There is one other negative fact which we gather from the bones of these refuse-heaps--no human bones are mingled with them; so we may conclude that these men were not cannibals. In fact, cannibalism is an extraordinary perversion of human nature, arising it is difficult to say exactly how, and only showing itself among particular people and under peculiar conditions. There is no doubt that, among a very large proportion of the savage nations which at present inhabit our globe, cannibalism is practised, and of this fact many explanations have been offered; but they are generally far-fetched and unsatisfactory; and it is certainly not within our scope to discuss them here. How little natural cannibalism is even to the most savage men is proved by the fact that man is scarcely ever, except under urgent necessity, found to feed upon the flesh of carnivorous or flesh-eating animals, and this alone, besides every instinct of our nature, would be sufficient to prevent him from eating his fellow-men. We have many proofs of the great antiquity of the shell mounds. Their position gives one. Whilst most of them are confined to the immediate neighbourhood of the seashore, some few are found at a distance of several miles inland. These exceptions may always be referred to the presence of a stream which has gradually deposited its mud at the place where it emptied itself into the sea, or to some other sufficient cause of the protrusion of the coast-line; so that these miles of new coast have come into existence after the shell mounds were raised. On the other hand, there are no mounds upon those parts of the coast which border on the Western Ocean. But it is just here that, owing to a gradual depression of the land at the rate of two or three inches in a century[8] the waves are slowly eating away the shore. This is what happens on every sea-coast. Almost all over the world there is a small but constant movement of the solid crust of the earth, which is, in fact, only a crust over the molten mass within. Sometimes, and in some places, the imprisoned mass makes itself felt, in violent upheavals, in sudden cracks of the inclosing surface, which we call earthquakes and volcanoes; but oftener its effect is slight and almost unnoticed. This interchange of state between the kingdoms of the land and of the ocean helps to show us the time which has passed between the making of the kitchen-middens and our own days. There seems little doubt that all along the Danish coast of the North Sea, as well as on that of the Baltic, these mounds once stood; but by the gradual undermining of the cliffs the former series have all been swept away, while the latter have, as it appears, been moved a little inland; and we have seen that when there was another cause present to form land between the kitchen-middens and the sea, the distance has often been increased to several miles.