ORIGIN OF WHALES. 251 



Pair-hoofed anim-als (Aitiodactyla), comprises those hoofed 

 animals in which the middle (third) and fourth toe of the 

 foot are almost equally developed, so that the space between 

 the two forms the central line of the entire foot. The order 

 is divided into two sub-orders — the Pig-shaped and the Cud- 

 chewing, or Ruminating. The Pig-shaped (Choeromorpha) 

 comprise in the first place the other branch of Primary- 

 Hoofed-animals, the Anoplotheria, which we consider as the 

 common primary form of all Pair-hoofed animals, or Artio- 

 dactyla (Dichobune, etc.) Out of the Anoplotheria arose, as 

 two diverging branches, the primseval swine, or Anthraco- 

 theria, on the one hand, forming the transition to swine and 

 river-horses, and the Xiphodonta on the other hand, forming 

 the transition to Ruminating animals. The oldest Rumin- 

 ating animals (Ruminantia) are the Primjeval Stags, or Dre- 

 motheria, out of which, possibly, the stag-shaped (Elaphia), 

 the hollow-horned (Cavicornia), and camels (Tylopoda), have 

 developed as three diverging branches. Yet these latter are, 

 in many respects, more allied to the Odd-hoofs than to the 

 genuine Pair-hoofs. The accompanying systematic survey 

 on p. 252, will show how the numerous families of Hoofed 

 animals are grouped, in correspondence with this genea- 

 logical hypothesis. 



It is probable that the remarkable legion of Whales 

 (Cetacea) originated out of Hoofed animals, which accustomed 

 themselves exclusively to an aquatic life, and thereby became 

 transformed into the shape of fish. Although these animals 

 seem externally very like many genuine Fish, yet they are, 

 as even Aristotle perceived, genuine Mammals. By their 

 whole internal structure — in so far as it has not become 

 changed by adaptation to an aquatic life — they, of all known 



