ut
any just distinction 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 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 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 could feel any great
surprise at hearing that the Megalonyx 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 of death--to feel no surprise at sickness--but
when the sick man dies to wonder, and to believe that he died through
violence."[69]

Geographical distribution is an important element in this question of
extinction. A species that is spread over a wide region is far more
likely to survive than one which is confined to a limited district; and
extraneous influences acting prejudicially will exterminate a species
which is confined to an island much sooner than if it had a continent to
retire upon. We have seen how the _Nestor_ Parrot became extinct in New
Zealand, while it survived in Norfolk Island, because the former was
colonised by the Maori race, while the latter remained in its virginity.
But how quickly did the poor Parrot succumb as soon as man set his foot
on Norfol