EVOLUTION TRACED BIOCHEMICALLY 39 



mouthed jar and the mouth covered w^ith thin gauze per- 

 mitting free entrance of air and w^armth but excluding 

 flies and other insects, no larvae of any form made their 

 appearance in the medium. 



The results of similar carefully made experiments disposed 

 of a number of instances of supposed spontaneous generation, 

 but the theory was maintained, nevertheless, and it was 

 generally held that the microorganisms which the micro- 

 scope revealed to observers of the seventeenth and eighteenth 

 centuries did not arise from preexisting forms, a view 

 stoutly maintained by Needham and Buffon. Needham, 

 when he heated infusions of animal and vegetable matter 

 so as to destroy germs existing therein and kept them for a 

 few days at room temperature, found these to contain 

 swarms of animalculae which he concluded must have 

 arisen from non-hving matter. Bufl'on repeated Needham's 

 experiments and confirmed his results and conclusions. 

 Spallanzani, however, found that no organisms appeared 

 in such infusions if they were heated for half an hour and 

 kept in flasks hermetically sealed by fusing their necks in the 

 flame. This result was regarded as decisive until after the 

 discovery of oxygen by Priestley in 1774, when it was 

 maintained that from Spallanzani's infusions oxygen of the 

 air was excluded, and that this element was necessary 

 there to permit of spontaneous generation of organisms 

 in it. The discussion on the subject was continued, conse- 

 quently, until in 1856 Schroeder and von Dusch, and finally, 

 in 1859, Schroeder, found that in infusions treated as in 

 Spallanzani's experiments, but contained in flasks closed 

 by plugs of cotton wool which aUowed free access of air, 

 but no air-borne organisms, to the infusions, no hving forms 

 developed therein. This result has been confirmed a countless 

 number of times in the last seventy years, and today no 

 one beheves that Hving forms do spontaneously originate 

 under the conditions which now obtain on the surface 

 of the earth. 



This turned attention to other possible explanations of 

 the origin of terrestrial Hfe, and in 1871 Lord Kelvin, then 

 Sir William Thomson, advanced the view that hfe began 

 on the earth when fragments of shattered hfe-bearing 



