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 tropical hippopotamus, it managed to survive the long and cold winters which suited the arctic species.

Moreover, no such calculations can really be made with accuracy:  we do not know what other astronomical facts may have to be taken into consideration, nor can we say when such “periods” as those which are so graphically described, began or ended.

In this very instance, we know that the mammoth only became extinct in comparatively recent times, since specimens have been found in Siberia, with the hair, skin, and even flesh, entirely preserved.  Granted that the intense cold of the Siberian ice effected this, it is impossible to admit more than a limited time for the preservation—­not hundreds of thousands of years.  Professor Boyd Dawkins is surely right in stating that the calculations of astronomy afford us no certain aid at present in this inquiry.

As regards the geological indications of age, the best authority seems to point to the first appearance of man in the post-glacial times:  that is to say, that the gravels in which the palaeolithic implements are found were deposited by the action of fresh water after the great glacial period, when, at any rate, Northern Europe, a great part of Russia, all Scandinavia, and part of North America were covered with icefields, the great glaciers of which left their mark in the numerous scoopings out of ravines and lake beds and in the raising of banks and mounds, the deposit of boulders, and the striation of rocks in situ, which so many districts exhibit.

The few instances in which attempts have been made, in Italy or elsewhere, to argue for a pliocene man (i.e. in the uppermost group of the tertiary) have ended in failure, at least in the minds of most naturalists competent to judge.

One of the most typical instances of the position of the implement age has been discovered by Fraas at Shuessenried in Suabia; here the remains of tools and the bones of animals (probably killed for food) were found in holes made in the glacial debris.

But here, again, it is impossible to say when this glacial age terminated, and whether man might not have been living in other more favoured parts while it was wholly or partially continuing.

In Scandinavia no palaeolithic stone implements have been found, from which it may be inferred that the glacial period continued there during the ages when palaeolithic man hunted and dwelt in caves in the other countries where his remains occur.