STRUGGLE IN NATURAL CONDITIONS 19 



of the given locality. Even when the species that has first established 

 itself is somewhat weaker than another species in the same habitat, 

 it can for a comparatively long time resist its stronger competitor 

 simply because it was the first to occupy this place. Only in case 

 of a considerable weakness of the first comer will its domination be 

 merely a temporary one, and the effect of the first accidental appear- 

 ance will be rapidly eliminated (E. Warming ('95), Du-Rietz ('30)). 



(6) Let us recapitulate briefly our discussion up to this point. 

 Botanists have endeavored to investigate the struggle for existence 

 by experimentation and under simplified conditions, but they are 

 only beginning to analyze these phenomena. Their experiments are 

 commonly limited to the process of ontogenetic development, and 

 in only a few cases, chiefly concerned with competition in cereals, has 

 displacement of some forms by others been traced through a series of 

 generations (Montgomery ('12) and others). As concerns animals we 

 have simply no exact data, and can only mention a few general prin- 

 ciples which have been developed by zoologists in connection with the 

 phenomena of competition. 



One of these ideas is that of the "niche" (see Elton, '27, p. 63). A 

 niche indicates what place the given species occupies in a community, 

 i.e., what are its habits, food and mode of life. It is admitted that as 

 a result of competition two similar species scarcely ever occupy simi- 

 lar niches, but displace each other in such a manner that each takes 

 possession of certain peculiar kinds of food and modes of life in which 

 it has an advantage over its competitor. Curious examples of the 

 existence of different niches in nearly related species have recently 

 been obtained by A. N. Formosov ('34). He investigated the ecology 

 of nearly related species of terns, living together in a definite region, 

 and it appeared that their interests do not clash at all, as each species 

 hunts in perfectly determined conditions differing from those of 

 another. This once more confirms the thought mentioned earlier, 

 that the intensity of competition is determined not by the systematic 

 likeness, but by the similarity of the demands of the competitors upon 

 the environment. Further on we shall endeavor to express all these 

 relations in a quantitative form. 



(7) The above mentioned observations of A. N. Formosov on 

 different niches in nearly related species of terns can be given here 

 with more detail, as the author has kindly put at our disposal the 

 following materials from his unpublished manuscript: According to 



