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 between a species destroyed by man
or by the increase of its natural enemies. (8/13. See the excellent
remarks on this subject by Mr. Lyell in his "Principles of
Geology.") 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
to be 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 and another closely-allied species rare in the
same district--why should we feel such great astonishment at the
rarity being carried a step farther to extinction? An action going
on, on every side of us, and yet barely appreciable, might surely
be carried a little farther without exciting our observation.
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