24 



THE STRUGGLE FOR EXISTENCE 



ance of the prey in years especially favorable for its multiplication. 

 This frequently happens with wild mice, and it gives us the possibility 

 of tracing the process of their being devoured by the predator. In 

 this connection we may mention the following observations recently 

 made by Kalabuchov and Raewski ('33) in the North Caucasus: 

 "The picture of the destruction of mice by different predators is a 

 curious one. At the beginning of the destruction about the same 

 number of rodents is devoured daily. But as the density of rodents 

 diminishes it becomes more and more difficult to catch them, and 

 the number of mice devoured gradually decreases. Finally a time 

 comes when the relation between the density of the rodents, the pres- 

 ence of cover or refuge (burrows, vegetation, etc.) and the biological 



Fig. 2. The destruction of mice by different predators in heaps of chaff 



peculiarities of the predators becomes such that the latter can devour 

 the rodents only in rare cases. In this way the number of the rodents 

 remains about constant. 



"The data on the change in the number of mice near the village 

 of Kambulat are a good illustration of this regularity. Having estab- 

 lished that the destruction of mice in this locality was due to owls, 

 polecats and other predators that devoured them, we obtained the 

 following picture of the change in the number of the rodents in heaps 

 of chaff (Fig. 2). This figure shows that with a density of 2.5 - 0.8 

 mice per mt 3 of chaff we have conditions in which the destruction of 

 the rodents by the predators became so rare that their number 

 scarcely varied." 



