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 1813-14 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 rodents 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 :— 
No. of Litters. No. of Young. | No. of Litters. No. of Young. 
| 
I 9 | 5 4 
5 7 | 8 3 
8 6 I 2 
9 5 I I 
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‘ 
1 Ann, Scott. Nat. Hist., 1896, 206. * See above, p. 418. 
* Glenbervie, of. cz7. + Fernand Lataste. 
