84 THE STRUGGLE FOR EXISTENCE 



alcohol produced by it is so to say "more toxic" for Saccharomyces 

 than the alcohol produced by the latter itself. All this tends to 

 imply that the situation is here complicated by the influence of certain 

 other waste products getting into the surrounding medium in small 

 quantities. The relations between species in these experiments are 

 therefore not so simple as has been supposed at the beginning of this 

 section. 



(1) The above described experiments of 1931 were repeated in 

 1932, and the new data confirmed all the observed regularities. In 

 these new experiments the influence of oxygen upon the growth of a 

 mixed population of the same two species of yeast was investigated, 

 and this enabled us to further somewhat our understanding of the 

 nature of the competitive process. 



The experimental data given in the preceding section have to do 

 with the growth of a yeast population under "anaerobic conditions," 

 i.e., in test tubes. In order to study the influence of oxygen on the 

 growth of the yeast population, together with experiments in test 

 tubes we arranged other experiments under conditions of somewhat 

 better aeration. The technique of such "aerobic" and "anaerobic" 

 experiments has already been described at the beginning of this 

 chapter. Here it must only be remarked that in the "aerobic" series 

 the access of oxygen was very limited, and a part of the available 

 energy was, as before, obtained by our species through alcoholic 

 fermentation. As a result, a considerable amount of alcohol accumu- 

 lated in the nutritive medium (as will be seen in the corresponding 

 tables), and in its essential features the mechanism limiting the 

 growth of the yeast population remained the same. The experiments 

 of 1932 consisted of two aerobic and two anaerobic series. In them 

 168 separate microcosms were studied. 



In all the experiments of 1932 nutritive medium of the same prep- 

 aration was used. It was made according to the usual method, but 

 the dry beer yeast was of another origin. As a result, the absolute 

 values of growth were somewhat different. It must also be remarked 

 that in all the new experiments the centrifuged volume of yeast was 

 always reduced to 10 cm 3 of nutritive medium. 



(2) Figure 16 represents the growth curves of Saccharomyces, 

 Schizosaceharomyces and of the mixed population according to two 



