156 



CHAPTERS ON EVOLUTION. 



producing the so-called " tarso-metatarsal " bone (Fig. 76, w). It is 

 this bone which becomes so greatly elongated in the waders, such as 

 the storks and ibises. As seen in the young fowl (Fig. 776), the shin or 

 leg-bone (/) bears at its lower extremity the " astragalus " (a) of the 



ankle, shortly to be firmly 

 united to the leg by bony 

 union (A). The latter con- 

 dition is seen in the left- 

 hand figure (A), where the 

 astragalus (a) has become 

 united to the tibia, or 

 chief leg-bone (/); the 

 other bone of the leg, or 

 fibula (/), being rudi- 

 mentary. Such a com- 

 plete union of ankle-bones 

 with the leg is not seen in 

 any living reptiles (see Fig. 

 85, c). Whilst the latter 

 have four toes as their 

 least complement, birds 

 have never more than 

 four, the fifth toe being 

 invariably wanting. And 

 whilst in birds, the bones 

 of the instep unite with 

 the lower half of the ankle 

 to form a single bone 

 (Figs. 85, A, tm, and 76, 

 #'), in reptiles the instep 

 FIG. 7 6.-s K EtETON OF BIRD. bones (or metatarsals) 



(Fig. 85 c, i, 2, 3, 4) are not united together, and are distinct from 

 those of the ankle. 



Thus much for dry details. The reader who has taken the trouble 

 to follow this category of the personal characters of birds as compared 

 with those of reptiles, will probably find that the somewhat extended 

 examination will assist his comprehension of certain abnormalities 

 in the structure of several extinct forms of bird and reptilian life ; 

 since many of the characteristic features of each class just detailed 

 will be found to have been curiously modified and often united in 

 the "missing links" which bind these two groups of animals together. 

 It may be firstly asserted that the ostriches, cassowaries, and their 

 relatives, differ from all other birds in possessing a flat shield-like 

 breast-bone instead of the normal "keeled" structure (Fig. 76, f,g) 

 proper to the class. Their " merrythought " is likewise incomplete, 



