OF THE ATTITUDES AND OF LOCOMOTION. 151 



the projectile. Were the soil to yield under the feet, it is 

 obvious that no leap could take place. With quadrupeds, it 

 is principally the hinder extremities which act as the spring 

 or force ; and hence, in animals of great speed, as the ante- 

 lope, horse, &c., these limbs are long, and flexed, and slender. 

 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 



Fig. 104.* 



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. 



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 



* Skeleton of the foetus of the Greenland whale. The specimen was re- 

 moved for me from the uterus of its mother, a full-sized Mysticetus or 

 Greenland whale, by Mr. Robert Auld, one of my students. The skeleton 

 was long in the museum in Old Surgeons' Hall, and is now in that of the 

 University of Edinburgh. 



