CHAPTER IX. 



LINEAR SERIES continued. 



TEETH. 



FROM the consideration of numerical Variation in mammae we 

 may proceed to an examination of like phenomena in the case of 

 the teeth of vertebrates. The modes of Variation in these organs 

 are, as might be expected, in many ways similar, but several cir- 

 cumstances combine to make the Variations of teeth more com- 

 plicated than those of mammae. 



Teeth arise developmentally by special differentiation at points 

 along the jaws, much as the mammae arise by differentiation at 

 points along the mammary lines ; and as in the case of mammae, 

 so in the case of teeth, we are concerned first with changes in the 

 number of points at which such differentiation takes place, and 

 next with the general changes or accommodations which occur in 

 the series in association with numerical changes. As in mammae, 

 so also in teeth, numerical Variation may occur sometimes by the 

 division of a single member of the series into two, and sometimes 

 by a reconstitution of at least a considerable part of the series. 



Between the case of mammae and that of teeth, there is how- 

 ever an important point of distinction. The series of mammas is 

 practically an uudifferentiated series. There is between mammas 

 standing in one mammary line no obvious qualitative differentia- 

 tion. Though not all identical in structure, the differences between 

 them are of size and of quantity, not of form or quality. If 

 such qualitative difference is present it must be trifling. In con- 

 sidering Variation in mammae we have thus to deal only with 

 changes in number, and with the geometrical and perhaps mecha- 

 nical question of the relative positions of the mammae. The teeth 

 of most Vertebrates, however, are differentiated to form a series of 

 organs of differing forms and functions, and the study of Variation 

 in teeth may thus be complicated by the occurrence of qualitative 

 changes in addition to simply numerical ones. In teeth, in fact, 

 there are not only Meristic variations, but Substantive variations 



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