20 THE STRUGGLE FOR EXISTENCE 



the observations in 1923, the island Jorilgatch (Black Sea) is in- 

 habited by a nesting colony of terns, consisting of many hundreds of 

 individuals. The nests of the terns are situated close to one another, 

 and the colony presents a whole system. The entire mass of individ- 

 uals in the colony belongs to four species (sandwich-tern, Sterna 

 cantiaca; common-tern, S. fluviatilis; blackbeak-tern, S. anglica; and 

 little-tern, S. minuta), and together they chase away predators (hen- 

 harriers, etc.) from the colony. However, as regards the procuring 

 of food, there is a sharp difference between them, for every species 

 pursues a definite kind of animal in perfectly definite conditions. 

 Thus the sandwich-tern flies out into the open sea to hunt certain 

 species of fish. The blackbeak-tern feeds exclusively on land, and 

 it can be met in the steppe at a great distance from the sea-shore, 

 where it destroys locusts and lizards. The common-tern and the 

 little-tern catch fish not far from the shore, sighting them while flying 

 and then falling upon the water and plunging to a small depth. The 

 light little-tern seizes the fish in shallow swampy places, whereas the 

 common-tern hunts somewhat further from the shore. In this man- 

 ner these four similar species of tern living side by side upon a single 

 small island differ sharply in all their modes of feeding and procuring 

 food. 



(8) Another ecological notion is also important in connection with 

 our experiments. We have in view the degree of isolation of the 

 microcosm. The point is that our experimental researches have been 

 mainly made in isolated microcosms, i.e., in test tubes filled with 

 nutritive medium and stopped with cotton-wool. It must be re- 

 membered that the degree of isolation of different communities in 

 natural conditions is very different. Such a system as a lake is almost 

 isolated, but at times some of the animals inhabiting it go on land. 

 An oasis in the desert would also seem to be isolated, but for instance 

 some of the species of birds fly away for the winter, and consequently 

 there is no real isolation. The habitats not so sharply separated from 

 the surrounding life-area are, therefore, still less isolated. All this 

 emphasizes the idea already expressed, that the regularities observed 

 in isolated microcosms hold true only under certain fixed conditions, 

 and are not sufficient to explain all the complicated phenomena tak- 

 ing place in nature. We shall have an opportunity to appreciate the 

 role of this factor when experimentally studying the predator-prey 

 relations. 



