It takes the plain man a long time to find out that it is no use asking
the pre-historian, who is proudly displaying a skull or a stone
implement, "Please, how many years ago exactly did its owner live?"
I remember hearing such a question put to the great savant, M.
Cartailhac, when he was lecturing upon the pre-historic drawings found
in the French and Spanish caves; and he replied, "Perhaps not less
than 6,000 years ago and not more than 250,000." The backbone of our
present system of determining the series of pre-historic epochs is
the geological theory of an ice-age comprising a succession of periods
of extreme glaciation punctuated by milder intervals. It is for the
geologists to settle in their own way, unless, indeed, the astronomers
can help them, why there should have been an ice-age at all; what was
the number, extent, and relative duration of its ups and downs; and
at what time, roughly, it ceased in favour of the temperate conditions
that we now enjoy. The pre-historians, for their part, must be content
to make what traces they discover of early man fit in with this
pre-established scheme, uncertain as it is. Every day, however, more
agreement is being reached both amongst themselves and between them
and the geologists; so that one day, I am confident, if not exactly
to-morrow, we shall know with fair accuracy how the boys, who left
their clothes lying about, followed one another into the field.

Sometimes, however, geology does not, on the face of it, come into
the reckoning. Thus I might have asked the reader to assist at the
digging out of a cave, say, one of the famous caves at Mentone, on
the Italian Riviera, just beyond the south-eastern corner of France.
These caves were inhabited by man during an immense stretch of time,
and, as you dig down, you light upon one layer after another of his
leavings. But note in such a case as this how easily you may be baffled
by some one having upset the heap of clothes, or, in a word, by