THE COMMON GRASS MOUSE 



451 



an area of not less than sixty miles by twelve to twenty. 

 Plagues on such a large scale must, however, be regarded as 

 rare in Britain ; on a small scale Service believed ^ them 

 to be frequent, and in fact that of 1891-93 is said to have 

 commenced in a small way in the autumn of 1887. Sometimes 

 other species also are involved, as the Bank and Field Mice ; 

 a "run" of these two was observed by Mr A. H. Cocks ^ in 

 1900, and in the Forest of Dean plague of 181 3-14 there 

 was one Field Mouse to every fifty Grass Mice.^ 



It is doubtful if the recurrence of mouse-years can be attri- 

 buted to the destruction by game preservers and others of the 

 natural enemies of the rodents. In a state of nature the 

 numbers of the latter fluctuate even more markedly and 

 probably with greater regularity than under the artificial 

 environment imposed by civilisation, and in some cases it 

 appears that the carnivores depend on the rodents rather than 

 the rodents on the carnivores. The reclaiming of woods, forests, 

 marshes, and rough pastures which protect small rofients pro- 

 ceeds side by side with the destruction of predaceous creatures, 

 and it is probable that a thorough and effective system of 

 agriculture is the best preventative of mouse plagues. 



There are few detailed accounts of the breeding habits of 

 this species. Mr J. G. Millais has seen both sexes collecting 

 material for a nest. The average number of young is 

 probably about five, that number having been reported by Mr 

 Adams as the result of examining thirty-eight litters made up 

 as follows : — 



9 5 



and these results agree fairly well with those of other observers. 

 Grass Mice do not, therefore, produce so many young at a time 

 as the common rats and mice ; still they rear a considerable 

 number annually in a sexual season lasting at least from April 

 to December. In times of "plague" the number of young may 

 run higher per litter. Service having met with ten. A pair* 



' Ann. Scott. Nat. Hist., 1896, 206. 2 See above, p. 418. 



■■' Glenbervie, op. cit. « Fernand Lataste. 



