half-past
one; I sat for more than an hour, but instead of a diminution of this
prodigious procession, it seemed rather to increase, both in numbers and
rapidity; and anxious to reach Frankfort before night, I rose and went
on. About four o'clock in the afternoon I crossed Kentucky River, at the
town of Frankfort, at which time the living torrent above my head seemed
as numerous and as extensive as ever. Long after this I observed them in
large bodies that continued to pass for six or eight minutes, and these
again were followed by other detached bodies, all moving in the same
south-east direction, till after six o'clock in the evening. The great
breadth of front which this mighty multitude preserved would seem to
intimate a corresponding breadth of their breeding-place, which, by
several gentlemen who had lately passed through part of it, was stated
to me at several miles."

From these various observations, Wilson calculated that the number of
birds contained in the mass of pigeons which he saw on this occasion was
at least two thousand millions, while this was only one of many similar
aggregations known to exist in various parts of the United States. The
picture here given of these defenceless birds, and their still more
defenceless young, exposed to the attacks of numerous rapacious enemies,
brings vividly before us one of the phases of the unceasing struggle for
existence ever going on; but when we consider the slow rate of increase
of these birds, and the enormous population they are nevertheless able
to maintain, we must be convinced that in the case of the majority of
birds which multiply far more rapidly, and yet are never able to attain
such numbers, the struggle against their numerous enemies and against
the adverse forces of nature must be even more severe or more
continuous.

_Struggle for Life between, closely allied Animals and Plants often the
most severe._

The struggle we have hitherto been considering has been mainly that
between an animal or plant and its direct enemies, whether these enemies
are other animals which devour it, or the forces of nature which destroy
it. But there is another kind of struggle often going on at the same
time between closely related species, which almost always terminates in
the destruction of one of them. As an example of what is meant, Darwin
states that the recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush.[12] The