ITT LECTURES ON EVOLUTION 105 



each side. The examination of the pelvis of a 

 chick, however, shows that each half is made up 

 of three bones, which answer to those which re- 

 main distinct throughout life in the crocodile. 

 There is, therefore, a fundamental identity of plan 

 in the construction of the pelvis of both bird and 

 reptile ; though the difference in form, relative 

 size, and direction of the corresponding bones in 

 the two cases are very great. 



But the most striking contrast between the 

 two lies in the bones of the leg and of that part of 

 the foot termed the tarsus, which follows upon the 

 leg. In the crocodile, the fibula (F) is relatively 

 large and its lower end is complete. The tibia (T) 

 has no marked crest at its upper end, and its lower 

 end is narrow and not pulley-shaped. There are 

 two rows of separate tarsal bones (As., Ca., cu) 

 and four distinct metatarsal bones, with a rudiment 

 of a fifth. 



In the bird, the fibula is small and its lower end 

 diminishes to a point. The tibia has a strong- 

 crest at its upper end and its lower extremity 

 passes into a broad pulley. There seem at first to 

 be no tarsal bones ; and only one bone, divided at 

 the end into three heads for the three toes which 

 are attached to it, appears in the place of the 

 metatarsus. 



In a young bird, however, the pulley-shaped 

 apparent end of the tibia is a distinct bone, which 

 represents the bones marked As., Ca., in the croco- 

 dile ; while the apparently single metatarsal bone 



