VARIATIONS IN THE NUMBERS OF ANIMALS 135 



density of population is reached ; secondly, the natural 

 tendency to increase which leads to recovery of their numbers 

 after the epidemic ; and thirdly, the climatic factor which in 

 some way controls this cycle. It might be thought that the 

 lemming is exceptional among mammals in having such marked 

 and regular fluctuations in its population. This is not the 

 case. All small rodents, in all parts of the world for which 

 there are any data, undergo periodic fluctuations of the order 

 of three or four years. Wild rats, mice, hamsters, mouse- 

 hares, gerbilles, etc., all appear to have this as their regular 

 mechanism of number-regulation. The general fact has been 

 noticed in a dozen different countries, e.g. England, Scandi- 

 navia, Central Europe, France, Italy, Palestine, Siberia, 

 Canada, United States, South Africa, and to some extent 

 Brazil and India. The larger rodents also undergo similar 

 fluctuations in which epidemics play an important part. 

 Squirrels in North America, Europe, and Asia have periodic 

 maxima in numbers separated by intervals of five to ten years. 

 Some of these maxima are associated with huge migrations 

 like those of the lemming. In 18 19 a vast army of grey 

 squirrels swam across the Ohio River a hundred miles below 

 Cincinnati ,101 while in 1897 a great swarm of the same sort 

 passed through Tapilsk in the Ural Mountains : a solid army 

 marched through for three days, only . stopping at night, 

 and they also swam across the river.^s In Denmark ^7 and 

 Norway ^Sc there are also periodic variations in squirrel 

 numbers. 



12. Rabbits and hares and jack- rabbits are subject to 

 violent fluctuations in numbers in most parts of the world 

 {e.g. North America from Alaska to Utah and California ; and 

 Siberia). The best-known example is that of the varying 

 hare or snowshoe rabbit {Lepus americanus) ^ which, inhsihits 

 Canada, and whose cyclical increase and decrease have long 

 attracted attention owing to the effect that they have upon the 

 numbers of valuable fur-bearing mammals which subsist on 

 rabbits. The increase is partly due to the natural recovery after 

 epidemics, and probably in part to climatic influences which 

 speed up the rate of reproduction in certain years.^^ 



