The fact is, the climate is now much more temperate and uniform than in the first stone age. Then the reindeer and the chamois, animals which belong naturally to regions of ice and snow, freely traversed, in winter at least, the valleys or the plains far towards the south of Europe.[7] But as the climate changed, the first was driven to the extreme north of Europe, and the second to the higher mountain peaks. The only extinct species belonging to the shell mounds is the wild bull (_bos primigenius_), which however survived in Europe until quite historical times. His remains appear in great numbers, as do those of the seal, now very rare, and the beaver, which is extinct in Denmark. No remains of any domesticated animal are found; but the existence of tame dogs is guessed at from the fact that the bones bear traces of the gnawing of canine teeth, and from the absence of bones of young birds and of the softer bones of animals generally. For it has been shown experimentally that just such portions are absent from these skeletons as will be devoured when birds or animals of the same species are given to dogs at this day. Dogs, therefore, we may feel pretty sure, were domesticated by the stone-age men; so here again we can see the beginning of a step in civilization which has been of incalculable benefit to man, the taming of animals for his use. The ox, the sheep, the goat, were as yet unknown; man was still in the hunter’s condition, and had not advanced to the shepherd state, only training for his use the dog, to assist him in pursuit of the wild animals who supplied part of his food. He was, too, utterly devoid of all agricultural knowledge. Probably the domestication of the dog marks a sort of transition state between the hunter and the shepherd. When that experiment has been tried, the notion must sooner or later spring up of training other animals, and keeping them for use or food. With regard to the dogs themselves, it is a curious fact that those of the stone age are smaller than those of the bronze period, while the dogs of the bronze age are again smaller than those of the age of iron. This is an illustration of the well-known fact that domestication increases the size and improves the character of animals, as gardening does that of plants.