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 and another closely allied species rare in the same
district--why should we feel such great astonishment at the rarity
being carried one step further to extinction? An action going on, on
every side of us, and yet barely appreciable, might surely be carried a
little further, without exciting our observation. Who would feel any
great surprise at hearing that the Magalonyx was formerly rare compared
with the Megatherium, or that one of the fossil monkeys was few in
number compared with one of the now living monkeys? and yet in this
comparative rarity, we should have the plainest evidence of less
favourable conditions for their existence. To admit that species
generally become rare before they become extinct--to feel no surprise
at the comparative rarity of one species with another, and yet to call
in some extraordinary agent and to marvel greatly when a species ceases
to exist, appears to me much the same as to admit that sickness in the
individual is the prelude to death--to feel no surprise at
sickness--but when the sick man dies to wonder, and to believe that he
died through violence.
[1] Mr. Waterhouse has drawn up a detailed description of this head,
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