ns, each remote from
the other, it has often been deemed inconceivable how they could all
become stocked with fish from one common source; but it has been
suggested, that the minute eggs of these animals may sometimes be
entangled in the feathers of water-fowl. These, when they alight to wash
and plume themselves in the water, may often unconsciously contribute to
propagate swarms of fish, which, in due season, will supply them with
food. Some of the water-beetles, also, as the Dyticidae, are amphibious,
and in the evening quit their lakes and pools, and, flying in the air,
transport the minute ova of fishes to distant waters. In this manner
some naturalists account for the fry of fish appearing occasionally in
small pools caused by heavy rains; but the showers of small fish, stated
in so many accounts to have fallen from the atmosphere, require farther
investigation.

_Geographical Distribution and Migrations of Testacea._

The Testacea, of which so great a variety of species occurs in the sea,
are a class of animals of peculiar importance to the geologist; because
their remains are found in strata of all ages, and generally in a higher
state of preservation than those of other organic beings. Climate has a
decided influence on the geographical distribution of species in this
class; but as there is much greater uniformity of temperature in the
waters of the ocean, than in the atmosphere which invests the land, the
diffusion of marine mollusks is on the whole more extensive.

Some forms attain their fullest development in warm latitudes; and are
often exclusively confined to the torrid zone, as _Nautilus_, _Harpa_,
_Terebellum_, _Pyramidella_, _Delphinula_, _Aspergillum_, _Tridacna_,
_Cucullaea_, _Crassatella_, _Corbis_, _Perna_, and _Plicatula_. Other
forms are limited to one region of the sea, as the _Trigonia_ to parts
of Australia, and the _Concholepas_ to the western coast of South
America. The marine species inhabiting the ocean on the opposite sides
of the narrow isthmus of Panama, are found to differ almost entirely, as
we might have anticipated, since a West Indian mollusk cannot enter the
Pacific without coasting round South America, and passing through the
inclement climate of Cape Horn. The continuity of the existing lines of
continent from north to south, prevents any one species from belting the
globe, or from following the direction of the isothermal lines.

Currents also flowing permanently in certa