The building of houses, after living for ages in stone caverns, would not be a very profound invention. The early houses were--as may be gathered from the many remains in Devonshire and Cornwall--mere rings of heaped stones, over which, most probably, was put a roof of branches or reeds, plastered with mud. They belong to the last part of the New Stone Age. In other places, chiefly Switzerland, Neolithic man lived in wooden huts built on piles in the shallow shores of lakes. It is an evidence that life on land is becoming as stimulating as we find it in the age of Deinosaurs or early mammals. These pile-villages of Switzerland lasted until the historical period, and the numerous remains in the mud of the lake show the gradual passage into the age of metal. Before the metal age opened, however, there seem to have been fresh invasions of Europe and changes of its culture. The movements of the various early races of men are very obscure, and it would be useless to give here even an outline of the controversy. Anthropologists have generally taken the relative length and width of the skull as a standard feature of a race, and distinguished long-headed (dolichocephalic), short-headed (brachycephalic), and middle-headed (mesaticephalic) races. Even on this test the most divergent conclusions were reached in regard to early races, and now the test itself is seriously disputed. Some authorities believe that there is no unchanging type of skull in a particular race, but that, for instance, a long-headed race may become short-headed by going to live in an elevated region. It may be said, in a few words, that it is generally believed that two races invaded Europe and displaced the first Neolithic race. The race which chiefly settled in the Swiss region is generally believed to have come from Asia, and advanced across Europe by way of the valley of the Danube. The native home of the wheat and barley and millet, which, as we know, the lake-dwellers cultivated, is said to be Asia. On the other hand, the Neolithic men who have left stone monuments on our soil are said to be a different race, coming, by way of North Africa, from Asia, and advancing along the west of Europe to Scandinavia. A map of the earth, on which the distribution of these stone monuments--all probably connected with the burial of the dead--is indicated, suggests such a line of advance from India, with a slighter branch eastward. But the whole question of these invasions is disputed, and there are many who regard the various branches of the population of Europe as sections of one race which spread upward from the shores of the Mediterranean.