TRANSFORMATION OF ORGANS OF ANIMALS 5*7 



(d) The Frog (Eana Esculenta). Adaptation to 

 leaping entails, as a rule, the elongation of the hind 

 limbs. When a large blade-like swimming foot is 

 required, the number of toes cannot be diminished 

 and length must be augmented, as in the tarsius, in 

 the region of the ankle (fig. 28). 

 But, in the frog, it is the 

 astragulus (a.) and the calcan- 

 eum (c.) which have elongated, 

 whereas in the tarsius it was 

 the calcaneum and navicular 

 bones. 



Besides the union of the 

 calcaneum and the astragulus 

 bones which may be regarded 

 as a phenomenon of degenera- 

 tion, as it entails the disappear- 

 ance of the intervening tissues 

 the adaptation of the frog to 

 leaping has been attended by a F iG.28.-mght.footof*aa 

 yet more characteristic sign of a> astragnfus?'"', caican- 

 degeneration : the tarsus, which 

 is a very complicated structure 

 in tailed amphibia, is reduced, 

 especially in the distal row, to a few minute 

 bones. 



3. Adaptation to running. The ruminants and 

 the horse furnish us with two excellent examples 

 among mammals of adaptation to running. 



Their common ancestor is known to have been a 



Frog ' } 



o/ f 



