740 THE SMALL-MAMMAL PROBLEM 
maintained by the pressure of competition or by extreme 
persecution; the greater the loss inflicted upon the rat 
population the more rapid the rate of recovery. 
Rodier, dealing primarily with the rabbit pest in Australia, 
has advanced an ingenious plan for controlling the numbers 
of rodents. He thinks we should aim at producing a vast 
excess of males. Rabbits, rats, or mice should be trapped 
alive ; the females should be killed and the males given their 
liberty. By this proceeding a great disparity in the numbers 
of the sexes will be produced in due course, and a keen 
competition will arise among the males for the possession of 
the surviving females. The males will fight each other con- 
tinuously, and they will, at all times, relentlessly pursue and 
harass the females. The nursing does will be unable to rear 
their families, and any species attacked by this system will 
become rare if not extinct. The present system of indis- 
criminate trapping and poisoning, according to Rodier, has 
a directly opposite effect; it ensures the destruction of the 
surplus males, and results in fertile unions for all females. 
Rodier’s scheme has been advocated recently by Mr G. 
Jennison,’ who has been experimenting for some years at 
the Manchester Zoological Gardens. He states that ‘‘our 
present system of destruction helps the rat in the struggle 
for existence. The more rats killed, the more food for the 
remainder ; the more males killed, the greater the chance for 
the doe to breed quietly and raise her offspring. These two 
facts together neutralise all the good effects of indiscriminate 
slaughter. The rats can be reduced quickly to a certain 
point beyond which it is almost impossible to make further 
progress, and from which they soon reach their former numbers 
if at all neglected, ¢.g., Copenhagen caught 100,000 in four 
months, 8th August to 8th December 1904; they could still 
catch 99,000 in the three months of July quarter 1908, under 
the new rat law.” Applying the Rodier system to the Bellevue 
Gardens, Manchester, Jennison reduced the number of rats 
caught there from about 34 per month at the end of rors, 
to 18.5 per month in the first six months of 1920. He 
says ‘the best plan for rat destruction appears to me plain. 
1 G. Jennison, “ Rat Repression by Sexual Selection,” 7. RX. Sav. Jnst., xli., 358. 
