m the eighth and third
degrees of north latitude to the first and fourth of south latitude,
directing their course south-south-east. In September, when the Orinoco
decreases and re-enters its channel, these birds return
northwards.[903]

The insectivorous swallows which visit our island would perish during
winter, if they did not annually repair to warmer climes. It is supposed
that in these aerial excursions the average rapidity of their flight is
not less than fifty miles an hour; so that, when aided by the wind, they
soon reach warmer latitudes. Spallanzani calculated that the swallow can
fly at the rate of ninety-two miles an hour, and conceived that the
rapidity of the swift might be three times greater.[904] The rate of
flight of the eider duck (_Anas mollissima_) is said to be ninety miles
an hour; and Bachman says that the hawk, wild pigeon (_Columba
migratoria_), and several species of wild ducks, in North America, fly
at the rate of forty miles an hour, or nearly a thousand miles in
twenty-four hours.[905]

When we reflect how easily different species, in a great lapse of ages,
may be each overtaken by gales and hurricanes, and, abandoning
themselves to the tempest, be scattered at random through various
regions of the earth's surface, where the temperature of the atmosphere,
the vegetation, and the animal productions, might be suited to their
wants, we shall be prepared to find some species capriciously
distributed, and to be sometimes unable to determine the native
countries of each. Captain Smyth informs me, that, when engaged in his
survey of the Mediterranean, he encountered a gale in the Gulf of Lyons,
at the distance of between twenty and thirty leagues from the coast of
France, which bore along many land birds of various species, some of
which alighted on the ship, while others were thrown with violence
against the sails. In this manner islands become tenanted by species of
birds inhabiting the nearest mainland.

_Geographical Distribution and Dissemination of Reptiles._

A few facts respecting the third great class of vertebrated animals will
suffice to show that the plan of nature in regard to their location on
the globe is perfectly analogous to that already exemplified in other
parts of the organic creation, and has probably been determined by
similar causes.

_Habitations of reptiles._--Of the great saurians, the gavials which
inhabit the Ganges differ from the cayman of America, or the cr