Turtles migrate in large droves from one part of the ocean to another

during the ovipositing season; and they find their way annually to the

island of Ascension, from which the nearest land is about 800 miles

distant. Dr. Fleming mentions, that an individual of the hawk's bill

turtle (_Chelonia imbricata_), so common in the American seas, has been

taken at Papa Stour, one of the West Zetland Islands;[907] and,

according to Sibbald, "the same animal came into Orkney." Another was

taken, in 1774, in the Severn, according to Turton. Two instances, also,

of the occurrence of the leathern tortoise (_C. coriacea_), on the coast

of Cornwall, in 1756, are mentioned by Borlase. These animals of more

southern seas can be considered only as stragglers, attracted to our

shores during uncommonly warm seasons by an abundant supply of food, or

carried by the Gulf stream, or driven by storms to high latitudes.

Some of the smaller reptiles lay their eggs on aquatic plants; and these

must often be borne rapidly by rivers, and conveyed to distant regions

in a manner similar to the dispersion of seeds before adverted to. But

that the larger ophidians may be themselves transported across the seas,

is evident from the following most interesting account of the arrival of

one at the island of St. Vincent. It is worthy of being recorded, says

Mr. Guilding, "that a noble specimen of the _Boa constrictor_ was lately

conveyed to us by the currents, twisted round the trunk of a large sound

cedar tree, which had probably been washed out of the bank by the floods

of some great South American river, while its huge folds hung on the

branches, as it waited for its prey. The monster was fortunately

destroyed after killing a few sheep, and his skeleton now hangs before

me in my study, putting me in mind how much reason I might have had to

fear in my future rambles through the forests of St. Vincent, had this

formidable reptile been a pregnant female, and escaped to a safe

retreat."[908]

CHAPTER XXXIX.

LAWS WHICH REGULATE THE GEOGRAPHICAL DISTRIBUTION OF

SPECIES--_continued_.

Geographical distribution and migration of Fish--of Testaoea--of

Zoophytes--Distribution of Insects--Migratory instincts of some

species--Certain types characterize particular countries--Their

means of dissemination--Geographical distribution and diffusion of

man--Speculations as to the birth-place of the human

species--Progress of human population--Drifting of canoes to vast

distances--On the involuntary influence of man in extending the

range of many other species.

_Geographical Distribution and Migrations of Fish._

Although we are less acquainted with the habitations of marine animals

than with the grouping of the terrestrial species before described, yet

it is well ascertained that their distribution is governed by the same

general laws. The testimony borne by MM. Paron and Lesueur to this

important fact is remarkably strong. These eminent naturalists, after

collecting and describing many thousand species of marine animals which

they brought to Europe from the southern hemisphere, insist most

emphatically on their distinctness from those north of the equator; and

this remark they extend to animals of all classes, from those of a more

simple to those of a more complex organization--from the sponges and

Medusae to the Cetacea. "Among all those which we have been able to

examine," say they, "with our own eyes, or with regard to which it has

appeared to us possible to pronounce with certainty, there is not a

single animal of the southern regions which is not distinguished by

essential characters from the analogous species in the northern

seas."[909]

On comparing the freshwater fish of Europe and North America, Sir John

Richardson remarks, that the only species which is unequivocally common

to the two continents is the pike (_Esox lucius_); and it is curious

that this fish is unknown to the westward of the Rocky Mountains, the

very coast which approaches nearest to the old continent.[910] According

to the same author the genera of freshwater fish in China agree closely

with those of the peninsula of India, but the species are not the same.

