THE HOUSE MOUSE 639 



Field Mouse; the strongly recurved coronoid processes rise slightly 

 above the level of the condyles. 



The teeth have been described above under the genus. 



Local variation: — The yellow subterminal bands of the longer 

 hairs are usually better developed in outdoor House Mice, which, 

 therefore, have a more sandy appearance than those generally caught 

 in houses. This fact has long been known, as, to Macgillivray, who 

 states (251) that " individuals obtained in the fields are sometimes 

 almost as beautifully coloured as the Wood Mouse, there being much 

 yellowish-brown on their upper parts, and their lower being of a dull 

 cream-colour " ; and to Jenyns, who adds {Man., p. 32) that they " some- 

 times attain a larger size, measuring nearly four inches in length." 



The mice described by Jameson {Journ. Linn. Soc, xxvi., 1898, p. 

 465) from the sandhills of the North Bull, Dublin Bay, though differing 

 among themselves, are characterised by their very light colour, and may 

 be regarded as representing the extreme phase of the yellow outdoor 

 coloration ; these mice live on a barren sandy waste, where they are 

 exposed only to the attacks of enemies hunting by sight alone ; there- 

 fore, as suggested by Jameson, it is not unlikely that natural selection 

 has played and is playing an important part in the elimination of the 

 darker individuals of the colony. 



Adams ^ (MS.) has caught tawny bellied House Mice in Surrey and 

 Sussex ; these were taken usually in cornfields and hedge-banks — often 

 300 or 400 yards from any building, sometimes in ricks at threshing, 

 and once or twice in country houses. He notes that the tails were 

 often relatively longer than in indoor House Mice. A few years ago 

 W. Evans observed that the House Mice living out of doors on the Isle 

 of May (Firth of Forth) were lighter in colour than ordinary indoor 

 examples; he sent one of these to Barrett-Hamilton. It seems probable 

 that the original colour of wild M. viusculus was some shade of yellow 

 or tawny ; and that in this case, as Adams suggests, the tawny hue of 

 present outdoor families may be explained either as a reversion to 

 type, if such families have descended from a domestic stock, or as a 

 retention of the ancestral coloration, if they have always been feral. 

 In either case the difference between the indoor and outdoor coloration, 

 whatever its "protective" value may be, is probably to be explained 

 as a result of the wide difference in the light intensities to which the 

 two stocks are respectively exposed. 



Kinnear {Ann. Scott. Nat. Hist., 1906, 65) describes the House Mice 

 of Pair Isle as being rather larger and more tawny in colour than main- 



' Adams {MS.) "noticed very markedly in the sunny summer of 191 1, that the 

 coats of the Common Shrew were lighter in colour than usual. There was much 

 more albinism in ears than usual— about 25 per cent, had white ears in that year, 

 whereas about 4 per cent, is the normal condition." 



