STRUGGLE IN NATURAL CONDITIONS 



17 



to judge by the percentage of perished individuals, it is not the local 

 biotypes that appear the most resistant in the struggle for existence, 

 but those introduced from other regions, (c) The struggle for exist- 

 ence in mixed cultures of various biotypes is not so keen as that in 

 pure cultures of separate biotypes with the same density. 



The analysis of the struggle for existence in mixed populations of 

 plants is now only at its very beginning. The exact data are few in 

 number, but there exist numerous observations on the stratified 

 distribution of plants (see Alechin, '26) , considering these strata as a 

 result of complex processes of competition and adaptation of the 

 plants to one another in mixed cultures. 



(4) Plants are also very favorable for the study of the influence of 



TABLE III 



Percentage of eliminated individuals in three biotypes, A, B and C of Taraxacum 



officinale 

 From Sukatschev ('27) 



environment upon the struggle for existence in mixed populations. 

 DeCandolle was already interested in this question in 1820, but as 

 regards exact experimental researches very little has been done until 

 quite recently, when the works of Tansley ('17) and others appeared. 

 These investigations show that in pure cultures two species can grow 

 for a certain time on various soils, but each of the species has an 

 advantage over the others in particular soil conditions. Therefore in 

 mixed population in some soils the first species displaces the second, 

 but in others the second species displaces the first. 1 The actual rela- 



1 Among animals such observations have been recently made by Timofeeff- 

 Ressovsky ('33), who studied the competition between the larvae of Drosophila 

 melanogaster and Drosophila funebris under different temperatures. We can 

 mention also certain interesting data of Beauchamp and Ullyott ('32) : "When 



