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 rearrangement.