Living Organisms 733 



from which living organisms could arise later. The living organisms 

 which later arose in his flasks probably arose from living materials which 

 had not been killed by boiling. 



B. Biogenesis (Life From Life) 



Spallanzani (1729-1799) stated that Needham's results were incon- 

 clusive because they were obtained by insufficient sterilization of the in- 

 fusions in the flasks. The chemists also entered the controversy by stat- 

 ing that free oxygen which is an essential material for life processes was 

 excluded by Spallanzani in his experiments, thus preventing the possi- 

 bility of spontaneous generation. To answer the latter point, various 

 investigators during the first half of the last century showed that thor- 

 oughly sterilized infusions never developed living organisms even when 

 sterile air was admitted. The air was sterilized by heating or by passage 

 through acids in order to remove the suspended "dust particles" on 

 which living substances were attached. 



Francisco Redi (1626-1698), an Italian scientist, by a simple experi- 

 ment of protecting decaying meat from contamination by flies, demon- 

 strated that the maggots of these flies did not arise spontaneously from 

 the meat, but that they developed from living eggs deposited by living 

 flies. Redi's results definitely formulated the theory that "life must arise 

 from living organisms" and cannot arise spontaneously from nonliving 

 sources. 



Louis Pasteur (1822-1895), by means of a special type of flask, con- 

 vincingly proved that the source of life, which so rapidly appeared in 

 infusions exposed to the air, was the air itself, or rather the life existing 

 in the air. It was thus contended that much of the dust of the air was 

 made up of microorganisms in a dormant condition ready to become 

 actively alive when suitable environmental conditions, such as moisture, 

 temperature, and food, were encountered. He stated that such organ- 

 isms were the causes of certain chemical changes, fermentations, putre- 

 factions, and certain diseases. Pasteur thus laid a rather accurate foun- 

 dation for the germ theory of diseases and many chemical activities. 



XL ORIGIN OF LIFE ON THE EARTH 



Geologists and astronomers tell us that the condition of the earth at 

 one time was such that life could not exist but that life was finally estab- 

 lished hundreds of millions of years ago and has existed continuously 

 since. 



