forms of life appropriate to them, it would be going as much too
far in the other direction to claim that life can exist only with
the precise surroundings which nurture it on this planet. It is
very remarkable in this connection that while in one direction we
see life coming to an end, in the other direction we see it
flourishing more and more up to the limit. These two directions
are those of heat and cold. We cannot suppose that life would
develop in any important degree in a region of perpetual frost,
such as the polar regions of our globe. But we do not find any end
to it as the climate becomes warmer. On the contrary, every one
knows that the tropics are the most fertile regions of the globe
in its production. The luxuriance of the vegetation and the number
of the animals continually increase the more tropical the climate
becomes. Where the limit may be set no one can say. But it would
doubtless be far above the present temperature of the equatorial
regions.

It has often been said that this does not apply to the human race,
that men lack vigor in the tropics. But human vigor depends on so
many conditions, hereditary and otherwise, that we cannot regard
the inferior development of humanity in the tropics as due solely
to temperature. Physically considered, no men attain a better
development than many tribes who inhabit the warmer regions of the
globe. The inferiority of the inhabitants of these regions in
intellectual power is more likely the result of race heredity than
of temperature.

We all know that this earth on which we dwell is only one of
countless millions of globes scattered through the wilds of
infinite space. So far as we know, most of these globes are wholly