INTRODUCTION 



Eats and mice, with some of tlieir near relations, constitute a 

 great zoological family, the Muridae,"' which belongs to the order 

 Eodentia. This order comprises the mammals characterised by a 

 propensity for gnawing, a function performed by a pair of ever- 

 growing chisel-like teeth, or incisors, placed in the forepart of each 

 jaw. The distinction between a rat and a mouse is mainly one of 

 size ; larger species, such as those with a hind-foot measuring 

 more than 30 mm.f in length, are rats " ; smaller species, with 

 hind-feet less than 30 mm. long, are " mice." 



Besides some native wild Muridae, Great Britain possesses 

 three species which have been introduced, at different dates, from 

 abroad; these aliens are the Black Eat (Eattus rattus), the 

 Common Eat (B. norvegicus), and the House Mouse [Mus 

 muscuhis). Possibly these three species are the most highly 

 organised members of their family ; but unquestionably they are 

 the most successful of mammals. They are clearly of Asiatic 

 origin ; but uninvited, and unfortunately for us, they have linked 

 their fortunes with those of humanity. Human enterprise, in all 

 its phases, and human negligence have disturbed the balance of 

 Nature in favour of these species, have afforded them an unnatural 

 degree of protection from their many enemies, a large and 



* In a strict zoological sense the names " rat" and "mouse" are only 

 applied to members of the Murinae — one of the many sub-families into which 

 the Mtiridae are divided. The three species which are the subjects of this 

 work belong, of course, to the Murinae. 



For the convenience of those who may wish to acquire a slightly more 

 extensive knowledge of the Muridae, and would like to be able to distinguish 

 the species inhabiting Britain, the writer has prepared a short account of the 

 structure, classification, history and distribution of the principal divisions of 

 the family. This, together with a "key" to the characters of the British 

 forms, appears as an appendix (pp. 48). 



t 25 millimetres = 1 inch. 