"As in the distribution," he adds, "of marine fish, the interposition of

a continent stretching from the tropics far into the temperate or colder

parts of the ocean, separate different ichthyological groups; so with

respect to the freshwater species, the intrusion of arms of the sea

running far to the northwards, or the interposition of a lofty

mountain-chain, effects the same thing. The freshwater fish of the Cape

of Good Hope and the South American ones, are different from those of

India and China, &c."[911]

Cuvier and Valenciennes, in their "Histoire des Poissons," observe, that

very few species of fish cross the Atlantic. Although their statement is

correct, it is found that a great many species are common to the

opposite sides of the Indian Ocean, inhabiting alike the Red Sea, the

eastern coast of Africa, Madagascar, the Mauritius, the Indian Ocean,

the southern seas of China, the Malay archipelago, the northern coasts

of Australia, and the whole of Polynesia![912] This very wide diffusion,

says Sir J. Richardson, may have been promoted by chains of islands

running east and west, which are wanting in the deep Atlantic. An

archipelago extending far in longitude, favours the migration of fish by

multiplying the places of deposit for spawn along the shores of islands,

and on intervening coral banks; and in such places, also, fish find

their appropriate food.

The flying fish are found (some stragglers excepted) only between the

tropics: in receding from the line, they never approach a higher

latitude than the fortieth parallel. The course of the Gulf stream,

however, and the warmth of its water, enable some tropical fish to

extend their habitations far into the temperate zone; thus the chaetodons

which abound in the seas of hot climates, are found among the Bermudas

on the thirty-second parallel, where they are preserved in basins

inclosed from the sea, as an important article of food for the garrison

and inhabitants. Other fish, following the direction of the same great

current, range from the coast of Brazil to the banks of

Newfoundland.[913]

All are aware that there are certain fish of passage which have their

periodical migrations, like some tribes of birds. The salmon, towards

the season of spawning, ascends the rivers for hundreds of miles,

leaping up the cataracts which it meets in its course, and then retreats

again into the depths of the ocean. The herring and the haddock, after

frequenting certain shores, in vast shoals, for a series of years,

desert them again, and resort to other stations, followed by the species

which prey on them. Eels are said to descend into the sea for the

purpose of producing their young, which are seen returning into the

fresh water by myriads, extremely small in size, but possessing the

power of surmounting every obstacle which occurs in the course of a

river, by applying their slimy and glutinous bodies to the surface of

rocks, or the gates of a lock, even when dry, and so climbing over

it.[914] Before the year 1800 there were no eels in Lake Wener, the

largest inland lake in Sweden, which discharges its waters by the

celebrated cataracts of Trolhattan. But I am informed by Professor

Nilsson, that since the canal was opened uniting the river Gotha with

the lake by a series of nine locks, each of great height, eels have been

observed in abundance in the lake. It appears, therefore, that though

they were unable to ascend the falls, they have made their way by the

locks, by which in a very short space a difference of level of 114 feet

is overcome.

Gmelin says, that the Anseres (wild geese, ducks, and others) subsist,

in their migrations, on the spawn of fish; and that oftentimes, when

they void the spawn, two or three days afterwards, the eggs retain their

vitality unimpaired.[915] When there are many disconnected freshwater

lakes in a mountainous region, at various elevations, each remote from

the other, it has often been deemed inconceivable how they could all

become stocked with fish from one common source; but it has been

suggested, that the minute eggs of these animals may sometimes be

entangled in the feathers of water-fowl. These, when they alight to wash

and plume themselves in the water, may often unconsciously contribute to

propagate swarms of fish, which, in due season, will supply them with

food. Some of the water-beetles, also, as the Dyticidae, are amphibious,

and in the evening quit their lakes and pools, and, flying in the air,

transport the minute ova of fishes to distant waters. In this manner

some naturalists account for the fry of fish appearing occasionally in

small pools caused by heavy rains; but the showers of small fish, stated

in so many accounts to have fallen from the atmosphere, require farther

investigation.

_Geographical Distribution and Migrations of Testacea._

The Testacea, of which so great a variety of species occurs in the sea,

are a class of animals of peculiar importance to the geologist; because

their remains are found in strata of all ages, and generally in a higher

state of preservation than those of other organic beings. Climate has a

decided influence on the geographical distribution of species in this

class; but as there is much greater uniformity of temperature in the

waters of the ocean, than in the atmosphere which invests the land, the

diffusion of marine mollusks is on the whole more extensive.

