ITS DISPUTED AFFINITY. 435 



alike rudimentary. The pair of developed incisors are ap- 

 proximated and placed collaterally, as in the placental Bo- 

 dents ; and commonly they are projected forwards with but 

 a very slight upward inclination. They are unequally op- 

 posed in the upper jaw by two or more incisors on either 

 side. Why there should be this plurality of incisors above, 

 and only two invariably occupying the same position below, 

 is wholly unknown to us ; but the constancy of the structure 

 makes it certain that there must be a sufficient cause for it 

 in nature; and we employ the generalization, empirically 

 arrived at, with as much confidence as we do the law of 

 necessary correlation.' In many critical cases, where the 

 evidence is limited or defective, the empirical is even a safer 

 guide than the rational law, since it is freer from the risk of 

 errors of interpretation. Applied to the instance before us, 

 it is manifest that the principle on which the incisors in 

 Plagiaulax are framed, in regard of number, order of sup- 

 pression, collateral position, and relation to the premolars, 

 corresponds exactly with the type of the Marsupial Herbi- 

 vores, such as Halmaturus, Hypsiprymnus, and Pliascolarctus, 

 and that it is wholly at variance with the Carnivorous type. 



Let us now test the opinion in its professed character as a 

 physiological deduction. Throughout the Mammalia, where 

 teeth perform the functions of canines ' to pierce, retain, and 

 kill,' they are held well apart through the interposition of a 

 line of incisors— the end being obvious ; the points of pene- 

 tration are doubled, the grasp is strengthened by widening 

 the base, and the dilacerating and killing powers are multi- 

 plied. To arrange them collaterally in the axis would be to 

 place them at a disadvantage to the end to be attained. But 

 when a gnawing power is required, the middle incisors are 

 powerfully developed, and placed collaterally in the axis of 

 the jaws, one on each side, above and below, as typically ex- 

 emplified in the placental Rodents and Cheiromys. Doubt- 

 less a Rat when seized can inflict a smart wound on the 

 hand ; but the power is a secondary attribute, complemen- 

 tary to the main function. Regarded in this aspect, it is 

 negatively stamped upon the incisors of Plagiaulax by their 

 collateral position, that they are not constructed upon the 

 carnivorous plan of design, nor in rational correlation 

 thereto. 



It is obvious that this position of the teeth in Plagiaulax 

 was not overlooked by the author of ' Paleontology ' ; for, on 

 the first occasion he describes the incisor of P. BecMcsii as 

 being ' very large, shaped like a canine, but implanted by a 



1 Cuvier, 'Discours Preliminaire,' p. 51. 



F F 2 



