wild cattle
probably kept under cover of the trees during the day and went out to
the grassy uplands only when darkness came on. The feeding grounds of
the grass-eating animals determined the haunts of the beasts of prey.
When wild cattle are attacked, the larger animals in the herd surround
the younger and weaker so as to present a wall of horns to the
assailant. This habit is not peculiar to wild cattle, but is quite
characteristic of all grass-eating animals.

_The Wild Horse._ The steps in the evolution of the horse are stated so
fully in the text that it is not necessary to repeat them here. Almost
any good text in geology gives the same facts. It should be remembered
that horses with more than one toe on each foot did not live when the
Tree-dwellers did, but during earlier periods. The teacher who wishes to
read further regarding the wild horse will find materials in the
_Century Dictionary_ under _horse_, in _Chambers' Encyclopedia_,
in H. N. Hutchinson's _Creatures of Other Days_, in N. S. Shaler's
_Domesticated Animals_, and in _McClure's Magazine_, Vol. 15, p. 512.

_The Musk Sheep._ The appearance of the musk sheep in western Europe
during the mid-Pleistocene period marked the change that was beginning
to take place in the climate. As the climate increased in severity all
the arctic species came down from the north and occupied the land during
the late Pleistocene period. The musk sheep is the most arctic in its
habits of any of the herbivores, and at the present time is restricted
to the high latitudes of North America. It thrives in desolate,
treeless, barren grounds, not even being driven from its haunts by the
extremest cold. It is closely allied to the species which is the parent
of our domestic sheep, although that species did not appear in western
Europe until prehistoric times. The musk sheep goes in herds of from
twenty to thirty individuals, and when alarmed the animals huddle
together like frightened sheep. Its food is grass, lichens, moss, and
tender shoots of the willow and pine. It is much sought after for its
skin, which makes a fine robe. It is sometimes known as the musk ox and
occasionally as the musk bison.

_Plant Life._ The characteristic trees of the mid-Pleistocene period
were evergreen. Of these the most abundant forms were the spruce, the
fir, and the yew tree. The trees which shed their foliage were
represented by the oak and the birch. The banks of rivers were shaded by
thickets