oe Se ee Se 
ORIGIN OF WHALES, 251 
Pair-hoofed animals (Artiodactyla), 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 (Chceromorpha) 
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, ete.) Out of the Anoplotheria arose, es 
two diverging branches, the primzeval 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 Primeeval 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. 
Jt 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 
