TRANSFORMATION OF ORGANS OF ANIMALS 53 



In the Opossum (Didelphys) we find a normal 

 five-toed foot, in which the third toe predominates, 

 bnt which is fully adapted to arboreal life, the first 

 toe being opposable and provided with a flat nail. 



The foot of the Phalanger 

 (Phalangista) (fig. 24) is 

 also five-toed, and the great 

 toe is opposable, but here it 

 is the fourth toe (iv.) which 

 predominates owing to the < 

 degeneration of the second 

 (ii.) and third (iii.) 



The Kaola (Phascolarctos) 

 exhibits the same type of 

 foot, but with an even more 

 marked diminution of the 

 second and third toes. 



In Hypsiprymnodon the 

 first toe has become rudi- 

 mentary. 



In Perameles the first toe 

 is only represented by a 

 metatarsal with phalanges. 



In the kangaroo (fig. 25) 

 it has completely disappeared. 



Finally, in Chceropus (fig. 26) the fifth toe is 

 reduced to a mere thread, as are also the second 

 (ii.) and third (iii.) toes. 



This last stage, which represents the present and 

 most perfect adaptation of marsupials to leaping, 



FIG. 24. Right fore-paw of 



Phalanger. 



c, calcanenm; a, astragalus; n, 

 os centrale ; cb, cuboid ; el, c2, c 15 , 

 first, second, and third cuneiforms ; 

 I, 11, III, IV, V, 1st, 2nd, 3rd, 

 4th and 5th d'gits, of which the 

 second and third are degenerating. 

 (After Flower, Introduction to the 

 Osteology of the Mammalia.) 



