16 THE STRUGGLE FOR EXISTENCE 



data concerning wild-growing plants have been recently published 

 by Sukatschev ('27) in his "Experimental studies on the struggle for 

 existence between biotypes of the same species." First of all he 

 studied the competition between local biotypes of the plant, Taraxa- 

 cum officinale Web., from the environs of Leningrad. These biotypes 

 were cultivated in similar conditions with a fixed distance between 

 the individuals, and the experiments led Sukatschev to the following 

 conclusions: (a) One must rigorously distinguish the conditions of 

 the struggle for existence in a pure population, formed by a single 

 biotype, and in a mixed population, consisting of various biotypes. 

 (b) It is to be noted that a biotype which shows itself to be the most 

 resistant in an intrahiotic, struggle for existence, may turn out to be 

 the weakest one in an interhiotic, struggle between different biotypes 

 of the same species, (c) The increase in mutual influence of plants 

 upon each other with an increase in density of the plant cultures, may 

 completely reverse the relative stability of separate biotypes in the 

 process of the struggle for existence. The biotypes yielding the great- 

 est percentage of survivors under a small density of cultivation may 

 occupy the last place in this respect in conditions of a dense culture. 

 This can be illustrated by Table III. 



If we arrange the biotypes mentioned in Table III according to de- 

 creasing stability, we shall find that in the conditions of a not dense, 

 pure culture : C > A > B, i.e., the biotype C gives the smallest percent- 

 age of non-survivals and is the most resistant, whilst the biotype B is 

 the weakest of all. In dense pure cultures the relations are entirely 

 different: B > A > C, i.e., the biotype B is the most stable one. 

 Lastly, for dense, but mixed cultures, we have: C > A > B. Almost 

 similar data have been obtained by Montgomery ('12) in studying 

 the competition between two races of wheat. 



In another series of experiments Sukatschev ('27) studied the 

 struggle for existence between biotypes of various geographical origin 

 (from various parts of U. S. S. R.), and inferred the following: (a) 

 Judging by the percentage of non-survivals, one can say that in pure, 

 as well as in mixed not-dense cultures, the dying-off is chiefly due to 

 the influence of physico-geographical factors. Therefore, the bio- 

 types originating from geographical regions strongly differing from 

 a given region in their climate, turn out to be less resistant, as com- 

 pared with the local biotypes. But these relations can change under 

 the influence of aggregation, (b) In dense mixed cultures, if we are 



