hey are also found at the bottom of
various natural caverns.

No human bones have been found as yet with the implements, but the bones
of large numbers of animals have. And it seems certain that the men who
made the implements were contemporaries of the animals, because in the
later part of the age, at any rate, they drew or scratched likenesses of
the animals on bone. Among these representations are figures of the
_mammoth_ an extinct form well known to the reader by description and
museum specimens of remains.

The animals contemporary with these primeval men were the mammoth,
species of rhinoceros and hippopotamus, the "sabre-toothed" lion, the
cave-bear, the reindeer, besides oxen, horses, and other still surviving
forms.

In his address to the British Association in 1881 Sir John Lubbock
called attention to the fact that these animals appear to indicate both
a hot and a cold climate, and he referred to the fact (known to
astronomers) that the earth passes through periods of slow change in the
eccentricity of its orbit, and in the obliquity of the ecliptic. The
result of the latter condition is, to produce periods of about 21,000
years each, during one-half of which the Northern hemisphere will be
hotter, and in the other the Southern. At present we are in the former
phase.

But the obliquity of the ecliptic does not act alone; the eccentricity
of the orbit produces another effect, namely, that when it is at a
minimum the difference between the temperatures of the two hemispheres
is small, and as the eccentricity increases, so does the difference. At
the present time the eccentricity is represented by the fraction .016.
But about 300,000 years ago the eccentricity would have been as great as
.26 to .57. The result, it is explained, would have been not a uniform
heat or cold, but extremes of both; there would probably have been short
but very hot summers, and long and intensely cold winters.

This, Sir John Lubbock thought, might account for the co-existence of
both hot and arctic species, like the hippopotamus and rhinoceros on the
one hand, and the musk-ox and the reindeer on the other.

But such considerations really help us little. In the first place, it is
only an assumption that the fossil hippopotamus _was_ an animal of a hot
climate--it does not in any way follow from the fact that the now
existing species is such; nor if we make the assumption, does it explain
how, if the hot summer sufficed for the t