88 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 affinities, or 

 the fundamental differences, existing between similar organs, 

 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, 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 poste- 

 rior 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 one, 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 

 this account, the albatross, and some other aquatic birds 



