Studies of the Mammalian Dentition.–Differentiation of the Post‐canine Dentition

Summary. 1. The theory is developed that teeth are repeated organs that occupy different positions in a continuous morphogenetic field. The field can undergo changes in structure and position in the course of evolution, and thus changes in the differentiation of the dentition may be explained. 2. The differentiation of teeth in size seems to be to some extent independent of their differentiation in form. 3. The characteristics of the field or gradient can be described by plotting dental form against the numerical position of the tooth in the series. Two main types of evolutionary change can be detected: (a) retardation of the gradient in the molar region, so that some mammals have more molariform teeth than others; (6) shifting of the molar region backwards and forwards along the dentition. 4. The upper molar patterns of Cenozoic mammals are considered to be of four types–zalambdodont, dilambdodont, tritubercular, and quadrituber‐cular. The serial differentiation of a number of dentitions of which the molars belong to each of these types is described. 5. Each element of the molar has its own peculiar mode of change as it is traced along the series of teeth, and different elements reach their maximum development on different teeth. 6. Some variations are distributed along the gradient in such a way that they appear most frequently on certain teeth and less frequently on teeth lying farther away from the point of maximum incidence. 7. Other variations, in form and dental formula, can be interpreted as due to changes in the degree and position of the gradient. 8. In dentitions that differ in molar pattern, homologous (or homodynamous) elements show similar serial changes and the mode of serial change can be used as a test of homology (or homodynamy). Marsupials and Placentals are similar in this respect. 9. In evolution the serial change of one element can get out of step with that of another, so that changes can take place at one point in the series that occurred at several different points in the ancestor and vice versa. 10. In evolution, new types of change can be superimposed upon the old, e. g. , the reduction of the paraconid in the lower molars. 11. The premolars differ from the milk‐molars in a limited number of ways, and the relation between the two series can undergo evolutionary changes. 12. The two jaws are usually similar in the degree of differentiation between corresponding adjacent teeth, and between the premolars and milk‐molars. 13. A description of'Potamogale has necessitated certain corrections in the interpretation of the Centetoid molar pattern as described in the previous paper. 14. This study makes very improbable the view that individual teeth are independent of each other in variation and evolution.