OF THE ATTITUDES AND OF LOCOMOTION. 



143 



In some, as in the jerboa and kangaroo, the anterior limbs 

 are but little, if at all, used in progression. 



292. Swimming or Flying. These movements are 

 analogous to leaping, the only difference being in fact in the 

 medium in which they take place. The points to be con- 

 sidered are, first, the medium, which in the case of flying is 

 the air, and this, by reason of its rarity and the facility of its 

 displacement, requires being struck with much greater force 

 and rapidity than if it were water, or the still more resisting 

 soil. Hence the great force required by birds in the muscular 

 apparatus by which flight is effected. Second, diminution 

 of the surface of the motory organs during the advance 

 of the body, so as to offer less resistance to the passage of the 

 body through the air. Now, these two conditions we shall 

 find uniformly take place in animals which fly or swim 

 naturally ; the expanded foot of the seal diminishes during 

 the moment of advance, and the wing of the bird approaches 

 the sides ; the flanges attached to the archimedean screw are 

 but poor imitations of the tail-fin of the whale. 



W 



',7 



293. The palmated feet of the otter and seal, of the 

 swan and duck, represent and explain the mechanism by which 

 nature provides for the wants of an animal requiring at 

 times to swim at others to walk on the soil ; the otter also 

 furnishes a good example of this mechanism. But nature does 

 not for all this depart from her great plan in the construction 

 of animals ; the skeleton of the hand and foot of the seal 

 * The bones are lettered as in F;g. 82. 



