measure of our 'yellow-peril'. But where works of art can travel, ideas
can travel too; and can travel right across the frontiers of race and
language and even of religion; meaning at all events by these, the
customary observance of each region, and of its endemic population. A
few merchants, or craftsmen, or philosophers, work transformations in
culture and bring about uniformities, of which language, or
cult-edifices give us no indication at all, or at best an aftermath of
decadence.

It is not a merely ephemeral interest which draws attention at this
point to the significance of engines of war, among this class of
transferable inventions. Little has been done in a systematic way on
this topic, but the rapidity with which a really important change in
equipment and organization passes from camp to camp, and revolutionizes
not only armies but states, when it is a question of survival or defeat,
has its illustration in many phases of warfare, and ranks among the
great levellers of national or regional pride.

The recorded movements of peoples in historic times, and the previous
movements inferred from language, and other symptoms, indicate a
long-established distribution of what might be described in
meteorological phrase as _man-pressure_; certain regions being
characterized either always or repeatedly by high man-pressure, and an
outward flow of men into the cyclonic areas or vortexes of low
man-pressure in the human covering (or biosphere) of the planet. Typical
high-pressure regions are the Arabian peninsula with its repeated crises
of Semitic eruption, and the great Eurasian grasslands. Typical regions
of low man-pressure, and repeated irruption, are the South European
peninsulas. Occasionally a region plays both parts, alternately
accepting inhabitants, and unloading them on to other lands; examples