MICROBE AXD LIFE 



atmosphere, gave rise to the appearance of the ozone layer, screening 

 out the biocidal short-wavelength ultraviolet solar radiation, and thus 

 for the first time offering life the opportunity to leave the hydrosphere 

 and to invade the mainland. The first terrestrial inhabitants will also 

 have exhibited a photo-autotrophic metabolism, and consequently 

 belonged to the green plant kingdom. But the most beneficial effect 

 of the birth of free oxygen on earth was undoubtedly that this gas 

 lends itself pre-eminently to serve in its turn as hydrogen acceptor. 

 The oxygen, and the organic matter that becomes available as hydro- 

 gen donator upon the death of photoautotrophic organisms, together 

 constitute the system on which is based the aerobic mode of life. It 

 must furthermore be realized that not until the gaseous hydrogen ac- 

 ceptor, oxygen, had appeared did the conquest of the atmosphere as 

 a realm of life for the animal organism become possible. 



Herewith I bring to an end my sketch of evolution; if one desires to 

 acquire an idea of the manner in which aerobic life evolved from its 

 inception to the present-day plant and animal kingdoms, one has to 

 sit at the feet of the paleontologist. I have wished merely to indicate 

 that a knowledge of microbial metabolism such as, even to-day, it 

 presents itself to us in its surprising diversity, can unquestionably con- 

 tribute to the acquisition of some insight into the first, and as yet still 

 obscure, phases of evolution. 



My audience may pardon me this effusion. I realize that the discussion 

 concerning the evolution of life on earth must have left many of you 

 with the impression that I was spinning a fairy tale. I shall not excuse 

 myself by referring to the fact that the eminent savant who addressed 

 the meeting of the combined sections a year ago also requested your 

 attention for a fairy tale; but I should like to remark that, if a fairy 

 tale be defined as a story that contains elements not immediately ac- 

 ceptable to human reason, a survey of results of investigations in very 

 diverse branches of natural science must almost always create the 

 impression of a fairy tale in the outsider. Let me illustrate this with 

 an example. 



I shall choose the physicist's concept of the structure of matter. The 

 layman, examining for instance a sugar crystal, will perceive something 

 altogether static, and conclude to a fully homogeneous composition. 

 Nevertheless the physicist, with a pitying expression, will explain to 



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