68 MODES OF PROGRESSION. 



190. CLIMBING- is merely walking upon the surface of an 

 inclined or even upright object. It is more frequently ac- 

 complished by means of sharp nails ; and hence many 

 carnivorous animals climb with great facility, such as the 

 cat tribe, the lizards ; and many birds, the woodpecker, for 

 instance. Others employ their arms for this purpose, like 

 the bears, when they climb a tree ; or their hands, and even 

 their tails, like the monkeys ; or their beaks, like the par- 

 rots. Lastly, there are some whose natural mode of pro- 

 gression is climbing. Such are the sloths, with their arms 

 so long, that when placed upon the ground, they move very 

 awkwardly ; and yet their structure is by no means defect- 

 ive, for in their accustomed movements upon trees, they 

 can use their limbs with very great adroitness. 



191. Most quadrupeds can both walk, trot, gallop, and 

 leap ; birds walk and leap ; lizards neither leap nor gallop, 

 but only walk and run, and some of them with great rapid- 

 ity. No insect either trots or gallops, but many of them 

 leap. Yet their leaping is not always the effect of the mus- 

 cular force of their legs, as with the flea and grasshopper ; 

 but some of them leap by means of a spring, in the form of 

 a hook, attached to the tail, which they bend beneath the 

 body, and which, when let loose, causes them to bound to a 

 great distance, as in the Podurellse. Still others leap by 

 means of a spring, attached beneath the breast, which 

 strikes against the abdomen when the body is bent ; as the 

 spring-beetles (Elaters). 



192. FLIGHT is accomplished by the simultaneous action 

 of the two anterior limbs, the wings, as leaping is by that of 

 the two hinder limbs. The wings being expanded, strike and 

 compress the air, which thus becomes a support, for the 

 moment, upon which the body of the bird may rest itself. 

 But as this support very soon yields, owing to the slight 

 density of the air, it follows that the bird must make the 

 greater and more rapid efforts to compensate for this dis- 



