198 MEMSTIC VARIATION. [part i. 



Pike, the Alligator, or the Toothed Whales, for example, the teeth 

 stand in a regular and usually continuous series, differing from 

 each other chiefly in size, ranging from small teeth in front, 

 through large teeth, and often down to small teeth again at the 

 back of the jaw. Such a 'homodont' series as a rule passes through 

 only one maximum. Most mammals, however, are 'heterodont,' 

 that is to say, the teeth can be distinguished into at least two 

 groups, the incisors and canines on the one hand, and the pre- 

 molars and molars on the other ; and in a large number of animals 

 having this arrangement the anterior members of the series of 

 premolars and molars are small, increasing regularly in size from 

 before backwards, reaching a maximum usually in some tooth 

 anterior to the last. Though instances will be given of Variation, 

 and especially of reduplication, occurring in most of the teeth, 

 even in those which stand well in the middle of the series of back- 

 teeth, such as the upper carnassials of the Cat, or the fourth pre- 

 molars of the Seal, yet on the whole Variation in heterodont forms 

 is more common at the anterior and posterior ends of the series of 

 back-teeth. In view of this fact it is of some importance to 

 recognize that the small members at the beginning of the pre- 

 molar series are as regards their relatively small size, in the con- 

 dition of terminal members of series, and exhibit the variability of 

 terminal members almost as much as the last molars. 



With these remarks by way of preface, evidence as to the 

 numerical Variation of teeth in certain groups will be given in full. 

 This account will for the most part be confined to a brief description 

 of the conditions presented by the specimens. In the next chapter 

 the principles which may be perceived to underlie these facts and 

 the general conclusions to which they appear to lead will be 

 separately discussed. 



The evidence here given relates to certain selected groups 1 of 

 Mammals, and chiefly to the Primates (excepting Lemuroidea), Car- 

 nivora (CanidaB, Felidse, Viverridse, Mustelidse and Pinnipedise), and 

 Marsupialia (Phalangerida?, Dasyurida^, Didelphyidas, part of Macro- 

 podidae, &c). 



The facts to be given relate chiefly to increase in number 

 of teeth. In the case of terminal members of series, such as the 

 most anterior premolar or the last molar, some reliable facts as to 

 cases of absence were found, but for the most part the evidence as 

 to the absence of teeth is ambiguous and each case requires 

 separate treatment. 



The evidence is in this chapter arranged according to the 



1 Evidence as to the dental variations of Man is not here introduced. Con- 

 siderable collections of such facts have been made by Magitot {Anom. du syst. 

 dent.), Brscn (Dent. Monats. f. Zahnh. 188fi, iv.), and others, and illustrative 

 specimens are to be found in most museums. I do not know that among these 

 human variations are included phenomena different in kind from those seen in 

 other groups, except perhaps certain cases ot teeth united together, a condition 

 rarely if ever recorded in other animals. 



