22 THE STRUGGLE FOR EXISTENCE 



Carp are not used as extensively for food in America as in Europe and 

 in our smaller lakes are not generally fished for commercially. The 

 other case is the introduction of the perch (Perca flavescens) into cer- 

 tain lakes in the Adirondacks and in Maine, which were naturally 

 populated with the trout (Salvelinus fontinalis). The competition 

 for food is believed to be one of the causes for the decrease in the num- 

 ber of the trout." 



"These cases are both matters of general observation. I do not 

 know of any papers describing them nor in fact, dealing with this 

 subject in American waters." 



(11) Another example of competition is the replacing of one species 

 of cray-fish by another in certain waters of Middle Russia. Some 

 observations on this were made by Kessler (75) and recently by Bir- 

 stein and Vinogradov ('34). Two species of cray-fish inhabit the 

 waters of European Russia: the broad-legged (Pota?nobius astacus L.) 

 and long-legged (Potamobius leptodactylus Esch.). The broad-legged 

 cray-fish is distributed in the western part, and the long-legged in the 

 south-eastern one, but the areas of their distribution largely overlap 

 one another. It is observed that the long-legged cray-fish displaces 

 the broad-legged one and spreads gradually more and more to the 

 west. It has been possible to establish this replacement with particu- 

 lar distinctness in White Russia (in the western part of U. S. S. R.). 

 The cray-fish are found there in lakes isolated from each other, and 

 most of the lakes are inhabited only by the broad-legged cray-fish. 

 In some cases long-legged cray-fish were put into such lakes from 

 other waters. As a result the broad-legged cray-fish began to de- 

 crease, and finally disappeared completely leaving the lake populated 

 exclusively by the long-legged species. The following examples can 

 be given. (I) Black lake (White Russia) was populated only by the 

 broad-legged cray-fish. In 1906, 500 specimens of long-legged cray- 

 fish were introduced, and now (1930) only this species remains. (II) 

 Forest lake (same region). Up to 1920 there were no long-legged 

 cray-fish there. Later on they were introduced, and at present (1930) 

 there is a considerable number of this species. The causes why one 

 species of cray-fish is replaced by another have scarcely been studied. 



(12) Curious are the observations reported by Goldman ('30) on 

 the competition among predators belonging to different species. 

 Thus, according to a resident of Telegraph Creek near the Stikine 

 River, Canada, no coyotes were known in that section prior to 1899. 



