147 



end of the foot would be kept fixed upon the ground*, become clearly intelU- 

 gible, and their final purpose explicable on the above-developed idea of the 

 exertions during which the well-based hind-limbs worked in steadying, and 

 reacting upon, the trunk of the Megatherian wrestling with its passively but 

 sturdily resisting source of subsistence. And thus the expanse of the pelvis, 

 the superior bulk and strength of the hind-legs compared with the fore-legs, 

 the peculiar length and organization of the hind-feet, the proportions and con- 

 struction of the tail, all of which form a combination of characters common 

 to the Megatherioids, and present in no other animals, yield the required ex- 

 planation of the uses of the fore-extremities, which too nearly resemble those 

 of other claviculate Edentata, to indicate, apart, their own proper functions in 

 the Megatherioids. 



If the foregoing physiological interpretation of the osseous frame-work of the 

 gigantic extinct Sloths be the true one, they may be supposed to have com- 

 menced the process of prostrating the chosen tree by scratching away the soil from 

 the roots ; for which office we find in the Mylodon the modern scansorial fore-foot 

 of the Sloth modified after the type of that of the partially fossorial Ant-eater. 

 The compressed or subcompressed form of the claws, which detracts from their 

 power as burrowing instruments, adds to their fitness for penetrating the inter- 

 spaces of roots, and for exposing and liberating them from the attached soil. 

 This operation having been duly effected by the alternate action of the fore-feet 

 aided probably by the unguiculate digits of the hind-feet, the long and curved 

 fore-claws, which are habitually flexed and fettered in the movements of exten- 

 sion, would next be applied to the opposite sides of the loosened trunk of the 

 tree : and now the Mylodon would derive the full advantage of those modifica- 

 tions of its fore-feet by which it resembles the Bradypus ; the correspondence 

 in the structure of the prehensile instruments of the existing and extinct Sloths, 

 extending as far as was compatible with the different degrees of resistance to be 

 overcome. In the small climbing Sloth the claws are long and slender, having 

 only to bear the weight of the animal's light body, which is approximated 



* This use of the large hind-claw was first pointed out by Dr. Buckland (Bridgewater Treatise, 

 i. p. 158) ; but the full value of the structure to the Megatherium could not be appreciated, when the 

 hind-legs were supposed to merely aid in supporting the trunk, whilst one fore-leg was used to dig 

 up roots. 



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