INTERRELATIONSHIPS OF GENERA 35 



Among the Microtinaj (excluding Brachytarsomys) the Lemmi 

 are at once the oldest and fundamentally the most primitive. The 

 remarkable shortness of the lower incisor in all lemmings proves 

 that the group diverged from the primitive Murine stock before 

 that tooth had attained its full development; the development 

 of the incisor in the lemmings was arrested by the decline of the 

 gnawing habit sooner than in the voles. Other circumstances 

 which point to the relatively greater antiquity of the lemmings 

 are firstly, the fact that they are all peripheral forms with a 

 relatively restricted and a waning distribution; secondly, that 

 whereas the Microti are represented by numerous genera, including 

 many in which the molars in adult stages of growth develop roots, 

 the Lemmi are now represented only by a few genera in which, 

 without exception, the cheek-teeth have acquired fully the power 

 of persistent growth. In other words, only the most highly 

 specialized Lemmi have survived; the more primitive forms 

 with rooted molars having utterly disappeared before the com- 

 petition of the newer and less primitive group of voles. 



DiCROSTONYX is fundamentally the most primitive of the 

 lemmings, and is by its structure one of the most isolated genera 

 of the subfamily. Its cheek-teeth, when quite unworn, have 

 members of the three primitive longitudinal rows of tubercles 

 distinctly developed; they retain, in adult stages of wear, many 

 of the primitive terminal elements which are suppressed in most 

 other genera, and their re-entrant folds lack cement. This 

 primitive type has survived either by colonizing, or else by remain- 

 ing in, the high north, where it has adapted itself to the rigorous 

 conditions now obtaining in that region and to subsistence upon 

 the unusually harsh and poor diet offered by that inhospitable 

 land. The primitive characters are therefore hidden beneath a 

 mask, the product of intense and two-fold specialization. The fur 

 has become very thick and subject to both seasonal and geo- 

 graphical variations of length, density, and colour, being shorter, 

 thinner, and darker in summer, in warmer regions, and in the young ; 

 longer, denser, and whiter or paler in winter, in the bleaker regions, 

 and in old age. The peripheral parts are shortened and with- 

 drawn to the shelter either of the dense fur or of the general integu- 

 ment of the trunk ; thus the outer ears are reduced each to a mere 

 fold of naked skin hidden in the fur ; the arms and legs protrude 

 but little from the general covering of the body; the caudal 

 vertebrae are shorter than the dense brush which envelops them. 

 The hands and feet have become exceptionally broad, partly for 

 digging, and partly for locomotion upon snowy wastes ; their 

 palms and soles are densely clothed with crisp, curling fur, and 

 the pads, having ceased to be of functional importance, are 

 represented only by some feeble vestiges which can be found by 

 clipping the hair away from the soles. The claws have become 

 large and sharp, those of the third and fourth manual digits being 

 highly modified for digging and subject to a remarkable and unique 



