152 'XHE MODES OF PROGRESSION. 



which are divided, like those of the hand, into three parts, the 

 tarsus, metatarsus, and toes. Their modifications are generally 

 less marked than in the arm, inasmuch as there is less diversity 

 of function ; for in all animals, without exception, the poste- 

 rior extremities are used exclusively for support or locomotion. 

 288. The anterior extremity of the vertebrata, however 

 varied in form, whether it be an arm, a wing, or a fin, is com- 

 posed of essentially the same parts, and constructed upon the 

 same general plan. This affinity does not extend to the in- 

 vertebrata, for although in many instances their limbs bear a 

 certain resemblance to those of the vertebrata, and are even 

 used for similar purposes, yet they have no real affinity. Thus 

 the leg of an insect (fig. 34), and that of a camel (fig. 169), 

 the wing of a butterfly, and the wing of a bat, are quite similar 

 in form, position, and use ; but in the bat (fig. 1 63) and the camel 

 (fig. 169), the organ has an internal bony support, which is a 

 part of the skeleton ; while the leg of the insect has merely 

 a horny covering, proceeding from one of the rings of the 

 body, and the wing of the butterfly is merely a fold of the 

 skin ; showing that the limbs of the articulata are constructed 

 upon a different plan. It is by ascertaining and regarding 

 these real affinities, or the fundamental differences existing 

 between similar organs, that the true natural grouping of ani- 

 mals is to be attained. 



2. OF STANDING, AND THE MODES OF PROGRESSION. 



289. STANDING, or the natural attitude of an animal, de- 

 pends on the form and functions of the limbs. Most of the 

 terrestrial mammals, and the reptiles, both of which employ 

 all four limbs in walking, have the back-bone horizontal, and 

 resting at the same time upon both the anterior and posterior 

 extremities. Birds, whose anterior limbs are intended for a 

 purpose very different from the posterior, stand upon the latter, 

 when at rest, although the back-bone is still very nearly hori- 

 zontal. Man alone is designed to stand upright, with his head 

 supported on the summit of the vertebral column. Some 

 monkeys can rise erect upon their hind legs ; but this is evi- 

 dently a constrained posture, and not their habitual attitude. 



290. In standing, it is requisite that the limbs should 

 be so disposed that the centre of gravity may fall within the 

 space included by the feet. If the centre of gravity be with- 



