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At present I know of no infection which would answer these requirements. As
already stated, the various viruses which have been used are known to introduce more
or less fatal epidemics among rats, but the disease is not fatal to all, and some even
acquire immunity and act as carriers with possible danger to public health, since the
organisms used are certainly not harmless to human beings.
Plague is notoriously fatal to rats, so much so that in plague-infested areas rats
desert the district and so introduce the disease to a previously non-infected area.
Plague is, of course, a fatal disease to which human beings are liable, and therefore
cannot be used. I do not consider that possibilities in the above direction have been
exhausted.
(30) (ii.) An attempt has been made to reduce the number of rats by the
introduction into a district of antagonistic species not normally indigenous to that
area. The introduction to new areas of animals which have successfully acclimatised
themselves to their new surroundings has not invariably been devoid of objection, e.g.,
rabbits in Australia, in which instance the introduced animal has constituted itself
practically a plague. The mongoose was also introduced into certain parts of the West
Indies to diminish the rat. It certainly effected its purpose, but I understand that this
animal is now almost as objectionable as the rat, for the rat has acquired arboreal
habits, and by limiting the supply available for the mongoose, the latter has turned to
the chicken runs. 1 have also been informed that the mongoose causes considerable
damage to the sugar canes.
In this country the weasel, stoat and owl, and perhaps the hawk, are the known
enemies of rats, but none of these animals or birds could be used in buildings. Some
use might be made in City warehouses of the mongoose, and I attach a description by
Mr. Jennison on how the mogoose should be tended.
(31) (iii.) The introduction of some factor which will interfere materially with rat
propagation, as, for example, that which would result from a disturbance of the
numerical sex relationship has been advocated; this latter under the designation of the
Rodier System. This method is advocated by Rodier for use against rabbits, and he
claims with success. As regards the application in the case of rats, the only experience
in this country I am acquainted with is that gained by Mr. Jennison at the Zoological
Gardens, Manchester.
I believe that it is possible for animals in the wild state to change from the
monogamous to polygamous state, but not the reverse. Take for instance the bird of
paradise, which was formerly monogamous, but to-day, owing to the slaughter of the
male birds, a polygamous state has developed. I believe also that limited polygamy in
nature can develop into increased polygamy under certain forms of selection, as for
example in the case of sheep. The herds of wild sheep in the Rockies are constituted
in the proportion of one male to eight females, but in the sheep flocks in this country
the average is about one male to 30 females. The introduction of an additional ram to
flocks of sheep results in interference with the number of lambs produced, and similarly
one finds that in a pigeon loft, free cock birds will materially affect the breeding in that
loft—three odd cocks will ruin the breeding of three nests. In a poultry yard excess
of cock birds will materially interfere with, if not entirely stop, the laying of eggs by
the hens of that community. (In this connection 1 am informed that the male bird of
the Jungle Fowl controls four or five hens, whilst in the ordinary domestic variety the
hens may number up to 20.) For somewhat similar reasons cock pheasants must not
be in excess in the nesting season, so they are shot off whilst the hen birds are spared
at the end of the ^shooting season. Also a comparatively small proportion of males
among guinea pigs kept in confinement together results in a reduction in numbers bred
and the final extinction of the females.
These facts tend to show that sexual relationship can b.e extended from the limited
range to the wider range, but when once the wider range has become established,
reduction of females and proportionate increase of males results in conditions which are
unfavourable to the procreation of the species.