EVOTOMYS 215 



for life within the Arctic Circle. In small size, delicacy of skull 

 and lightness of the cheek-teeth it remains primitive, more 

 primitive indeed than are most of the living members of the 

 glareolus group. The internal changes are not great; m^ is a 

 little simplified as a rule and the auditory bullae have become 

 somewhat enlarged, but neither character is carried beyond what 

 is found in some members of the glareolus grouj). On the other 

 hand the peripheral parts, external ears, limbs and tail, have all 

 been shortened or withdrawn, and they have acquired an un- 

 usually dense covering of hair. In the highest members of the 

 group the colour is greatly brightened, and while the rufous 

 mantle becomes less and less diffuse, more restricted to the spinal 

 region, it extends further backwards to end by spreading along 

 the dorsal surface of the tail. 



The nageri group seems also to have arisen from low members 

 of the glareolus groujj ; m^ for one thing has retained much of 

 its primitive complexity. For some reason, possibly enforced 

 subsistence upon rather tougher and less nutritious vegetable 

 substances than those usually devoured by E. glareolus, the cheek- 

 teeth have become larger and taller-crowned, the jaw muscles 

 more powerful, the skull more robust and more strongly ridged, 

 and the general size has increased. These changes have been 

 brought about by a gradual and long-continued course of adaptive 

 evolution ; for the least modified members of the nageri group are 

 scarcely to be distinguished from some of the larger members 

 of the glareolus group, and from them many gentle gradations lead 

 upwards to the highest forms, e.g., E. ccesarius and E. n. nageri. 



In the Far East and high North by a course of much more 

 intense adaptive modification the rufocanus group has been 

 evolved apparently from members of the nageri group. The 

 modifications follow the same general course as in the latter, but 

 they go much further. The animal has become unusually robust 

 and has acquired unusually heavy cheek-teeth, powerful jaw 

 muscles, and a skull which in sti'ength and angularity rivals that 

 of many species of Microtus. In young individuals the skull and 

 cheek-teeth (with their confluent dentinal spaces) resemble those 

 of ordinary Evoioniys, and m^ is frequently complex with four or 

 five sahent angles on each side ; but in adults the dentinal spaces 

 become rather tightly closed and the teeth bear a strong resem- 

 blance to those of Microtus, particularly noticeable in ln■^^, while 

 fn^ is simplified. The animal looks mature long before it is full- 

 grown and long before the molars show the slightest sign of root- 

 ing. So great is the difficulty of estimating the ages of in- 

 dividuals in this group, and so different are the young from the 

 adults in skull, dentition, and to some extent in outward 

 appearance, that competent observers have commonly referred 

 young and adults of one and the same form to different genera, 

 under the impression that all their specimens were really mature. 



Among the American forms E. gapperi (and its numerous 



