INCREASE AND DECREASE OF ANIMALS. 137 



makes manifest. In the years 1819 and 1820, all 

 the country about us was overrun with mice ; they 

 harboured under the hassocks of our coarse grasses 

 (air a caspitosa) , perforated the banks of ditches, 

 occasioned much damage by burrowing into our 

 potato heaps, and coursed in our gardens from bed 

 to bed even during daylight. The species were 

 the short-tailed meadow mouse, and the long-tailed 

 garden mouse, and both kinds united in the spring 

 to destroy our early-sown pease and beans. In the 

 ensuing summer, however, they became so greatly 

 reduced, that few were to be seen, and we have not 

 had anything like such an increase since that period. 

 It is probable that some disease afflicted them, and 

 that they perished in their holes, for we never found 

 their bodies, and any emigration of such large com- 

 panies would certainly have been observed ; yet the 

 appearance and disappearance of creatures of this 

 kind lead us to conclude that they do occasionally 

 change their habitations. 



A large stagnant piece of water in an inland 

 county, with which I was intimately acquainted, 

 and which I very frequently visited for many years 

 of my life, was one summer suddenly infested with 

 an astonishing number of the short-tailed water rat, 

 none of which had previously existed there. Its 

 vegetation was the common products of such places, 

 excepting that the larger portion of it was densely 

 covered with its usual crop, the smooth horsetail 

 (equisetum limosum). This constituted the food 

 of the creatures, and the noise made by their champ- 



