lso 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.

Perhaps, however, it was not the form of the dolmen which was brought by
commerce, but simply the art of architecture in general, and this was
adapted to burial purposes. To this there are serious objections. In the
first place it does not expl