VARIATIONS IN THE NUMBERS OF ANIMALS 141 



case whether disease is the cause of this scarcity, but it seems 

 very probable. Again, Fleming,^i« speaking of the wild deer 

 in England in 1834, refers to an epidemic *' which from time 

 immemorial had broken out at irregular intervals and swept 

 off thousands of these animals." Percival i^k says of the 

 African zebra that ** the free animal is carried off in considerable 

 numbers by periodic outbreaks of disease, which has been 

 traced to a lungworm." 



18. It should be sufficiently clear by now that the numbers 

 of many animals are subject to great fluctuations from year to 

 year, and that, in the majority of cases which have been in- 

 vestigated, these fluctuations can be traced ultimately to pulsa- 

 tions or changes in the environmental conditions affecting the 

 animals. If we follow up the implications of these facts it 

 becomes possible to see why so many animals appear at intervals 

 in vast plagues, often of great economic importance. Every 

 herbivorous animal is adapted to increase at such a rate that 

 enough extra individuals are produced to satisfy the require- 

 ments of its carnivorous enemies ; while the latter, being 

 usually larger, cannot increase so fast as their prey, and are 

 accordingly adapted to a certain rate of increase which will not 

 cause them to over-eat their food-supply. There are therefore 

 small herbivores increasing fast, and larger carnivores increasing 

 more slowly. The larger they are the more slowly they in- 

 crease ; this is a result of size- differences, but at the same time 

 acts as an adaptation, in that it makes possible the existence 

 of the food-cycle at all. Now, suppose one of these small 

 herbivores — a mouse or an aphid — is suddenly able to accele- 

 rate its rate of increase, either as a population or as an individual. 

 The change might be caused by a favourable winter which 

 would enable the population to start in spring with a larger 

 capital of numbers than usual. Again, it might be brought 

 about by increased numbers of young being produced in each 

 brood, as in the case of the varying hare or the short-eared 

 owl. If the herbivore accelerates its rate of increase in this 

 way, the carnivore, being larger and with slower powers of 

 increase, is entirely unable to control the numbers of its prey 

 any longer. And if the latter continues to increase at this 



