374 MURIDA 
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 7he Golden Bough (pt. v., vol. ii., 1912). 
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, Zz Northern Labrador, 1912, App., 
287-292; Thompson Seton, Zhe Arctic Pratries, 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 polycestrous 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 
