THE MODES OF PROGRESSION. 



153 



out these limits, the animal falls to that side towards which 

 centre of gravity inclines. On this account, the albatros, 

 -omo other aquatic birds which have their feet placed very 

 far back, cannot use them for walking. 



1 . The more numerous and the more widely separated 

 the points of support are, the firmer an animal stands. On 

 this account, quadrupeds are less liable to lose their balance 

 than birds. If an animal has four legs it is not necessary 

 that they should have a broad base. Thus we see that most 

 quadrupeds have slender legs touching the ground by only a 

 small surface (fig. 169). Broad feet would interfere with 

 each other, and only increase the weight of the limbs, without 

 adding to their stability. Birds are furnished with long toes, 

 which as they spread out, subserve the purpose of tripods. 



Fig. 169. The Skeleton of the Camel. 



v o, cervical vertebrae ; v d, dorsal vertebrae ; v I, lumbar vertebras ; v s, 

 the sacrum ; v g, caudal vertebrae ; c, the ribs ; o, scapula ; h, the humerus; 

 : , the carpus ; m c, the metacarpus ; p k, the phalanges; cu, the radius 

 and ulna ; / e , the femur ; r o, the patella ; t i, the tibia ; t a, the tarsus : 

 m t, the metatarsus. 



