477 
over 75 miles. ‘The price paid by the local government for these animals, 
which were bought from the people, in Tokyo alone has amounted to 
160,000 yen. 
However, the extermination of rats is complicated by the fact that 
the rodent increases at an enormous rate; as a rule within one month of 
pregnancy the female gives birth to five young ones at a litter, and the 
young reach puberty and become pregnant at the age of three months; 
thus these animals multiply in a geometric progression. Furthermore, 
if rats are destroyed by artificial means, such destruction only lessens the 
struggle for existence and then the rate of multiplication is much in- 
creased. In Tokyo more than 4,800,000 rats have been killed; yet we 
can hardly notice any considerable decrease in the number of these 
animals. 
In Japan, for killing rats, poisons such as arsenic and phosphorus, 
together with traps, are chiefly used. The typhoid bacillus of the rat, 
which has been effectively used for killing field mice, has been found 
useless for house rats and therefore we no longer employ it. 
Someone has offered the following suggestion for the disposal of the 
usual breed of household rats (Mus rattus), which unfortunately is the 
species most liable to be attacked by plague. This suggestion is to in- 
troduce M. decumanus, which is a persistent enemy of M. rattus, and 
which is comparatively more resistant to the infection. Such a sug- 
gestion appears to me impracticable for from my actual observation it 
is evident that so far as the epidemics in Japan are concerned, the kind 
of rat has been of very little importance in propagating plague. More- 
over, the results of biological researches tend to confirm the fact that 
although two distinct species of rats are found, the one most prevalent 
in Japan is a race which is a mixture of the two and which is also suscep- 
tible to plague. Such a fact rather contradicts the assumption that the 
two species are natural enemies to each other. Therefore, the above rec- 
ommendation can hardly be taken as a basis for preventive measures 
against plague. 
The best way to destroy rats in connection with the restriction of 
pest is to prevent their abiding in or at least to expel them from human 
habitations. In order to accomplish this purpose I suggest that all 
buildings be rebuilt according to a plan which will serve this end. This 
particularly applies to Japanese houses, because in most cases the latter 
are built of wood, and such buildings are liable to provide quarters for 
rats. I understand that the United States is planning a reconstruction 
of “Chinatown” in San Francisco. I highly approve of such a step in 
view of the situation of that port, which is constantly threatened with 
a plague invasion. In Manila, also, the cause of the recent fortunate 
decline of the plague epidemic, I believe, can chiefly be attributed to 
the rebuilding of the city since the Islands became a possession of the 
United States. 
