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 w^ar 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. 11. 3^3 



