Between the earlier and the later stone age, between man of the drift period and man of the neolithic era, occurs a vast blank which we cannot fill in. We bid adieu to the primitive inhabitants of our earth while they are still the contemporaries of the mammoth and woolly rhinoceros, or of the cave lion and the cave bear, and while the very surface of the earth wears a different aspect from what it now wears. With a changed condition of things, with a race of animals which differed not essentially from those known to us, and with a settled conformation of our lands and seas not again to be departed from, comes before us the second race of man--man of the polished stone age. We cannot account for the sudden break; or, what is in truth the same thing, many different suggestions to account for it have been made. Some have supposed that the palæolithic men lived at a time anterior to the last glacial era, for there were many glacial periods in Europe, and were either exterminated altogether or driven thence to more southern countries by the change in climate. Others have imagined that a new and more cultivated race migrated into these countries, and at once introduced the improved weapons of the later stone age; and lastly, others have looked upon the first stone age as having existed before the Deluge, and hold that the second race of man, the descendants of Noah, began at once with a higher sort of civilization. Two of these four theories, it will be seen, must suppose that man somewhere went through the stages of improvement necessary to the introduction of the newer sort of weapons, and they therefore take it for granted that the graduated series of stone implements, indicating a gradual progress from the old time to the newer, though they have not yet been found, are to be discovered somewhere. The first and last theories would seem to be more independent of this supposition, and therefore, as far as our knowledge yet goes, to be more in accordance with the facts which we possess. It is, however, by no means safe to affirm that the graduated series of implements required to support the other suppositions will never be found. * * * * * [Sidenote: The kitchen-middens.]