, a
season of unusual rigor should occur? Does not heat thus confine within
a fixed boundary the spread of these plants? And so, again, how many
others there are which grow luxuriantly in a temperate climate, but are
parched up and killed if fortuitously carried beneath a hot tropical
sun. To every one there is a climate which best suits the condition of
its life, and certain limits of heat and cold beyond which its existence
is not possible.

If the mean annual heat of the earth's surface were slowly to rise, and,
in the course of some centuries, the temperature now obtaining in
Florida should obtain in New York, the orange and lemon would certainly
be found here. [Sidenote: Boundary of organisms by heat.] With the
increasing heat those plants would commence a northward march, steadily
advancing as opportunity was given. Or, if the reverse took place, and
for any reason the heat of the torrid zone declined until the winter's
cold of New York should be at last reached under the equator, as the
descent went on the orange and lemon would retreat within a narrow and
narrower region, and end by becoming extinct, the conditions of their
exposure being incompatible with the continuance of their life. From
such considerations it is therefore obvious that not only does heat
arrange the limits of the distribution of plants, erecting round them
boundaries which, though invisible, are more insuperable than a wall of
brass, it also regulates their march, if march there is to be--nay, even
controls their very existence, and to genera, and species, and
individuals appoints a period of duration.

[Sidenote: Animals localized as well as plants.] Such observations apply
not alone to plants; the animal kingdom offers equally significant
illustrations. Why does the white bear enjoy the leaden sky of the pole
and his native iceberg? Why does the tiger restrict himself to the
jungles of India? Can it be doubted that, if the mean annual temperature
should decline, the polar bear would come with his iceberg to
corresponding southern latitudes, or, if the heat should rise, the tiger
would commence a northward journey? Does he not, indeed, every summer
penetrate northward in Asia as far as the latitude of Berlin, and retire
again as winter comes on? Why is it that, at a given signal, the birds
of passage migrate, pressed forward in the spring by the heat, and
pressed backward in the autumn by the cold? The annual migration of
birds illustrate