PROGRESSIVE MOTION IN QUADRUPEDS. 343 



and extent of its bounds; for which it is admirably qualified 

 by prodigious power of its muscles, enabling it to spring 

 forwards upon its victim witii an impetus which nothing 

 can resist. 



The speed with which a quadruped is capable of advancing 

 depends more on the disposition of the muscles and the ex- 

 tent of the articulations, and more especially on the power 

 of the extensors of the hind extremities, than on the form 

 of the body. Great length and muscularity in the hind legs 

 are generally attended with considerable power of leaping. 

 This is exemplified in the Jerboa and the Kanf;nroo^ ani- 

 mals, which, from the disproportionate shortness of their 

 fore legs, are totally incapacitated from walking; and for 

 the same reason, they cannot run with any degree of swift- 

 ness. It is only in climbing up a steep acclivity that the 

 jerboa is enabled to employ all its limbs: in a descent, on 

 the contrary, it uses only its fore legs, the hinder being 

 dragged after them. But, when pursued, these animals are 

 capable, for a long continuance, of taking leaps of nine feet 

 distance, and of repeating these leaps so quickly, that the 

 Cossacks, though mounted on the swiftest horses, are unable 

 to overtake them. 



The Kanguroo, in almost all his movements, brings into 

 action his powerful tail, which is furnished with very strong 

 muscles, and maybe considered as constituting a fifth limb. 

 It is of great assistance to the animal in taking leaps, and 

 during its repose, contributes, together with the hind feet, 

 to support the weight of the body, as on a tripod, and to 

 leave at liberty the fore legs, which may then be employed 

 as arms. 



The Hare and the Babbit furnish other instances of an 

 extraordinary length of the hinder legs depriving the ani- 

 mal of the power of walking, and obliging it to move for- 

 wards only by a succession of leaps. The hare may be said, 

 indeed, to walk with its fore legs only, while it gallops with 

 the hinder: but this disadvantage is amply compensated by 

 its amazing swiftness when runi;^ing at full speed. 



