92 MODES OF PROGRESSION. 



190. CLIMBING is merely walking upon an inclined or 

 even upright surface. It is usually accomplished 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 mon- 

 keys ; or their beaks, like the parrots. Lastly, there are some 

 whose natural mode of progression is climbing. Such are 

 the sloths, with their arms so long, that, when placed upon 

 the ground, they move very awkwardly ; and yet their struc- 

 ture is by no means defective, for in their accustomed move- 

 ments upon trees they can use their limbs with very great 

 a.droitness. 



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 rapidity. 

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

 Yet their leaping is not always the effect of the muscular 

 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, propels them to a great 

 distance, as in the Podurellae. 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 bird is sustained. 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 



