they
recognized in them a troop of the wild oxen, called Yak by the
Thibetans.[152] There were more than fifty of them incrusted in the ice.
No doubt they had tried to swim across at the moment of congelation, and
had been unable to disengage themselves. Their beautiful heads,
surmounted by huge horns, were still above the surface, but their bodies
were held fast in the ice, which was so transparent that the position of
the imprudent beasts was easily distinguishable; they looked as if still
swimming, but the eagles and ravens had pecked out their eyes."[153]

The foregoing investigations, therefore, lead us to infer that the
mammoth, and some other extinct quadrupeds fitted to live in high
latitudes, were inhabitants of Northern Asia at a time when the
geographical conditions and climate of that continent were different
from the present. But the age of this fauna was comparatively modern in
the earth's history. It appears that when the oldest or eocene tertiary
deposits were formed, a warm temperature pervaded the European seas and
lands. Shells of the genus Nautilus and other forms characteristic of
tropical latitudes; fossil reptiles, such as the crocodile, turtle, and
tortoise; plants, such as palms, some of them allied to the cocoa-nut,
the screw-pine, the custard-apple, and the acacia, all lead to this
conclusion. This flora and fauna were followed by those of the miocene
formation, in which indications of a southern, but less tropical climate
are detected. Finally, the pliocene deposits, which come next in
succession, exhibit in their organic remains a much nearer approach to
the state of things now prevailing in corresponding latitudes. It was
towards the close of this period that the seas of the northern
hemisphere became more and more filled with floating icebergs often
charged with erratic blocks, so that the waters and the atmosphere were
chilled by the melting ice, and an arctic fauna enabled, for a time, to
invade the temperate latitudes both of N. America and Europe. The
extinction of a considerable number of land quadrupeds and aquatic
mollusca was gradually brought about by the increasing severity of the
cold; but many species survived this revolution in climate, either by
their capacity of living under a variety of conditions, or by migrating
for a time to more southern lands and seas. At length, by modifications
in the physical geography of the northern regions, and the cessation of
floating ice on th