On the other hand, many find it preferable to suppose that over this same distance there extended a vast trade route or a series of trade routes, along which travelled the influences which account for the presence of precisely similar dolmens in Denmark, Spain, and the Caucasus.  Yet although much has been written about neolithic trade routes little has been proved, and the fact that early man occasionally crossed large tracts of land and sea in the great movements of migration does not show that he also did so by way of trade, nor does it prove the existence of such steady and extensive commercial relations as such a theory of the megalithic monuments would seem to require.  Immigration is often forced on a race.  Change of climate or the diverting of the course of a great river may make their country unfit for habitation, or they may be expelled by a stronger race.  In either case they must migrate, and we know from history that they often covered long distances in their attempt to follow the line of least resistance.  Thus there is nothing a priori improbable in the idea that the megalithic monuments were built by a single invading race.

There are other considerations which support such a theory.  It will be readily admitted that the commonest and most widely distributed form of the megalithic monument is the dolmen.  Both this and its obvious derivatives, the Giant’s Grave, the allee couverte, and others, are known to have been tombs, while other types of structure, such as the Maltese temple, the menhir, and the cromlech, almost certainly had a religious purpose.  It is difficult to believe that these types of building, so closely connected with religion and burial, were introduced into all these regions simply by the influence of trade relations.  Religious customs and the burial rites connected with them are perhaps the most precious possession of a primitive people, and they are those in which they most oppose and resent change of any kind, even when it only involves detail and not principle.  Thus it is almost incredible that the people, for instance, of Spain, because they were told by traders that the people of North Africa buried in dolmens, gave up, even in isolated instances, their habit of interment in trench graves in favour of burial in dolmens.  It is still more impossible to believe that this unnatural event happened in one country after another.  It is true that the use of metal was spread by means of commerce, but here there was something to be gained by adopting the new discovery, and there was no sacrifice of religious custom or principle.  An exchange of products between one country and another is not unnatural, but a traffic in burial customs is unthinkable.