90 THE COMMON FROG. [chap. 



The same observation may be repeated with far 

 greater force as regards the ankle. 



In all beasts, as in man, the motion of the leg on 

 the foot takes place by means of a joint between the 

 shin-bone of the leg and the small bones of the ankle ; 

 and though in some beasts (as in the orang) there is 

 considerable power of motion between the first and 

 the second row of ankle-bones, this is small compared 

 ^vith the mobility of the foot and ankle taken together 

 upon the leg. 



In all birds, on the contrary, not only is there no 

 motion between the. ankle-bones (as a whole) and the 

 shin-bone, but the two rows of ankle-bones actually 

 anchylose respectively with adjacent parts — the row 

 nearer the leg coming to form one with the shin-bone; 

 the second row coming to form one with the bones of 

 the foot. Thus in birds the motion of the foot on the 

 leg takes place not between the ankle and the shin- 

 bone, but between the two rows of ankle-bones. 



The same thing to a less degree takes place in 

 reptiles ; the ankle-bones do not indeed anchylose with 

 the shin-bone and foot respectively, but they never- 

 theless unite with those parts so firmly that motion 

 takes place between the bones of the ankle and not 

 between the whole ankle and the leg. 



Now in the frog's class, e.g. in the order Urodela, 

 we meet with a condition which is mammalian rather 

 than reptilian or avian. Motion takes place freely 

 between the leg and the whole tarsus. Moreover, the 

 number and proportions of the ankle-bones them- 

 selves far more closely agree with the condition of 

 the same parts exhibited to us by certain beasts than 



