468 The Plague of Field Mice, by Sir Walter Elliot. 



Microtus, and six in Arvicola (except in A. amphibius), but he does 

 ■ not rely on the spaces between the zigzag ridges of enamel, and 

 the number of their angles on the surface of the molars, on which 

 the main specific distinction of the Arvicola; appear to rest. 



I cannot help thinking that too much weight is attached to this 

 character likewise, and that a more extended comparison of many 

 specimens would shew these minute differences to be by no means 

 constant. The distinction between A. agrestis and A. arvalis is 

 supj)Osed to lie in the second upper molar, but Fatio, himself, is 

 puzzled by finding "une dentition mixte" in individuals of A. 

 agrestis. He attributes these, and certain other divergencies, to 

 " the effects of age, alimentation, season, and different conditions 

 of level and habitat in which such individuals are forced to live " 

 (page 71 and figs. 19, 24, 25 of Plate I.), but he considers that 

 the constant form of the second upper molar covers all deficien- 

 cies. 



Allowing this to be so, I cannot admit that it constitutes a 

 structural character of sufficient value, on which to establish a 

 species, and looking to the wide distribution of the field vole, 

 to its identity of habit in every situation, and under very divers 

 conditions, I cannot bring myself to believe that A. arvalis is dis- 

 tinct from the species originally established by Linnaeus, under 

 the name of A. agrestis. Fatio arranges his true Arvicolce as in- 

 habiting plains (Pratieola) or forests (Sglvicola), A. agrestis 

 being the sole example of the latter, a fallacious distinction, for 

 in this country it is more frequently found in meadows than in 

 woods. A. amphibius, which stands among the praticolce, was at first 

 separated from certain individuals living at a distance from water, 

 which were formed into a distinct species by Linnaeus himself, as 

 A. terrestris, in contradistinction to amphibius, and it was accepted 

 by Savi, Fred. Ouvier, Schinz, and others. It has also been 

 recognised as inhabiting hills, and named monticola, but all have 

 merged eventually into a single species as A. amphibius. 



I feel justified, therefore, in concluding that A. agrestis and A. 

 arvalis form but one species, which should stand under the name 

 originally assigned to it in the Sy sterna Naturoe of A. agrestis. 



