10G 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 tin- 

 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 

 modelled in reptiles. 



On comparing the pelvis and hind limb of the 

 ornithoscelidan with that of the crocodile, on tin- 

 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. Tin 1 

 lower end of the fibula is much more slender, 

 proportionally, than in the crocodile. The in 



il 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 

 < Min|iarable to that of an unhatched chick. 



Taking all these facts together, it is obvious 

 that tin- view, which was entertained by Mantell 

 and the probability of which was demonstrated by 



