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
.
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