THE SMALL-MAMMAL PROBLEM 735 
is found. This variation, when not purely individual, proves to be of a 
colonial character and has little geographical value. It is therefore not 
possible to define sub-species or geographical races among the dark- 
bellied forms. In some districts, as in Kumaon, N. W. India, such rats 
seem to have little or no connection with the local white-bellied forms, 
in other places they differ from their white-bellied companions merely in 
colour and to a trifling extent in skull—the cranial differences being 
susceptible of physiological explanation; finally, in still other districts, 
the difference is purely one of colour, and even that sometimes breaks 
down. One may conclude therefore that the dark-bellied rats are of 
diverse origin; some seem to have been produced, in the localities where 
they are now found, from the local white-bellied race; others have found 
their way to their present habitations from other more or less remote 
districts of the country, or even from abroad; and lastly, many are 
doubtless to be regarded as the mixed descendants of both native and 
imported stocks. 
The work now done, incomplete as it is, affords a perfect explanation 
of the conflict of opinion, with regard to the value of the species and 
sub-species recognised in recent zoological literature, which has arisen 
between systematic zoologists, and observers like Hossack and Lloyd 
studying rats in connection with plague in large towns or ports like 
Calcutta or Bombay. In such places it is hopeless to attempt to 
disentangle the history of the rats, for the urban rat population is a 
motley horde, representing the progeny of the truly native rats crossed 
with the descendants of old wanderers and with newcomers not only 
from the neighbouring hinterland but from all parts of the world. It 
is only in the rural districts that we can expect some measure of 
success to crown such efforts. 
THE SMALL-MAMMAL PROBLEM. 
During the war a great increase in the rat and mouse 
population of Britain became visible both in town and country. 
This rapid growth gave rise to alarm, which culminated in 
the passing of the “ Rats and Mice (Destruction) Act, 1919.” 
The abnormal increase was due to a combination of two 
entirely distinct sets of causes. Firstly, neglect and active 
folly pursued throughout a long course of years in the pre- 
war period, had together brought the rat and mouse resistance 
of the country down to a low degree. Thus few of our older 
buildings were, or have been made, rat-proof; and the stores 
of foodstuffs, and other materials attractive to rodents, in our 
warehouses, markets, and shops, remained without adequate 
VOL. II. s44 
