warm room. 

 Apparently these animals arise spontaneously or spring, 

 as it were, from nothing in the way of animal Ufe. Formerly 

 zoologists called this sudden appearance of so many active 

 animals a case of spontaneous generation. 



Ancient zoologists held that flies were spontaneously 

 generated in the flesh of dead animals; and that frogs, 

 toads, and reptiles might be produced from the moist, slimy 

 earth of rivers and seas. We now know that flies, frogs, 

 and toads develop from eggs deposited in favorable places 

 by the female parents. And Pasteur has shown that when 

 infusions of hay are boiled and hermetically sealed they may 

 be kept indefinitely without a trace of animal Hfe. More- 

 over, it has been demonstrated that many plants and ani- 

 mals produce great numbers of tiny cells known as spares 

 or germs, and that these fall upon grass and in the water. 

 Therefore, when these two substances are put together in a 

 suitable temperature, the spores germinate and produce 

 swarms of minute animals; but if the spores are killed by 

 boiling, no animals appear in the infusion. It would seem, 

 then, that there is no such thing as spontaneous generation, 

 but that all hfe comes from some life that went before. 



18 



