536 BIOLOGY FOR BEGINNERS 



At that time Pasteur and Tyndall showed that bacteria would 

 not develop except when the medium had been exposed, and so 

 proved, even for these minute plants, that bacteria were produced 

 by bacteria, and in no other way. 



The idea that life could come from dead matter was called the 



FIG. 163. William Harvey. 1578-1667. From Locy. 



theory of " spontaneous generation," and died hard. This is now 

 replaced by the belief that " all life comes from life." 



Oxidation. We talk freely of oxygen and oxidation, but oxygen 

 was not discovered until 1774 when Priestley obtained it and 



