484 Dr. Falconer on Cuvier's Laws of Correlation, 



Cuvier), the Sloth has a veri/ broad scapula, an enormously ptdi 

 longed acromion, and a clavicle. A portion of the functions of 

 its fore-arm is modified after the plan presented by the Tiger, 

 instead of that of the Ungulata. The habits of the Sloth, although 

 herbivorous, required it, and the necessity under the law of cor- 

 relation worked out the means*. The detailed demonstration 

 would be tedious ; but it is wholly unnecessary, as every com- 

 parative anatomist is familiarly acquainted with it, and probably 

 no one better than Mr. Huxley. So far as the applicability of 

 this objection to the case in point is concerned, it is clearly of a 

 still more exceptionable character than that of the Bears contra 

 the typical Carnivora. 



But the special force of Mr. Huxley^s objection lies in the 

 absence of enamel from the teeth of the herbivorous Sloth, 

 The adduction of the instance is ingenious ; yet the objection in 

 reality is more specious than valid. The molars of the Sloth 

 consist of an irregular cylinder of soft and open-grained ivory 

 (vascular dentine of Owen), which is so permeated by vascular 

 or medullary canals, that it has been compared structurally to 

 the teeth of certain fishes ; this central mass is encased in a 

 shell of compact hard ivory (unvascular dentine of Owen), 

 closely resembling (it is said) that of the human tooth f; and 

 outside of this shell there is a layer of cement harder than the 

 central mass, but softer than the shell of ivory. The cement by 

 use wears away, so as " to form a bevelled edge,^^ while the cen- 

 tral mass becomes depressed, the edge of the shell projecting 

 between them. The crown thus presents three alternate mate- 

 rials of unequal hardness, resulting in an unequally worn surface, 

 being the very end to be attained, in the case put by Cuvier ; 

 the only difference being, that in the Sloth a shell of hard ivory 

 is substituted for the ordinary shell of hard enamel. And so 

 exactly does this shell, to the naked eye, simulate the appearance 

 of enamel, that Cuvier and every other naturalist down to 1837 

 described it either as being enamel or analogous to it. Enamel 

 is equally absent from the teeth of the whole of the Megatheroid 



* " Toutes ces choses se deduisent Tune de I'autre selon leur plus ou 

 moins tie generallte, et de maniere que les unes sont essentielles et exclu- 

 siveraent propres aux animaux a sabot, et que les autres, quoique egale- 

 ment necessaires dans ces animaux, ne leur seront pas exclusives, mais 

 pourront se retrouver dans d'autres animaux, oil le rests des conditions per- 

 mettra encore celles-l^.'' (Cuvier, Discours prelim, p. 50, "^to edit.) Alter 

 the words " animaux a sabot " into " animaux carnivores," and the clause 

 in italics is applicable to the fore-arm of the Sloths. It were easy to show, 

 that the construction of the Sloths, so far from weakening the evidence as 

 to the law of necessary correlation, does, in fact, furnish the strongest 

 arguments in favour of it. 

 , t Owen, Odontography, vol. i. p. 330. 



