of population exceeds two hundred or more per square
mile. The reason is obvious. Life seeks that environment which yields
the greatest amount of nutrition with the least expenditure of energy.

The study of a good relief map shows that, as a rule, the Pacific Ocean
is bordered by a rugged highland, which has a more or less abrupt slope,
and a narrow coast-plain. Indeed, the latter is absent for the greater
part. The slopes of the Atlantic, on the other hand, are long and
gentle--being a thousand miles or more in width throughout the greater
part of their extent. The area of productive land is correspondingly
great, and the character of the surface features is such that
intercommunication is easy.

[Illustration: A RIVER FLOOD-PLAIN--A REGION ADAPTED TO CULTIVATION]

The result of these conditions is evident. The Atlantic slopes, though
not everywhere the most densely peopled areas, contain the great centres
of the world's activities and economies. In the past 400 years they have
not only overtaken the Pacific coast races, but have far surpassed them.
They are now entering upon a commercial invasion of the Pacific nations
that is resulting in a reorganization of the entire industrial world.

=Topography and Trade Routes.=--As the settlement and commerce of a
country grow, roads succeed trails, and trails are apt to follow the
paths of migrating animals. Until the time of the Civil War in the
United States, most of the great highways of the country were the direct
descendants of "buffalo roads," as they were formerly called.

In the crossing of divides from one river-valley to another, the
mountain-sections of the railways for the greater part follow the trails
of the bison. This is especially marked in the Pennsylvania, the
Baltimore and Ohio, and the Chesapeake and Ohio railways; in some
instances the tunnels through ranges have been constructed directly
under the trails. The reason is obvious; the instinct of the bison led
him along routes having the minimum of grade.

Throughout the Mississippi Valley and the great plains the Indian trails
usually avoided the bottom-lands of the river-valleys, following the
divides and portages instead. This selection of routes was probably due
to the fact that the lowlands were swampy and subject to overflow; the
portages and divides offered no steep grades, and were therefore more
easily traversed.

[Illustration: WHERE COMMODITIES ARE EXCHANGED--NEW YORK CITY
WATER-FRONT]