We shall consider first the theory that the idea of megalithic building was evolved among several races independently, i.e. that it was a phase of culture through which they separately passed. On the whole, this idea has not found favour among archaeologists. The use of stone for building might have arisen in many places independently. But megalithic architecture is something much more than this. It is the use of great stones in certain definite and particular ways. We have already examined what may be called the style of megalithic architecture and found that the same features are noticeable in all countries where these buildings occur. In each case we see a type of construction based on the use of large orthostatic slabs, sometimes surmounted by courses of horizontal masonry, with either a roof of horizontal slabs or a corbelled vault. Associated with this we frequently find the hewing of underground chambers in the rock. In almost all countries where megalithic structures occur certain fixed types prevail; the dolmen is the most general of these, and it is clear that many of the other forms are simply developments of this. The occurrence of structures with a hole in one of the walls and of blocks with 'cup-markings' is usual over the whole of the megalithic area. There are even more remarkable resemblances in detail between structures in widely separated countries. Thus the Giants' Tombs of Sardinia all have a concave facade which forms a kind of semicircular court in front of the entrance to the tomb. This feature is seen also in the temples of Malta, in the tomb of Los Millares in Spain, in the _naus_ of the Balearic Isles (where, however, the curve is slight), in the Giant's Grave of Annaclochmullin and the chambered cairn of Newbliss in Ireland, in the tomb of Cashtal-yn-Ard in the Isle of Man, in the barrow of West Tump in Gloucestershire, and in the horned cairns of the north of Scotland. These parallels are due to something more than coincidence; in fact, it is clear that megalithic building is a widespread and homogeneous system, which, despite local differences, always preserves certain common features pointing to a single origin. It is thus difficult to accept the suggestion that it is merely a phase through which many races have passed. The phases which occur in many races alike are always those which are natural and necessary in the development of a people, such as the phase of using copper. But there is nothing either natural or necessary in the use of huge unwieldy blocks of stone where much smaller ones would have sufficed.