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doubt exists in my mind that the slaughter of rats by terriers is as humane a method
of destruction as that of poisoning by strychnine, arsenic, barium carbonate or
sulphur dioxide.
(27) A study of the excellent return of Zuschlag respecting rat repressive
measures in Copenhagen and Frederiksberg in Denmark supports the view that
although the normal rat population may be easily reduced, considerably increased
efforts are necessary to reduce the numbers below a certain level, and this level
probably is sufficiently serious to allow the animals rapidly to return to their maximum
if laxity is allowed.
The following table shows the number of rats caught in Copenhagen in each
quarter of the year from 1907, in which year organised efforts were first put into
practice. It will be seen that in the first year and in the first quarter of the second
year, which was the July quarter, more rats were caught than in any subsequent year
or quarter. The reduction after the first two years is not very pronounced. No
figures are available to show how long it would take, if the rats at this reduced level
were left alone, before they resumed their normal state of excessive prevalence.
During the period July 1st, 1907, to June 30th, 1909, eight ore were paid for each
rat killed. From 1909 to 1919 the payment was increased to ten ore, and consideration
is being given to a still further increase.

I shall be interested to note the number of rat3 actually caught in Copenhagen
when food supplies are again in a normal state. The notable decrease in the years
1915-1919 compared with 1911-1914 might be due to food supplies being diminished
in consequence of the war, or to diminished activity—I have no information, however,
on these points.
(28) The work that has been done above, and which would correspond to energies
in the City, is undoubtedly beneficial, but the rat problem still remains.
Work and effort on the lines already indicated must be continued, but it is clear
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that we should not rest satisfied with these measures. Research should be instituted,
and as in other preventive branches, the results of research would probably prove a
much better investment than acceptance of the present position, which is expensive and
leads to no finality.
It is difficult to lay down lines on which research could be carried out, but as
indicative of what is meant, it is perhaps permissible to point out three directions in
which it has been suggested that the problem could be approached with this broader
conception in view.
(29) (i,) It has been thought possible to introduce amongst the rat population
some specific infectious disease which might prove fatal amongst the rats and yet be
practically harmless to domestic animals or human beings.