m the Reconstruction of extinct Vertebrate Forms. 489 



Edentata, as from the Sloths. But so little do the united in- 

 stances furnish a case of means inadequate to the end, that Clift, 

 in 1836, supplied Buckland with a drawing of the teeth of Me- 

 gatherium in opposition, in which the hard shell is figured and 

 described as enamel, and the harder parts of the reversed teeth 

 are shown to be brought in contact with the softer, in such a 

 manner, that mastication is performed and maintained by a series 

 of wedges " like the alternate ridges on the rollers of a crush- 

 ing-mill,^^ and accompanied by a property, the perfection of all 

 machinery, namely, that of maintaining itself perpetually in 

 perfect order by the act of performing its work*. 



Enamel, therefore, although structurally absent, is functionally 

 present in the substituted shell of hard ivory. The force of Mr. 

 Huxley^s objection is thus narrowed to the use in the Sloth of 

 a material different from the ordinary one. Does this furnish 

 any good argument against the law of correlation ? In physics 

 analogous cases of substitution are met with ; for instance, in 

 MitscherlicVs isomorphous salts, wherein certain bases may be 

 substituted indifferently, but the combinations will always result 

 in the same function, i. e. crystallize in the same geometrical 

 form. No one has on this account doubted the constancy of the 

 laws of crystallization. In predaceous birds, the teeth and jaws 

 of the Carnivora are replaced by the mandibles and hooked 

 bill ; but the claw of the Eagle is, notwithstanding, as much in 

 correlation with the bill, as the retractile claw with the scissorial 

 carnassial tooth in the Tiger, the tj^pes of construction being 

 different. 



Mr. Huxley^s next objection is starthng. He asks : " Why 

 should not ungulate animals be carrion-feeders ? or even, if 

 living animals were their prey, surely a horse could run down 

 and destroy other animals with at least as much ease as a wolf.^' 

 There are certain Ungulata which do sometimes eat flesh and 

 carrion. The Hog is an example. Cases have been asserted on 

 respectable evidence of its even having eaten young children. 

 But the molar teeth, unlike those of the typical Ungulata, are 

 tubercular or mammillated, not flat, and they differ otherwise. 

 " Among the extinct aberrant forms" (in the Suidse) " the 

 Hippohyus presents almost a ruminant pattern of the grinding 

 surface, while the Choer'opotamus manifests in its whole dentition 

 a close resemblance to the plantigrade Carnivora." " Nothing 

 as yet is known of the incisors of the Choeropotamus ; the rest of 

 the dentition closely resembles that of the Peccari ; but the pre- 

 molars are more simple, and the canines, by their size, shape 

 and direction, and the lower jaw by the backward prolongation 

 of its angle, alike manifest a marked approximation to the 

 * Buckland, Bridgewater Treatise, p. 148. 



