 and surely the departure from existence of a species, of a type of
being, that has subsisted in contemporary thousands of individuals, for
thousands of years, is not less imposingly mysterious than that of the
individual exemplar.

We do not know with any precision what are the immediate causes of death
in a species. Is there a definite limit to life imposed at first? or is
this limit left, so to speak, to be determined by accidental
circumstances? Perhaps both: but if the latter, what are those
circumstances?

Professor Owen says:--"There are characters in land animals rendering
them more obnoxious to extirpating influences, which may explain why so
many of the larger species of particular groups have become extinct,
whilst smaller species of equal antiquity have survived. In proportion
to its bulk is the difficulty of the contest which the animal has to
maintain against the surrounding agencies that are ever tending to
dissolve the vital bond, and subjugate the living matter to the
ordinary chemical and physical forces. Any changes, therefore, in such
external agencies as a species may have been originally adapted to exist
in, will militate against that existence in a degree proportionate to
the size which may characterise the species. If a dry season be
gradually prolonged, the large mammal will suffer from the drought
sooner than the small one; if such alteration of climate affect the
quantity of vegetable food, the bulky herbivore will first feel the
effects of stinted nourishment; if new enemies be introduced, the large
and conspicuous animal will fall a prey while the smaller kinds conceal
themselves and escape. Small quadrupeds, moreover, are more prolific
than large ones. Those of the bulk of the mastodons, megatheria,
glyptodons, and diprotodons, are uniparous. The actual presence,
therefore, of small species of animals in countries where larger species
of the same natural families formerly existed, is not the consequence of
degeneration--of any gradual diminution of the size--of such species,
but is the result of circumstances which may be illustrated by the fable
of 'the Oak and the Reed;' the smaller and feebler animals have bent and
accommodated themselves to changes to which the larger species have
succumbed."[68]

"We do not steadily bear in mind," remarks Mr Darwin, "how profoundly
ignorant we are of the condition of existence of every animal; nor do we
always remember that some check is constantly preventing the too rapid
increase of every organised being left in a state of nature. The supply
of food, on an average, remains constant; yet the tendency in every
animal to increase by propagation is geometrical; and its surprising
effects have nowhere been more astonishingly shewn, than in the case of
the European animals run wild during the last few centuries in America.
Every animal in a state of nature regularly breeds; yet in a species
long established, any _great_ increase in numbers is obviously
impossible, and must be checked by some means. We are nevertheless
seldom able with certainty to tell in any given species, at what period
of life, or at what period of the year, or whether only at long
intervals, the check 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 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 Norfolk and Philip Islands! And how brief was the lease of life
accorded to the _Didunculus_, when once the "Pussies" found their way to
the little Samoa isles!

Very many islands have a fauna that is to a great extent peculiar to
themselves. I know that, in Jamaica, the Humming-birds, some of the
Parrots, some of the Cuckoos, most of the Pigeons, many of the smaller
birds, and, I think, all of the Reptiles, are found nowhere else. Nay,
more, that even the smaller islands of the Antilles have each a fauna of
its own, unshared with any other land;--its own Humming-birds, its own
Lizards and Snakes; its own Butterflies and Beetles, its own Spiders,
its own Snails, its own Worms. How likely are some of these very limited
species to become extinguished! By the increasing aggressions of
clearing and cultivating man; by slight changes of level; even by
electric and meteoric phenomena acting very locally. I find that, in
Jamaica, many of the animals peculiar to the island are not spread over
the whole surface, limited as that is, but are confined to a single
small district. In some cases, the individuals are but few, even in that
favoured locality; how easily we may conceive of a season drier than
ordinary, or wetter than ordinary, or a flood, or a hurricane of unusual
violence, or a volcanic eruption, either killing outright these few
individuals, or destroying their means of living, and so indirectly
destroying them by starvation. And then the species has disappeared!

The common Red Grouse, so abundantly seen during the season hanging at
every poulterer's and game-dealer's shop in London, is absolutely
unknown out of the British Isles. It could not live except in wide,
unenclosed, uncultivated districts; so that when the period arrives that
the whole of British land is enclosed and brought under cultivation, the
Grouse's lease of life will expire. We owe it to our hard-worked members
of Parliament to hope that this condition of things may be distant.

[1] See my _Omphalos_,--_passim_.

[2] The gradual but constant elevation of the bed of the Baltic, and the
subsidence of that of the Pacific Ocean, are examples on