112 COMPARATIVE ANATOMY vin 



to show that the smaller size of the second and third toes, 

 and that being wrapped up to their ends in skin, added to 

 the efficiency of the foot as a climbing organ, though indeed 

 he might be at a loss to say why the American opossums, of 

 precisely similar habits, and with similarly opposable great 

 toes, have all their other four toes free and equal. 



The philosophical naturalist of the second of the types that 

 I have sketched out, in investigating this structure, will 



FIG. 3. Skeleton of the hind foot of the Virginian Opossum (Didelphys 

 Virginiana). 



institute a careful comparison between the foot of the koala 

 and that of other animals belonging to the same group. 



Among the marsupial animals of Australia, few are so well 

 known as the kangaroo. Like the koala, it is a vegetable 

 feeder, and these two forms have some dental and other 

 structural characters in common. But the mode of progres- 

 sion, and the limbs which effect this, offer the greatest possible 

 contrast. 



In the kangaroo the hind legs are disproportionately large. 

 The foot especially is long and narrow, and at first sight 

 appears composed of a single large toe. Its motion along the 

 ground consists of a series of leaps or hops effected entirely 

 by these powerful hind limbs. On looking more closely at 



