ge portion of the
amphibious quadrupeds and reptiles prey partly on aquatic plants and
animals, and in part on terrestrial; and a deficiency of one kind of
prey causes them to have immediate recourse to the other. The voracity
of certain insects, as the dragon-fly, for example, is confined to the
water during one stage of their transformations, and in their perfect
state to the air. Innumerable water-birds, both of rivers and seas,
derive in like manner their food indifferently from either element; so
that the abundance or scarcity of prey in one induces them either to
forsake or more constantly to haunt the other. Thus an intimate
connection between the state of the animate creation in a lake or river,
and in the adjoining dry land, is maintained; or between a continent,
with its lakes and rivers, and the ocean. It is well known that many
birds migrate, during stormy seasons, from the sea-shore into the
interior, in search of food; while others, on the contrary, urged by
like wants, forsake their inland haunts, and live on substances rejected
by the tide.

The migration of fish into rivers during the spawning season supplies
another link of the same kind. Suppose the salmon to be reduced in
numbers by some marine foes, as by seals and grampuses, the consequence
must often be, that in the course of a few years the otters at the
distance of several hundred miles inland will be lessened in number from
the scarcity of fish. On the other hand, if there be a dearth of food
for the young fry of the salmon in rivers and estuaries, so that few
return to the sea, the sand eels and other marine species, which are
usually kept down by the salmon, will swarm in greater profusion.

It is unnecessary to accumulate a greater number of illustrations in
order to prove that the stations of different plants and animals depend
on a great complication of circumstances,--on an immense variety of
relations in the state of the inanimate worlds. Every plant requires a
certain climate, soil, and other conditions, and often the aid of many
animals, in order to maintain its ground. Many animals feed on certain
plants, being often restricted to a small number, and sometimes to one
only; other members of the animal kingdom feed on plant-eating species,
and thus become dependent on the conditions of the _stations_ not only
of their prey, but of the plants consumed by them.

Having duly reflected on the nature and extent of these mutual relations
i