EVOLUTION OF MOLARS 103 



Direct evidence that such has been the general course of molar 

 evolution within the group is afforded by the vestiges of a tuber- 

 cular cap which occur, more or less well preserved, upon the 

 summits of the unworn cheek-teeth of Microtinse, and also by 

 the existence of certain Microtine genera in which the cheek-teeth 

 are, in one respect or another, noticeably less specialized than are 

 those of the highest forms alluded to above. 



A survey of the cheek-teeth of Microtinse shows that the molars 

 differ considerably in complexity in different genera. Before 

 we can form any idea of the history and relationships of the 

 various genera and before we can arrive at any more definite 

 notion than that outlined above of the dental characters of the 

 ancestor common to the whole group, we must determine the 

 direction of general progress within the group. Have the molars 

 tended to increase in complexity ? Have they tended to become 

 simpler by the reduction and atrophy of useless parts ? Or have 

 they tended to become more complex in some parts or directions, 

 and simpler in others? In attempting to find answers to 

 these questions, we leave the ground which is common to nearly 

 all workers and enter upon a field which has been in dispute for 

 many years. In this place it would, of course, be improper to 

 attempt a full discussion of the points at issue ; but it is necessary 

 to give a brief outline of my own opinion in order that the point 

 of view, from which the relationships of the Microtinae have been 

 discussed above may be appreciated. 



In the cheek-teeth of Microtinse, as in those of other mammals, 

 the apical portions of the crown are developed first. Growth 

 takes place at the base of the tooth, so that in descending through 

 the successive levels of the crown from the apex or wearing surface 

 to the pulp-cavities below we pass continuously from older to 

 newer horizons. These deeper and newer horizons are exposed 

 successively by wear upon the grinding surfaces of the teeth ; 

 and if we study a series of specimens illustrating a given tooth in 

 successive stages of wear, beginning with the unworn germ and 

 ending with the senile stump (when such exists), we find that the 

 characters of the grinding surface change from stage to stage 

 more or less conspicuously, according to the particular tooth and 

 to the particular genus or species chosen for examination. From 

 such a study it becomes apparent that more or less well-marked 

 differences of form and structure characterize successive levels of 

 the crown, and that each succeeding deeper horizon of the tooth 

 is the more or less modified descendant of the one immediately 

 above it. The apical parts of the crown are therefore not only 

 the oldest portions of the individual tooth, but they tend to be 

 the most conservative and primitive portions, the parts likely to 

 retain vestiges of structures which, although no longer of functional 

 importance, may have been inherited from ancestors in which 

 they were of functional importance. That this is a sound view 

 of the matter is immediately proved by the fact that in all rodents 



