ch the thirty-fifth parallel. The temperate climatic zones
extend from the annual isotherm of 20 deg. C. to that of 0 deg. C. (32 deg. F.),
which bears little relation to the polar circles forming the limits of
the solar Temperate Zone. The north temperate climatic zone has been
further sub-divided along the annual isotherm of 5 deg. C. (41 deg. F.),
distinguishing thus the warmer southern belt, which forms preeminently
the zone of greatest historical intensity. The areas beyond the annual
isotherms of 0 deg.C. belong to the barren cold zones. [See map page 612.]

[Sidenote: Historical effect of compressed isotherms.]

This isothermal division of the climatic zones is abundantly justified,
because the duration of a given degree of heat or cold in any region is
a dominant factor in its human, animal, and plant life. A map of the
mean annual isotherms of the earth is therefore eloquent of the relation
between historical development and this one phase of climate. Where the
lines run far apart, they enclose extensive areas of similar
temperature; and where they approach, they group together regions of
contrasted temperatures. The compression of climatic differences into a
small area enlivens and accentuates the process of historical
development. It produces the same sort of effect as the proximity of
contrasted reliefs. Nowhere else in the world do the tropical and
frigid climatic areas, as defined on the north and south by the annual
isothermal lines of 20 deg.C. and 0 deg.C. respectively, lie so near together as
in Labrador and northern Florida. Separated here by only twenty degrees
of latitude, on the opposite side of the Atlantic they diverge so
sharply as to include the whole western face of Europe, from Hammerfest
and the North Cape down to the Canary Islands and the crest of the Atlas
Mountains in Africa, a stretch of forty-two degrees of latitude. This
approximation of contrasted climatic districts in North America was an
immense force in stimulating the early economic development of the
Thirteen Colonies, and in maturing them to the point of political
autonomy. It gave New England commerce command of a nearby tropical
trade in the West Indies, of sub-tropical products in the southern
colonies, in close proximity to all the contrasted products of a cold
climate--dense northern forests for naval stores and lumber, and an
inexhaustible supply of fish from polar currents, which met a strong
demand in Europe and the Ant