ich arrests the progress of an
animal or plant in a particular direction, the individuals are fewer and
less vigorous as they approach the extreme confines of the geographical
range of the species. But these stragglers are ready to multiply rapidly
on the slightest increase or diminution of heat that may be favorable to
them, just as particular insects increase during a hot summer, and
certain plants and animals gain ground after a series of congenial
seasons.

In almost every district, especially if it be mountainous, there are a
variety of species the limits of whose habitations are conterminous,
some being unable to proceed farther without encountering too much heat,
others too much cold. Individuals, which are thus on the borders of the
regions proper to their respective species, are like the outposts of
hostile armies, ready to profit by every slight change of circumstances
in their favor, and to advance upon the ground occupied by their
neighbors and opponents.

The proximity of distinct climates produced by the inequalities of the
earth's surface, brings species possessing very different constitutions
into such immediate contact, that their naturalizations are very speedy
whenever opportunities of advancing present themselves. Many insects and
plants, for example, are common to low plains within the arctic circle,
and to lofty mountains in Scotland and other parts of Europe. If the
climate, therefore, of the polar regions were transferred to our own
latitudes, the species in question would immediately descend from these
elevated stations to overrun the low grounds. Invasions of this kind,
attended by the expulsion of the pre-occupants, are almost
instantaneous, because the change of temperature not only places the one
species in a more favorable position, but renders the others sickly and
almost incapable of defence.

_These changes inconsistent with the theory of transmutation._--Lamarck,
when speculating on the transmutation of species, supposed every
modification in organization and instinct to be brought about slowly and
insensibly in an indefinite lapse of ages. But he does not appear to
have sufficiently considered how much every alteration in the physical
condition of the habitable surface changes the relations of a great
number of coexisting species, and that some of these would be ready
instantly to avail themselves of the slightest change in their favor,
and to multiply to the injury of others. Even