40 



Rats a7id Mice 



of the females is a very long one. The period of gestation is 

 normally from nineteen to twenty-one days ; it may in certain 

 circumstances be shortened to twelve or thirteen days. Heat," 

 not lasting longer than twelve hours, rapidly succeeds parturition. 

 Many litters are born throughout the year, but fewer in the cold 

 months. The number of young per litter is between five and six, 

 but it may be as many as nine or as few as two. The young are 

 born blind, naked, and pink ; they grow rapidly, and are able to 

 leave the mother in less than three weeks. 



Economic Importance and Relation to Public Health. 



— Few houses in Britain are permanently free from House Mice. 

 When the number present is small they do comparatively little 

 harm, and many people rather welcome the occasional and sudden 

 appearance of the little beast on their hearths. But when a large 

 colony is present it is quite a different matter. The mice become 

 then an intolerable nuisance, eating large quantities of our food, 

 spoiling far more with their droppings, and tainting every place 

 and thing with which they come in contact with their strong and 

 unsavoury odour. Birdcages are robbed of their seed ; linen, 

 clothing of all kinds, and books are attacked, and holes are 

 gnawed in the woodwork in all directions. Traps and cats are 

 now brought in to fight the pests ; the members of the household 

 grow more careful in securing the food supplies and in placing 

 other things beyond the reach of the mice ; in a short time, as a 

 rule, the premises revert to their normal condition. The loss 

 occasioned by such a visitation is often quite considerable. 



In shops, warehouses and granaries, and on farms House Mice 

 are normally more abundant ; in such places they often do great 

 damage and cause much loss. In the materials stored they find 

 abundant food and shelter, and accordingly they breed at an 

 amazing rate. 



Most countries are visited periodically by " mouse plagues." 

 These plagues usually develop in summers following mild winters 

 and previous seasons of great plenty. Such favourable conditions 

 of climate and nutrition favour the mice by decreasing the normal 

 natural mortality, and by increasing the size and the frequency of 

 the litters born. Usually all the species of Muridae inhabiting the 

 district affected contribute to the formation of the mouse plague." 

 Thus in Britain what are called "vole plagues " are marked not 

 merely by a great increase in the number of voles present, but 



