W. B. Scott — Variations and Mutations. 365 



majority of placental mammals have another mode of dental 

 evolution, viz : that of reduction, the ancestor in nearly all 

 cases having a larger number of teeth than the descendant. 

 The earl} 7 Eocene representatives of all the placental orders, 

 except the rodents, have a uniform dental formula, i\ c^iPh 

 w-|x2 = 44, a maximum which is never exceeded. Ti'acing 

 the various phyla down from their Eocene progenitors, we 

 find that in nearly all cases there is a gradual reduction in the 

 number of teeth, never (with the one doubtful exception) an 

 increase. Throughout the history of the phyla we may and 

 often do find just such individual variations as those which 

 Bateson has described, of abnormal increase or decrease in the 

 number of teeth, but these in no way affect the march of suc- 

 cessive species and genera, as indicated by the character of the 

 normal or position of the center of stability, a march which is 

 almost invariably in the direction of reduction, though in a 

 few cases the original number is retained. Wortman's dic- 

 tum : "there are no cases known to me in which teeth have 

 been added. On the contrary, I am firmly of the opinion 

 that not so much as a single tooth has even been added to the 

 diphyodont mammalian dentition in the course of development, 

 but that specialization has invariably gone in the other direc- 

 tion," is perhaps too strongly expressed, but is, on the whole, 

 supported by the facts of palaeontology. 



Further than this, the individuality of the teeth is preserved 

 in a remarkable way. One tooth may, it is true, assume the 

 form and function of another ; thus, in the oreodonts the 

 lower canine has taken on the shape and character of the 

 incisors, while the place of the canine is assumed by the first 

 premolar, which has become caniniform, but as this tooth bites 

 oehind the upper canine, these changes do not in the least dis- 

 guise the homologies. Whenever the successive members of a 

 phylogenetic series have been recovered, there is no difficulty 

 in determining what teeth have been lost. While reduction 

 usually affects first the ends of the series, it does not always 

 do so. For example, the camels have retained the first pre- 

 molar, though losing the second. Throughout the whole his- 

 tory of the Carnivora, from the earliest dog-like forms with 

 undiminished dental formula to the cats, in which the cheek- 

 teeth have been greatly reduced in number {p -§-, m i) the indi- 

 viduality of the sectorials has been preserved. These teeth, 

 when present at all, are always the fourth upper premolar and 

 the first lower molar. This invariable homology is shown both 

 by the steps of the reduction, all of which can be followed in 

 many lines, and also by the relation of these teeth to the milk- 

 dentition. 



