156 THE MODES OP PROGRESSION. 



divisions, so as to grasp branches like a forceps. Others like 

 the bears employ their arms for this purpose ; monkeys use 

 their hands and tails ; and parrots their beaks. Lastly, there 

 are some whose natural mode of progression is climbing ; such 

 as the long-armed sloths, which, when placed upon the ground, 

 move very awkwardly ; yet their structure is by no means 

 defective, for in their accustomed movements upon trees, they 

 use their limbs with very great adroitness. 



299. 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 

 Podurellce. 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 (JElaters). 



300. 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 momentary 

 support, upon which the body of the bird rests. But as 

 this support very soon yields, owing to the slight density 

 of the air, it follows that the bird must make greater and 

 more rapid efforts to compensate for this disadvantage. 

 Hence it requires a much greater expenditure of strength to 

 fly than to walk ; and therefore, we find the great mass of 

 muscles in birds concentrated about the breast (fig. 77). To 

 facilitate its flight, the bird, after each stroke of the wings, 

 brings them against the body, so as to present as little re* 

 sisting surface to the air as possible, and for the same end 

 all birds have the anterior part of the body very slender. 



301. Some quadrupeds, as the flying squirrel, Galeopithe- 

 cus [and flying lizard, Draco volans], have a fold of the skin at 

 the sides, which in some extends to the legs, thereby enabling 

 them to leap from branch to branch with more facility. But 

 this is not flight, properly speaking, since none of the peculiar 

 operations of this act are performed. There are also some 



