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, which I hope he will publish in some Journal.
[2] A nearly similar abnormal, but I do not know whether
hereditary, structure has been observed in the carp, and
likewise in the crocodile of the Ganges: Histoire des Anomalies,
par M. Isid. Geoffroy St. Hilaire, tom. i. p. 244.
[3] M. A. d'Orbigny has given nearly a similar account of these
dogs, tom. i. p. 175.
[4] I must express my obligations to Mr. Keane, at whose house
I was staying on the Berquelo, and to Mr. Lumb at Buenos Ayres,
for without their assistance these valuable remains would never
have reached England.
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