106 LECTURES ON EVOLUTION m 



consists of three bones, which, early unite with one 

 another and with an additional bone, which repre- 

 sents the lower row of bones in the tarsus of the 

 crocodile. 



In other words, it can be shown by the study of 

 development that the bird's pelvis and hind limb 

 are simply extreme modifications of the same fun- 

 damental plan as that upon which these parts are 

 modelled in reptiles. 



On comparing the pelvis and hind limb of the 

 ornithoscelidan with that of the crocodile, on the 

 one side, and that of the bird, on the other (Fig. 6), 

 it is obvious that it represents a middle term be- 

 tween the two. The pelvic bones approach the 

 form of those of the birds, and the direction of the 

 pubis and ischium is nearly that which is charac- 

 teristic of birds ; the thigh, bone, from the direction 

 of its head, must have lain close to the body ; the 

 tibia has a great crest ; and, immovably fitted on 

 to its lower end, there is a pulley-shaped bone, 

 like that of the bird, but remaining distinct. The 

 lower end of the fibula is much more slender, 

 proportionally, than in the crocodile. The meta- 

 tarsal bones have such a form that they fit together 

 immovably, though they do not enter into bony 

 union; the third toe*is, as in the bird, longest and 

 strongest. In fact, the ornithoscelidan limb is 

 comparable to that of an unhatched chick. 



Taking all these facts together, it is obvious 

 that the view, which was entertained by Maritell 

 and the probability of which was demonstrated by 



