things which led him to the conclusion that force is best resorted
to for northern nations, reason for the middle, and superstition for the
southern.

[Sidenote: Effects of the seasons on animals and plants.]

In the month of March the sun crosses the equator, dispensing his rays
more abundantly over our northern hemisphere. Following in his train, a
wave of verdure expands towards the pole. The luxuriance is in
proportion to the local brilliancy. The animal world is also affected.
Pressed forward, or solicited onward by the warmth, the birds of passage
commence their annual migration, keeping pace with the developing
vegetation beneath. As summer declines, this orderly advance of light
and life is followed by an orderly retreat, and in its turn the southern
hemisphere presents the same glorious phenomenon. Once every year the
life of the earth pulsates; now there is an abounding vitality, now a
desolation. But what is the cause of all this? It is only mechanical.
The earth's axis of rotation is inclined to the plane of her orbit of
revolution round the sun.

Let that wonderful phenomenon and its explanation be a lesson to us; let
it profoundly impress us with the importance of physical agents and
physical laws. They intervene in the life and death of man personally
and socially. External events become interwoven in our constitution;
their periodicities create periodicities in us. Day and night are
incorporated in our waking and sleeping; summer and winter compel us to
exhibit cycles in our life.

[Sidenote: Individual existence depends on physical conditions.]

They who have paid attention to the subject have long ago ascertained
that the possibility of human existence on the earth depends on
conditions altogether of a material kind. Since it is only within a
narrow range of temperature that life can be maintained, it is needful
that our planet should be at a definite mean distance from the source of
light and heat, the sun; and that the form of her orbit should be so
little eccentric as to approach closely to a circle. If her mass were
larger or less than it is, the weight of all living and lifeless things
on her surface would no longer be the same; but absolute weight is one
of the primary elements of organic construction. A change in the time of
her diurnal rotation, as affecting the length of the day and night, must
at once be followed by a corresponding modification of the periodicities
of the nervous system