584 THE MECHANICAL FUNCTIONS. 



their mechanical support and progressive motion 

 while on land. 



The anterior extremities having been exclu- 

 sively appropriated to flight, and constructed 

 with reference to the properties of the atmos- 

 phere, the offices of sustaining and of moving 

 the body along the ground must be entrusted 

 wholly to the hind limbs. The centre of gravity, 

 before sustained by the wings, must now be 

 brought over the new basis of support formed 

 by the feet ; or rather, as it is placed far 

 forwards, the feet must be considerably ad- 

 vanced so as to be brought underneath that 

 centre. But as the bones of the posterior ex- 

 tremity have their origin from the remote part 

 of the pelvis, which is elongated backwards, at 

 a considerable distance from the wings, it be- 

 came necessary to lengthen some of their parts, 

 and to bend their joints at very acute angles. 

 We accordingly find that while nature, in the 

 formation of the limb, has preserved an accor- 

 dance with the vertebrated type, both as to the 

 number of pieces which compose it, and as to 

 their relative situations, she has deviated from 

 the model of quadrupeds in giving much greater 

 length to the division corresponding to the foot. 

 At the same time that the foot is brought for- 

 wards, the toes are lengthened, and made to 

 spread out so as to enclose a wide base, over 

 which the centre of gravity is situated. The 



