CHAP. X.] LEAST SIZE OF TEETH. 271 



Considered in the absence of evidence it might be supposed that 

 any tooth could be reduced to the smallest limits which are histo- 

 logically conceivable ; that a few cells might take on the characters 

 of dental tissue, and that the number of cells thus constituting a 

 tooth might be indefinitely diminished. Indeed on the hypothesis 

 that Variation is continuous this would be expected. Now of 

 course there is no categorical proof that this is not true, and that 

 teeth may not thus occur in the least conceivable size, but there is 

 a good deal of evidence against such a view. The facts on the 

 whole go to shew that teeth arising by Variation in particular 

 places, at all events when standing in series in the arcade, have a 

 more or less constant size on thus appearing. Within limits it 

 seems also to be true that the size in which such a tooth appears 

 has in many cases a relation to the size of the adjacent teeth and to 

 the general curves of the series. For example in the Orang, the 

 series of molars does not diminish in size from before backwards, 

 and extra molars when present are, so far as I know, commonly of 

 good size, not wholly disproportionate to the last normal molar. 

 The same is I believe true in the case of the Ungulates. In the 

 Dogs however the series of lower molars diminishes rapidly at the 

 back, and the extra molars added at the posterior end of the series 

 are of a correspondingly reduced size. As presenting some ex- 

 ception to this rule may be mentioned two cases in the Chimpanzee, 

 Nos. 178 and 181 and the case of Cebus robustus No. 194, in each 

 of which the extra molar is disproportionately small. 



The principle here indicated is of loose application, but speaking 

 generally it is usual for an extra tooth arising at the ends of series 

 to be of such a size as to continue the curves of the series in a 

 fairly regular way. It would at all events be quite unparalleled for 

 an extra tooth arising at the end of a successively diminishing 

 series, as the Dog's lower molars, to be larger than the tooth next 

 to it, and with the exception of cases of duplicate anterior pre- 

 molars (see Dogs Nos. 232 and Cat No. 268) I know no such case. 

 In these besides, the anterior tooth is very slightly larger than its 

 neighbour, and it should be remembered that the first premolar, 

 though the terminal member of the series of premolars, is not 

 actually a terminal tooth. 



Examples have been given of animals which seem to be oscil- 

 lating between the possession and loss of particular teeth, the first 

 premolar of the Badgers, p 1 of some species of Otter, &c. In these 

 cases we are not yet entitled to assume because in a given skull 

 the tooth is absent, that it has never been formed in it, though 

 this is by no means unlikely, but as already pointed out (p. 228), 

 the fact of its presence or absence may still indicate a definite 

 variation. Attention should be called to the case of Trichosurus 

 vulpecula, var. fuliginosa No. 378, in which the first premolar is 

 generally of good size if present, and there can be no doubt that it 

 has never been present in those skulls from which it is absent. 



