STRUGGLE IN NATURAL CONDITIONS 21 



Let us note another important circumstance connected with com- 

 petition. This phenomenon can be particularly pronounced during a 

 periodical food shortage connected with certain seasons, etc., whilst 

 at another time with an abundance of food it will scarcely take place. 

 This fact has frequently been pointed out in various discussions of 

 the struggle for existence. 



(9) It remains but to give some examples of the struggle for exist- 

 ence among animals in order to show with what problems the zo- 

 ologists have to deal, and how difficult it is to apply here exact quan- 

 titative methods. An instructive example of competition among 

 fishes has been recently described by Kashkarov ('28). It concerns 

 the supplanting of Schizothorax intermedins by wild carp, Cyprinus 

 albus L., in lakes of Middle Asia. Wild carp were introduced into the 

 lake Bijly Kul in 1909. Before that only Schizothorax intermedins 

 with white-fish (Leuciscus sp.) inhabited this lake. Formerly Schizo- 

 thorax intermedins were very numerous, but after the introduction of 

 wild carp their quantity diminished considerably. As an indicator 

 of the relatively small number of Schizothorax intermedins the follow- 

 ing data on the catch may serve: on May 15, 1926, 19 carp and 1 

 Schizothorax were caught in two nets; on May 16, 1926, in the same 

 place the catch was 24 carp and 2 Schizothorax. Schizothorax keeps 

 chiefly to the south-western part of the lake, where there are stones 

 making the casting of nets difficult. Now Schizothorax is disappear- 

 ing even there, as wild carp devour its spawn. The quantity of white- 

 fish also decreases because carp devour its young. The particular 

 interest of this example lies in the fact that a new species not found 

 in a given microcosm before (Cyprinus albus L.) was introduced, and 

 in this way a direct proof of one species displacing another was 

 obtained. 



(10) Processes of this kind can often be observed when fish are 

 introduced into waters to which they are new. Professor G. C. 

 Embody writes in a letter recently received: "Concerning the com- 

 petition between different species of fishes we have two cases in 

 particular in the eastern United States. The European carp was 

 introduced in the '70's, and has now in many streams and lakes 

 multiplied to such an extent that several native species are found in 

 greatly diminished number. This has probably been due to the high 

 reproductive capacity of the carp, food competition, destruction of 

 weed beds by carp, and the fact that very few of them are captured. 



