what miracle the gulf between it and himself was bridged. That is to lay
a superfluous strain on the imagination. The proper term of comparison
is the lowest type of human being known to us, since the higher types of
living men have confessedly evolved from the lower. But even the lowest
type of existing or recent savage is not the lowest level of humanity.
Whether or no the Tasmanian or the Yahgan is a primitive remnant of the
Old Stone Age, we have a far lower depth in the Java race. What we have
first to do is to explain the advance to that level, in the course of
many hundreds of thousands of years: a period fully a hundred times as
long as the whole history of civilisation. Time itself is no factor
in evolution, but in this case it is a significant condition. It means
that, on this view of the evolution of man, we are merely assuming that
an advance in brain-development took place between the Miocene and the
Pleistocene, not similar to, but immeasurably less than, the advance
which we know to have been made in the last fifty thousand years. In
point of fact, the most mysterious feature of the evolution of man was
its slowness. We shall see that, to meet the facts, we must suppose man
to have made little or no progress during most of this vast period, and
then to have received some new stimulation to develop. What it was we
have now to inquire.

CHAPTER XIX. MAN AND THE GREAT ICE-AGE

In discussing the development of plants and animals during the Tertiary
Era we have already perceived the shadow of the approaching Ice-Age.
We found that in the course of the Tertiary the types which were more
sensitive to cold gradually receded southward, and before its close
Europe, Asia, and North America presented a distinctly temperate aspect.
This is but the penumbra of the eclipse. When we pass the limits of
the Tertiary Era, and enter the Quaternary, the refrigeration steadily
proceeds, and, from temperate, the aspect of much of Europe and North
America becomes arctic. From six to eight million square miles of the
northern hemisphere are buried under fields of snow and ice, and even in
the southern regions smaller glacial sheets spread from the foot of the
higher ranges of mountains.

It is unnecessary to-day to explain at any length the evidences by which
geologists trace this enormous glaciation of the northern hemisphere.
There are a few works still in circulation in which popular writers,
relying on the obstinacy of