62 THE STRUGGLE FOR EXISTENCE 



in certain cases has already been verified, can serve as a guide for the 

 study of the very mechanism of the influence of one species on the 

 growth of another. 



ii 



(1) To verify our differential equations of the struggle for exist- 

 ence we had recourse to populations of yeast cells. Yeast cells were 

 cultivated in a liquid nutritive medium, where they were nourished 

 by various substances dissolved in water and excreted certain waste- 

 products into the surrounding medium. Owing to the considerable 

 practical importance of yeast for the food industry a great number of 

 papers has been devoted to investigation of its growth, and although 

 the majority deals with purely practical questions that do not at 

 present interest us, nevertheless it is pretty well ascertained what 

 substances yeast requires for its growth, and what is the chemical 

 composition of the waste-products it excretes. 



For the study of competition we took two species of yeast: (1) a 

 pure line of common yeast, Saccharomyces cerevisiae stock XII, 

 received from the Berliner Gahrungsinstitut, and (2) a pure line of 

 the yeast Schizosaccharomyces kephir, cultivated in the Moscow In- 

 stitute of the Alcohol Industry and obtained from Dr. Pervozvansky. 1 

 Both these species can grow under anaerobic conditions as well as 

 when oxygen is accessible. It is very well known that the processes 

 of life activity are connected with a continuous consumption of 

 energy which is supplied by certain chemical reactions. In the case 

 when the growth of yeast proceeds in the absence of oxygen it is the 

 decomposition of sugar into alcohol and carbon dioxide which fur- 



1 We began our experiments with yeast in 1930. The first group of experi- 

 ments on competition between species was made in September-December, 1931, 

 and appeared in the Journal of Experimental Biology (Gause, '32b). These 

 experiments were extended and repeated in September-December, 1932. Their 

 results coincided completely with the data of 1931. Later it appeared that the 

 yeast culture kept in the Museum of the Institute of the Alcohol Industry 

 under the name of "Schizosaccharomyces kephir" and used under the same 

 name in our experiments, has been incorrectly determined by the specialists of 

 the Museum and that it belonged to another species. The culture consists of 

 oval, budding yeast cells much more minute than Saccharomyces cerevisiae and 

 producing an alcoholic fermentation. An exact systematic determination 

 presented extreme difficulty and seemed not to be indispensable, as this culture 

 is kept in the Museum and can be obtained thence under the name of "Schizo- 

 saccharomyces kephir." 



