hroughout several thousand centuries, for more than 20 degrees
Fahrenheit; so that if a plant or animal be provided with an
organization fitting it to endure such a range, it may continue on the
globe for that immense period, although every individual might be liable
at once to be cut off by the least possible excess of heat or cold
beyond the determinate degree. But if a species be placed in one of the
temperate zones, and have a constitution conferred on it capable of
supporting a similar range of temperature only, it will inevitably
perish before a single year has passed away.

Humboldt has shown that, at Cumana, within the tropics, there is a
difference of only 4 degrees Fahr. between the temperature of the
warmest and coldest months; whereas, in the temperate zones, the annual
variation amounts to about 60 degrees, and the extreme range of the
thermometer in Canada is not less than 90 degrees.

The same remark might be applied to any other condition, as food, for
example; it may be foreseen that the supply will be regular throughout
indefinite periods in one part of the world, and in another very
precarious and fluctuating both in kind and quantity. Different
qualifications may be required for enabling species to live for a
considerable time under circumstances so changeable. If, then,
temperature and food be among those external causes which, according to
certain laws of animal and vegetable physiology, modify the
organization, form, or faculties, of individuals, we instantly perceive
that the degrees of variability from a common standard must differ
widely in the two cases above supposed; since there is a necessity of
accommodating a species in one case to a much greater latitude of
circumstances than in the other.

If it be a law, for instance, that scanty sustenance should check those
individuals in their growth which are enabled to accommodate themselves
to privations of this kind, and that a parent, prevented in this manner
from attaining the size proper to its species, should produce a dwarfish
offspring, a stunted race will arise, as is remarkably exemplified in
some varieties of the horse and dog. The difference of stature in some
races of dogs, when compared to others, is as one to five in linear
dimensions, making a difference of a hundred-fold in volume.[801] Now,
there is a good reason to believe that species in general are by no
means susceptible of existing under a diversity of circumstances, which