EVOLUTION 31 



hinder edge of the alveolus of mj. The cheek-teeth, as explained 

 fully on pp. 102-119, must have been low-crowned, rooted, and 

 niultitubercular ; their crowns were remarkably complex, m, 

 consisting of about twenty-one tubercles; each of the other 

 teeth was composed of about fifteen tubercles, the tubercles being 

 arranged in three longitudinal rows both in upper and lower 

 molars. As in other primitive Muridse the food was bruised and 

 crushed, the motion of the lower jaw during mastication being 

 transverse or oblique instead of longitudinal in direction. From 

 such a form all known Muridse living or extinct may well have 

 descended ; but in this work we have merely to trace the modi- 

 fications which have led to the evolution of the Microtinse from 

 this primitive type. 



Representatives of the primitive stock just described appear 

 at different times to have discovered the nutritive value of the 

 coarser vegetable substances such as moss, grass, tough leaves, 

 bark and roots, and by degrees they substituted such unattrac- 

 tive provender for the softer and more succulent fruits, berries, 

 nuts, tender foliage and green shoots eaten by their ancestors 

 and less enterprising rivals. Leaving the dainties to others, they 

 thus tapped vast and never-failing food supplies which have in 

 the course of time enabled their descendants, the Microtinse, 

 to colonize the Holarctic region more thoroughly and more 

 completely than any of the related groups. 



This gradual change in diet induced corresponding gradual 

 changes in the dentition, ahmentary canal, and all the related 

 organs. Gnawing, that fundamental habit of Rodentia, lost 

 some of its primitive importance, since the extraction of kernels 

 from hard shells ceased to be one of the chief operations in feeding, 

 and the incisors, free to develop in other directions, were used 

 sometimes as digging instruments, sometimes as forceps for the 

 extraction of seeds, etc., special uses which have led to various 

 modifications in the form of these teeth (see pp. 99-101). The 

 new food made great and ever-increasing demands upon the cheek- 

 teeth, and old normal methods of mastication had slowly to be 

 changed. Bruising and crushing no longer sufficed for the 

 reduction of the food, and were gradually replaced by shearing 

 and slicing. Low-crowned tuberculate teeth, admirably adapted 

 for the earher use, had to be transformed into tall-crowned, 

 prismatic structures fitted for the new purpose. Mere pressure 

 between the tooth-rows, accompanied by a transverse or oblique 

 rocking motion of the lower jaw, had to be replaced by a powerful 

 gliding stroke from behind forwards. The motion imparted to 

 the jaw by the masseter lateralis muscle, used by other Muridae 

 chiefly in gnawing, had now to become the chief motion in masti- 

 cation, and the anterior part of the temporalis muscle became 

 specially developed as an auxiliary to the masseter complex for 

 this purpose. The uneven surfaces of the primitive tubercular 

 teeth had at first to be worn flat in order to permit the lower jaw 



