374 MURID^ 



tive power caused them to attain importance in mythological 

 systems of different parts of the world from the earliest times, as 

 traced by Fraser in The Golden Bough (pt. v., vol. ii., 191 2). 

 In certain favourable seasons, recurring irregularly or in 

 an unknown cycle, their numbers multiply exceedingly, and 

 thus exert an extraordinary influence on other mammals and 

 birds to which they are an indispensable prey, especially in sub- 

 arctic regions (see Cabot, In Northern Labrador, 19 12, App., 

 287-292; Thompson Seton, The Arctic Prairies, 1912, 107); 

 in these years of increase they are specially disastrous to 

 agriculture, but "mouse-plagues" are fortunately not frequent 

 in Britain. Sometimes, as in the well-known case of the 

 lemmings, the superfluous population may attempt to 

 "migrate" to new districts, but rarely, if ever, with success. 

 " Mouse-years " are, perhaps, caused by unusual abundance of 

 food, and are characterised by exceptional fertility, in which the 

 predatory creatures temporarily participate. Fertility decreases 

 in the succeeding years of scarcity ; frequently also disease 

 appears, until the animals concerned almost vanish temporarily. 



Swimming: — Although several British species swim well, 

 they always use all four legs like a dog, none having acquired 

 a more specialised method of propulsion (Adams). 



Pairing habits:— As usual in mammals, sexual maturity is 

 attained before full size. In the females the urethra opens 

 separately through a prominent clitoris. The vagina is closed 

 in immature individuals, so that identification of the sexes 

 is difficult in the young, the only guide being the shorter 

 distance from clitoris to anus than from penis to anus. 

 After sexual maturity the open orifice of the vagina makes 

 identification easy. The mature male, if in good condition, is 

 almost always capable of pairing ; but in the females, although 

 there is a long polyoestrous sexual season, "heat" lasts for a 

 few hours only, at intervals which, in the absence of impregna- 

 tion, recur about every eleven days. After effective pairing 

 the vagina is closed, as in bats (see above, vol. i., p. 231), by 

 a vaginal stopper, which remains in position for from twelve to 

 twenty-eight hours, and is then expelled. This is a joint 

 production, but chiefly of the male, which provides the major 

 and central portion, consisting of a spontaneously coagulating 



