as Enemies of Mankind. 



51 



Cheek-teeth with low crowns composed of rounded tubercles, 

 rooted and of limited growth. 



Tubercles of upper cheek-teeth arranged in two primary 

 longitudinal series (Fig. 4 A). Cricetinae, or Hamsters. 



Tubercles of upper cheek-teeth arranged in three 

 primary longitudinal series (Fig. 5). Murinae, or Rats 

 and Mice. 



The natural distribution of these sub-families is interesting 

 and suggestive, for, coupled with the evidence of fossil remains, it 

 seems to indicate that the Muridae originated in the Old World 

 in early Tertiary times. The Cricetinae are the most ancient 

 and on the whole the most primitive of the sub-families ; they are 

 now poorly represented in Europe and Asia, absent from Africa, 

 though represented in Madagascar by certain very lowly forms — 

 living fossils " — which are doubtless the little modified de- 

 scendants of the parent stock of the whole family. They are 

 widely distributed throughout the New World, where they have 

 developed many peculiar generic types. Neither in Madagascar 

 nor in America has this sub-family, or its allies, had to compete 

 with the Murinae until the most recent times. The Microtinae 

 also appear to be fundamentally rather primitive Muridae ; they 

 have withstood the competition of the Murinae only in so far as 

 they have been able to acquire more earth-bound habits and the 

 power of living upon a hard diet of roots and coarse herbage ; they 

 seem to have originated in the temperate regions of Europe and 

 Asia and to have colonized North America by way of a former 

 land connection at the Bering Straits ; unlike the Cricetinae, they 

 have not succeeded in entering South America. The Murinae 

 comprise many genera and are on the whole the most highly 

 developed and certainly the most successful members of the 

 family ; they have probably originated in the w^armer regions of 

 the Old World, their range extending all through Africa and also 

 to Australia. They reached East Africa only after the separation 

 of Madagascar ; and north-eastern Asia, in time to enter Japan, 

 but too late to cross to America. Neither the Cricetinae nor the 

 more generalized of the Microtinae have been able to compete 

 with them successfully. 



The living British Muridae belong to the sub-families 

 Microtinae and Murinae; the Cricetinae also formerly inhabited 

 our country, but they became extinct here towards the close of the 

 Pleistocene period. 



