EVOLUTION 



29 



mentioned suggest that the dental, cranial, and myological 

 characters are inseparably linked with each other both in Brachy- 

 tarsomys aud in the voles. The problems connected with the 

 evolution of the patterns of the cheek-teeth and their bearing 

 upon relationships are more fully discussed below at p. 102. 



The Microtinfe owe their development, special characters, and 

 survival in the face of keen competition, chiefly to the fact that 

 they have acquired the power of subsisting upon coarser, tougher, 



Fig . 1 7 . — Myospalax fontanus Thomas. 

 Skull of adult, lateral view, enlarged. 



less inviting and less nutritious vegetable substances than those 

 devoured by their more generalized relatives and rivals. For 

 the most part, too, the Microtinse have become earth-bound, 

 burrowing creatures, and their fossorial habits have played a 

 great part in moulding both the outward form and the internal 

 structure of most of the living genera. 



The immediate ancestor of the Microtinse must have been a 

 generalized Murine with moderately large eyes and ears, long 

 tail, normal feet and hands, and normal fur. In these respects it 

 cannot have shown a greater degree of fossorial specialization than 

 that e.xhibited in Brachytarsomys. Its skull must have resembled 



