cially several North
American species. "The commercial relations," says Malte-Brun[947],
"between France and India have transported from the latter country the
aphis, which destroys the apple tree, and two sorts of Neuroptera, the
_Lucifuga_ and _Flavicola_, mostly confined to Provence and the
neighbourhood of Bourdeaux, where they devour the timber in the houses
and naval arsenals."

Among mollusks we may mention the _Teredo navalis_, which is a native of
equatorial seas, but which, by adhering to the bottom of ships, was
transported to Holland, where it has been most destructive to vessels
and piles. The same species has also become naturalized in England, and
other countries enjoying an extensive commerce. _Bulimus undatus_, a
land species of considerable size, native of Jamaica and other West
Indian islands, has been imported, adhering to tropical timber, into
Liverpool; and, as I learn from Mr. Broderip, is now naturalized in the
woods near that town.

In all these and innumerable other instances we may regard the
involuntary agency of man as strictly analogous to that of the inferior
animals. Like them, we unconsciously contribute to extend or limit the
geographical range and numbers of certain species, in obedience to
general rules in the economy of nature, which are for the most part
beyond our control.

CHAPTER XL.

THEORIES RESPECTING THE ORIGINAL INTRODUCTION OF SPECIES.

Proposal of an hypothesis on this subject--Supposed centres or foci
of creation--Why distinct provinces of animals and plants have not
become more blended together--Brocchi's speculations on the loss of
species--Stations of plants and animals--Causes on which they
depend--Stations of plants how affected by animals--Equilibrium in
the number of species how preserved--Peculiar efficacy of insects in
this task--Rapidity with which certain insects multiply or decrease
in numbers--Effect of omnivorous animals in preserving the
equilibrium of species--Reciprocal influence of aquatic and
terrestrial species on each other.

_Theory of Linnaeus._--It would be superfluous to examine the various
attempts which were made to explain the phenomena of the distribution of
species alluded to in the preceding chapters, in the infancy of the
sciences of botany, zoology, and physical geography. The theories or
rather conjectures then indulged now stand refuted by a simple statement
of facts; and if Linnaeu