64 OF STANDING AND PROGRESSION. 



affinity. Thus the leg of an insect (Fig. 44), and that 

 of a lizard (Fig. 45) ; the wing of a butterfly and the 

 wing of a bat are quite similar in form, position and 

 use ; but in the bat and the lizard, 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 

 (157). It is by ascertaining and regarding these real affini- 

 ties, that the true natural grouping of animals is to be 

 attained. 



2. Of Standing, and the Modes of Progression. 



181. STANDING, or the natural attitude of an animal, depends 

 on the form and functions of the limbs. Most of the ter- 

 restrial 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 horizontal. Man alone, is designed to stand upright, 

 with his head supported on the summit of the vertebral col- 

 umn. Some monkeys can rise upon the hind-legs into the 

 erect posture ; but it is evidently a constrained posture, and 

 not their habitual attitude. 



182. That an animal may stand, it is requisite that the 

 limbs should be so disposed that the centre of gravity, in 

 other words, the point about which the body balances itself, 

 should fall within the space included by the feet. If the 

 centre of gravity is outside of these limits, the animal 

 falls to the side to which the centre of gravity inclines. On 



