h not a tiger, was one of the most formidable of these transitory
races. Its upper canine teeth (the "sabres") were several inches in
length, and it had enormously distensible jaws to make them effective.
The great development of such animals, with large numbers of hyaenas,
civets, wolves, bears, and other Carnivores, in the middle and later
Tertiary was probably the most effective agency in the evolution of
the horse and deer and the extinction of the more sluggish races. The
aquatic branch of the Carnivores (seals, walruses, etc.) is little
represented in the Tertiary record. We saw, however, that the most
primitive representatives of the elephant-stock had also some characters
of the seal, and it is thought that the two had a common origin.

The Moeritherium was a marsh-animal, and may very well have been cousin
to the branch of the family which pushed on to the seas, and developed
its fore limbs into paddles.

The Rodents are represented in primitive form early in the Eocene
period. The teeth are just beginning to show the characteristic
modification for gnawing. A large branch of the family, the Tillodonts,
attained some importance a little later. They are described as combining
the head and claws of a bear with the teeth of a rodent and the general
characters of an ungulate. In the Oligocene we find primitive squirrels,
beavers, rabbits, and mice. The Insectivores also developed some of the
present types at an early date, and have since proved so unprogressive
that some regard them as the stock from which all the placental mammals
have arisen.

The Cetacea (whales, porpoises, etc.) are already represented in the
Eocene by a primitive whale-like animal (Zeuglodon) of unknown origin.
Some specimens of it are seventy feet in length. It has large teeth,
sometimes six inches long, and is clearly a terrestrial mammal that
has returned to the waters. Some forms even of the modern whale develop
rudimentary teeth, and in all forms the bony structure of the fore limbs
and degenerate relic of a pelvis and back limbs plainly tell of the
terrestrial origin. Dolphins appear in the Miocene.

Finally, the Edentates (sloths, anteaters, and armadilloes) are
represented in a very primitive form in the early Eocene. They are then
barely distinguishable from the Condylarthra and Creodonta, and seem
only recently to have issued from a common ancestor with those groups.
In the course of the Tertiary we find them--especially in South