SWIMMING ANIMALS. 61 



Rodents, and the Hippopotamus among the Ungulates. 

 Since, however, none of these depart very widely from 

 the normal type of structure, we may pass to the 

 consideration of two groups, in which all the members 

 have undergone more or less profound structural altera- 

 tions solely and simply for the purpose of swimming. 



The first of these groups is that of the Seals and 

 Walruses, which form a special division of the Carni- 

 vores. Through the specimens exhibited from time to 

 time in the Gardens of the Zoological Society, most of 

 us are more or less familiar with the external form of 

 the Seals (Fig. 19), and have also witnessed the exceed- 

 ing gracefulness of their evolutions in the water. In 

 all Seals the limbs are very short, and by the complete 

 webbing of the toes, are converted into paddle-like 

 organs. Although the amount of modification is greater 

 in the hind than in the fore limb, yet in both the 

 several digits of the feet still retain their external 

 distinctness. In the so-called Eared-Seals (from which 

 the seal-skin of commerce is albne obtained) and 

 Walruses the hind feet are turned forwards beneath 

 the body when the animal is on land, after the ordinary 

 manner. In the true Seals, however (Fig. 19), these 

 feet are always directed backwards, and thus act solely 

 as propellers in the water. All the Seal tribe are 

 clearly very closely allied to the ordinary land Carni- 

 vores; and the Eared- Seals and Walruses indicate 

 the mode in which such animals have undergone a 

 progressive modification until the extreme specialisation 

 of the true Seals has been attained. 



