CHAPTER 18 

 EVOLUTION 



During medieval and ancient times it was commonly believed that 

 living organisms arose spontaneously from nonliving materials. Nu- 

 merous cases which supported this belief could be cited. For example, 

 from where except clay banks could the swallows possibly come? and 

 how could you account for the appearance of numerous toads except to 

 understand that they arose from the mud of stagnant pools? Certainly 

 these seemed indisputable facts. These beliefs persisted for many centu- 

 ries until a series of experiments beginning with those of Redi and cul- 

 minating in the work of Pasteur finally overthrew these ludicrous as- 

 sumptions. 



It was in the year 1668 that Redi published the results of a series 

 of experiments which he had performed. These involved the use of 

 controls and might well be cited as models of the well-performed scien- 

 tific experiment. He placed small pieces of meat in a series of flasks. 

 Some of these flasks were left open, others were covered with gauze, 

 while those of a third group were tightly sealed. Fly larvae developed 

 in the open flasks and on the surface of the gauze, but not in the sealed 

 vessels. He thus conclusively proved that flies were not generated 

 from rotting meat. 



Despite the obvious validity of his experimentation, the results were 

 not universally accepted. In fact, his conclusions gave rise to a storm 

 of criticism which lasted until Pasteur conclusively proved that even 

 bacteria must arise from previously existing ones. This work of Pasteur's 

 was done in 1861, thus bringing an end to two hundred years of dissen- 

 sion. For his experiment, Pasteur utilized a flask with a long S-shaped 

 neck. Within this flask was a broth suitable for the growth of micro- 

 organisms; after the neck was drawn out, the broth was boiled to kill 

 any microorganisms which might be present. Other organisms in the 

 air were prevented from entering due to the shape of the neck. Thus 

 treated, the broth would not ferment. When the S-shaped neck was 



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