•516 THE MECHANICAL FUNCTIONS. 



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 

 advanced so as to be brought underneath that 

 centre. But as the bones of the posterior extremity 

 have their origin from the remote part of the pelvis, 

 which is elongated backwards, at a considerable 

 distance from the wings, it became 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 accordance with the vertebrate 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 

 forwards, 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 extent of 

 this base is so considerable that a bird can, in 

 general, support itself with ease upon a single foot, 

 without danger of being overset by the unavoidable 

 vacillations of its body. 



The femur is short compared with the tibia, 

 which is generally large, especially in the order of 

 GralltE, or wading birds : the fibula is exceedingly 

 slender, and always united, at its lower part, with 

 the tibia ; and there is a total deficiency of tarsal 

 bones, except in the Ostrich, where rudiments of 

 them may be traced. Already we have seen, in 

 ruminant quadrupeds, that these bones have 

 dwindled to a very small size ; but here they have 



