io8 ANIMAL ECOLOGY 



to multiply unchecked for several years. Although various 

 interesting calculations have been made about what w^ould 

 happen if unlimited increase took place, and alarming pictures 

 have been drawn of an earth entirely peopled with elephants 

 so closely packed together that they would be unable to sit 

 down except on each others' knees, there are as a matter of 

 actual fact a number of wild animals which habitually multiply 

 for short periods almost at the maximum rate which is theo- 

 retically possible. It is therefore unnecessary to give rein 

 to the imagination in this matter, since we can obtain actual 

 examples. Some small mammals increase in numbers for 

 several years at a very high speed until they reach such an 

 immense abundance that malignant epidemic diseases break 

 out and wipe out the major part of the population, and those 

 which are left start again on another cycle of increase. Mice 

 do this. Such an over-increase of mice has been described 

 very vividly by Holinshed,^^ who wrote of a mouse plague in 

 1581 : " About Hallontide last past, in the marshes of Danesy 

 Hundred, in a place called South Minster, in the county of 

 Essex . . . there sodainlie appeared an infinite number of 

 mice, which overwhelming the whole earth in the said marshes, 

 did sheare and gnaw the grass by the rootes, spoyling and taint- 

 ing the same with their venimous teeth, in such sort that the 

 cattell which grazed thereon were smitten with a murraine and 

 died thereof." In 1907 another such *' plague " occurred in 

 Nevada, during which 15,000 out of 20,000 acres of alfalfa 

 were completely destroyed.^^ The natural increase of the 

 field-mice {Microtus) was so terrific that the ground was in 

 many places riddled with holes for miles. A Frenchman, 

 describing a similar outburst in Europe, said that the ground 

 was so perforated with holes as to resemble a sieve. In Nevada 

 it was estimated that there were some 3,000 birds of prey and 

 carnivorous mammals at work in the " plague " district, that 

 these would be destroying about a million mice or more every 

 month, and that this made no appreciable difference to the 

 numbers. Again, one and a half million mice were killed in 

 a fortnight in one district in Alsace during a great out- 

 break in 1822,11^ while during the mouse plague in 19 17 in 



