VARIATIONS IN THE NUMBERS OF ANIMALS 137 



maximum abundance. Later on he says : " Eventually, evidence 

 of the inevitable decline arrives. Empire among the rabbits as 

 elsewhere has its rise and fall, and then is swept away. A 

 strange peril stalks through the woods ; the year of death 

 arrives. An odd rabbit drops off here and there, then twos 

 and threes, then whole companies die, until the appalling 

 destruction reduces the woods to desolation. . . . One year 

 (19 1 7) in the district of Sudbury, northern Ontario, the signs 

 of rabbits were everywhere, but not a single rabbit could I 

 start. It seemed incredible. Local inquiries disclosed that 

 a little over a year before the Lepus population was beyond 

 count. Now, as if by magic, they were gone. Needless to say, 

 however, a few individuals survive the epidemic. These now, 

 because of their paucity, are seldom encountered." 



13. Marmots, muskrats, and numerous other rodents also 

 fluctuate with a more or less regular periodicity. Seton 1^ 

 says " the muskrat's variation probably has relation chiefly to 

 the amount of water, which, as is well known, is cyclic in the 

 North-West." 



In the case of lemmings, some mice, the varying hare, and 

 the muskrat, it is practically certain that the cycle of numbers 

 is partly under the control of regular cycles in climate ; and 

 this is also probably the case with many other rodents about 

 which we have at present Httle information beyond the fact 

 that their numbers do fluctuate. At the same time there is 

 no reason why such wave-motions in the population should 

 not be automatic, in so far as each epidemic is followed by a 

 period of recovery which would culminate in another epidemic, 

 and so on indefinitely. It seems possible that this may apply 

 to the rodents in some countries which show no regular or 

 synchronous over-increase. The mouse-plagues in France 

 are often local and always irregular, although sometimes they 

 may occur simultaneously over large areas. Here variations 

 in climate probably play at some times a more important part 

 than at others. But on the whole, from what little we know 

 about the matter at present, it appears that periodic fluctuations 

 in the numbers of rodents are primarily caused by the irregular 

 behaviour of their environment. After all, any species must 



