104 LECTURES ON EVOLUTION III 



is called the sacrum, a number of vertebra may 

 unite together into one whole, and in this respect, 

 as in some details of its structure, the sacrum of 

 these reptiles approaches that of birds. 



But it is in the structure of the pelvis and of 

 the hind limb that some of these ancient reptiles 

 present the most remarkable approximation to 

 birds, and clearly indicate the way by which the 

 most specialised and characteristic features of the 

 bird may have been evolved from the corre- 

 sponding parts in the reptile. 



In Fig. 6, the pelvis and hind limbs of a croco- 

 dile, a three-toed bird, and an ornithoscelidan are 

 represented side by side ; and, for facility of com- 

 parison, in corresponding positions ; but it must be 

 recollected that, while the position of the bird's 

 limb is natural, that of the crocodile is not so. In 

 the bird, the thigh-bone lies close to the body, 

 and the metatarsal bones of the foot (ii., iii., iv., 

 Fig. 6) are, ordinarily, raised into a more or less 

 vertical position ; in the crocodile, the thigh-bone 

 stands out at an angle from the body, and the 

 metatarsal bones (i., ii., iii., iv., Fig. 6) lie flat 

 on the ground. Hence, in the crocodile, the body 

 usually lies squat between the legs, while, in the 

 bird, it is raised upon the hind legs, as upon 

 pillars. 



In the crocodile, the pelvis is obviously com- 

 posed of three bones on each side : the ilium (//.), 

 the Pubis (Pb.), and the ischium (/s.). In the 

 adult bird there appears to be but one bone on 



