falls; or, again, what is the precise nature of the check.
Hence probably it is, that we feel so little surprise at one, of
two species closely allied in habits, being rare and the other
abundant in the same district; or, again, that one should be
abundant in one district, and another, filling the same place
in the economy of nature, should be abundant in a neighbouring
district, differing very little in its conditions. If asked
how this is, one immediately replies that it is determined by
some slight difference, in climate, food, or the number of
enemies: yet how rarely, if ever, we can point out the precise
cause and manner of action of the check! We are
therefore, driven to the conclusion, that causes generally
quite inappreciable by us, determine whether a given species
shall be abundant or scanty in numbers.

In the cases where we can trace the extinction of a
species through man, either wholly or in one limited district,
we know that it becomes rarer and rarer, and is then lost:
it would be difficult to point out any just distinction [13]
between a species destroyed by man or by the increase of its
natural enemies. The evidence of rarity preceding extinction,
is more striking in the successive tertiary strata, as remarked
by several able observers; it has often been found that a shell
very common in a tertiary stratum is now most rare, and has
even long been thought extinct. If then, as appears probable,
species first become rare and then extinct -- if the too rapid
increase of every species, even the most favoured, is steadily
checked, as we must admit, though how and when it is hard to
say -- and if we see, without the smallest surprise, though
unable to assign the precise reason, one species abundant