88 ANIMAL LIFE AND SOCIAL GROWTH 



of cannibalism practised which has a definite rela- 

 tion to the degree of crowding. 



In nature the problem which faces animal 

 communities is never so simple. Many more 

 factors are continually fluctuating; temperature 

 values change in daily, yearly and in longer cycles. 

 Periods of relatively mild winters are followed by 

 years when the winters are unusually cold; sim- 

 ilarly there are cycles of drought that may be 

 seasonal or may run over periods of several years. 

 The exact number of factors in the physical 

 environment that show similar daily, seasonal 

 or longer cycles, would be difficult to list. These 

 may all affect the numbers of animals that may 

 be found living in a given habitat at any given 

 time. The problem which the animals have to 

 face is further complicated by the fact that the 

 numbers of plants or even of animals present also 

 affect the numbers that the habitat can support 

 both of their own and of related species. Some 

 of these interlocking and fluctuating relationships 

 in community life have been pointed out in the 

 opening chapter in connection with the animal 

 life of lakes; population cycles among many land 

 animals are even more striking. 



One of the best studied cases of apparent un- 

 balance in nature is that of the lemming popula- 

 tions of the arctic regions. Lemmings are small 

 animals related to rats and mice; they are about 



