ALTICOLA 305 



of the teeth, and in the size and form of the auditory bullae. By 

 the differences in the structure of 7n^ the species fall into two 

 rather natural groups, viz., one, the roylei group, in which m^ 

 remains comparatively complex, always possessing at least three 

 well-developed salient angles on the inner side; and the other, 

 the stracheyi groujj, in which in^ has been considerably reduced, pos- 

 sessing in adults normally only two salient angles upon the inner 

 side, although a vestige of the third angle is commonly present as an 

 ephemeral complication in immature teeth. Broadly speaking, 

 the external characters also follow this dental division between the 

 two groups; thus in the roylei group, side by side with the more 

 primitive m^, we find in one species or another the most primitive 

 external characters (darkest colour; barest feet; longest, least 

 clothed, or most distinctly bicoloured tail) known in the genus; 

 whereas in the stracheyi group, in association with the highly 

 specialized m!^, we find that all the forms show in their pallid 

 adult coloration, booted feet, very short, heavily clothed and 

 entirely white tails, a high degree of external specialization. 

 But although the members of thero^/ie t group are on the whole more 

 primitive than those of the stracheyi group, many of them are so 

 highly specialized in one or other external character that as regards 

 that particular character they approach or even rival the members 

 of the higher group. No hard and fast line can be drawn between 

 the two groups save that which depends upon the structure of the 

 m^; that even this is quite an arbitrary distinction is evident 

 when we consider that the reduction of that tooth from species to 

 species, or from youth to age (in the stracheyi group) proceeds by 

 very gentle gradations. As in other Microtinae the progressive 

 reduction of this tooth has proceeded apparently from behind 

 forwards, the three posterior (viz. one outer, two inner) of the six 

 triangles primitively intervening between the anterior and the 

 posterior loops successively dwindling in size until, losing their 

 independence, they have become completely blended with the 

 posterior loop, thus producing the peculiar form of m^ so charac- 

 teristic of A. stracheyi and its nearest relatives. Finally the 

 posterior loop, now a compound of the loop proper and the three 

 atrophied triangles, has been shortened from behind, thus result- 

 ing in the form of m^ found in A . stoliczkanus, somewhat similar 

 to that so characteristic of the genus Hyperacrms. 



If we examine the various species of Alticola critically we 

 find that it is impossible to bring them into any linear sequence 

 if attention be paid to more than one character at a time; for 

 each species shows its own peculiar blend of archaic and pro- 

 gressive features. Diilerences of altitude and latitude (probably 

 by their influence upon humidity, temperature, light intensity, 

 food and the other factors which constitute an environment) 

 appear to exercise a direct control over the outward habit of these 

 animals and to be the chief causes of the external differentiation 

 of the species. In some degree too, by controlling the nature of 



