THE BROWN OR COMMON RAT 627 
still, combined with trappers, professional or desultory, foxes, 
owls, and other creatures, together with the plan of bacillus 
infection, which appears to have met with some success 
recently, a large toll is annually taken, but seems to have 
no effect on their numbers. Sooner or later civilised man 
will have to face the problem of totally destroying these pests, 
but hitherto his efforts have met with practically no success. 
In Japan alone several hundred thousand to a million rats 
are said to be killed annually, but without producing any 
sensible diminution of the numbers present.' The cutting- 
off of the chief sources of food-supply, thus reducing the 
number of young, and the universal erection of rat-proof 
dwellings, as recommended by Mr Lantz,” if combined in a 
systematic manner with trapping, may prove more effective 
in the long run than the present desultory campaign. 
Rats are extremely prolific, and when living in houses in 
warmth and plenty, will produce young at every season of the 
year; but this, of course, does not indicate that any particular 
female will breed throughout the year. Those who live out of 
doors and are more poorly fed have a sexual season varying 
with their circumstances, but coinciding more or less with the 
warmer six or eight months of the year.’ Fertility is greatest 
in countries of mild climate free from extremes of heat or cold, 
but in exceptional cases winter litters are found even in severe 
weather in the open country.* 
Darwin (Desc. of Man, ed. 2, 247) was informed that the 
males are ‘in great excess,” while John Sinclair reported 
(Thompson, iv., 18) that 75 per cent. of the rats in litters he 
examined were males. Bonhote, however, found that in 
Egypt males were apparently fewer than females, constitut- 
ing only 42 per cent. of those he examined ; and of eighty-four 
examined at Kilmanock, on one occasion, only fourteen were 
bucks. 
Tame female white rats are said to be capable of breeding 
1 Professor Kitasako, quoted by C. Hart Merriam, U.S. Department Agric. Biol. 
Survey, Bull., 23, 1909, letter of transmittal. 
2 [bid., 10. Much valuable information as to the best means of destroying rats or 
of protecting property and food from their attacks is given in this paper (pp 36-54). 
* Heape found the dicestrous cycle to occupy about ten days. 
4 J. C. B. Noble, 7ze/d, 26th November 1904, 950. 
