s the causes of geological appearances and extinctions.
Do we not herein recognize the agent that determines animal
distribution? We must not deceive ourselves with any fancied terrestrial
impediment or restraint. Let the heat rise but a few degrees, and the
turkey-buzzard, to whose powerful wing distances are of no moment and
the free air no impediment, would be seen hovering over New York; let it
fall a few degrees, and he would vanish from the streets of Charleston;
let it fall a little more, and he would vanish from the earth.
Shell-fish, once the inhabitants of the British seas, retired during the
glacial period to the Mediterranean, and with the returning warmth have
gone back northward again.

[Sidenote: Control of animals by food.] Animals are thus controlled by
heat in an indirect as well as a direct way. Indirectly; for, if their
food be diminished, they must seek a more ample supply; if it fails,
they must perish. Doubtless it was insufficient food, as well as the
setting in of a more rigorous climate, that occasioned the destruction
of the mastodon giganteus, which abounded in the United States after the
drift period. Such great elephantine forms could not possibly sustain
themselves against the rigors of the present winters, nor could they
find a sufficient supply of food for a considerable portion of the year.
The disappearance of animals from the face of the earth was, as
Palaeontology advanced, ascertained to have been a determinate process, a
condition of their existence, and either inherent in themselves or
dependent on their environment. It was proved that the forms now
existing are only an insignificant part of the countless tribes that
have lived. [Sidenote: Nature of creations and extinctions.] The earth
has been the theatre of a long succession of appearances and removals,
of creations and extinctions, reaching to the latest times. In the
Pleistocene of Sicily, 35/124 of the fossil shells are extinct; in the
bone caverns of England, out of thirty-seven mammals eighteen are
extinct. But judging, from what may be observed of the duration of races
contemporary with us, that their life is prolonged for thousands of
years, successive generations of the same species in a long order
replacing their predecessors before final removal occurs, this again
resistlessly brought forward the same conclusion to which all the
foregoing facts had pointed, that there have transpired since the
introduction of animal l