Some forms attain their fullest development in warm latitudes; and are

often exclusively confined to the torrid zone, as _Nautilus_, _Harpa_,

_Terebellum_, _Pyramidella_, _Delphinula_, _Aspergillum_, _Tridacna_,

_Cucullaea_, _Crassatella_, _Corbis_, _Perna_, and _Plicatula_. Other

forms are limited to one region of the sea, as the _Trigonia_ to parts

of Australia, and the _Concholepas_ to the western coast of South

America. The marine species inhabiting the ocean on the opposite sides

of the narrow isthmus of Panama, are found to differ almost entirely, as

we might have anticipated, since a West Indian mollusk cannot enter the

Pacific without coasting round South America, and passing through the

inclement climate of Cape Horn. The continuity of the existing lines of

continent from north to south, prevents any one species from belting the

globe, or from following the direction of the isothermal lines.

Currents also flowing permanently in certain directions, and the influx

at certain points of great bodies of fresh water, limit the extension of

many species. Those which love deep water are arrested by shoals;

others, fitted for shallow seas, cannot migrate across unfathomable

abysses. The nature also of the ground has an important influence on the

testaceous fauna, both on the land and beneath the waters. Certain

species prefer a sandy, others a gravelly, and some a muddy sea-bottom.

On the land, limestone is of all rocks the most favourable to the number

and propagation of species of the genera Helix, Clausilia, Bulimus, and

others. Professor E. Forbes has shown as the result of his labours in

dredging in the AEgean Sea, that there are eight well-marked regions of

depth, each characterized by its peculiar testaceous fauna. The first of

these, called the littoral zone, extends to a depth of two fathoms only;

but this narrow belt is inhabited by more than one hundred species. The

second region, of which ten fathoms is the inferior limit, is almost

equally populous; and a copious list of species is given as

characteristic of each region down to the seventh, which lies between

the depths of 80 and 105 fathoms, all the inhabited space below this

being included in the eighth province, where no less than 65 species of

Testacea have been taken. The majority of the shells in this lowest zone

are white or transparent. Only two species of Mollusca are common to

all the eight regions, namely, _Arca lactea_ and _Cerithium lima_.[916]

_Great range of some provinces and species._--In Europe conchologists

distinguish between the arctic fauna, the southern boundary of which

corresponds with the isothermal line of 32 degrees F., and the Celtic,

which, commencing with that limit as its northern frontier, extends

southwards to the mouth of the English Channel and Cape Finisterre, in

France. From that point begins the Lusitanian fauna, which, according to

the recent observations of Mr. M'Andrew (1852), ranges to the Canary

Islands. The Mediterranean province is distinct from all those above

enumerated, although it has some species in common with each.

The Indo-Pacific region is by far the most extensive of all. It reaches

from the Red Sea and the eastern coast of Africa, to the Indian

Archipelago, and adjoining parts of the Pacific Ocean. To the geologist

it furnishes a fact of no small interest, by teaching us that one group

of living species of mollusca may prevail throughout an area exceeding

in magnitude the utmost limits we can as yet assign to any assemblage of

contemporaneous fossil species. Mr. Cuming obtained more than a hundred

species of shells from the eastern coast of Africa identical with those

collected by himself at the Philippines and in the eastern coral islands

of the Pacific Ocean, a distance equal to that from pole to pole.[917]

Certain species of the genus _Ianthina_ have a very wide range, being

common to seas north and south of the equator. They are all provided

with a beautifully contrived float, which renders them buoyant,

facilitating their dispersion, and enabling them to become active agents

in disseminating other species. Captain King took a specimen of

_Ianthina fragilis_, alive, a little north of the equator, so loaded

with barnacles (_Pentelasmis_) and their ova that the upper part of its

shell was invisible. The "Rock Whelk" (_Purpura lapillus_), a well-known

British univalve, inhabits both the North Atlantic and North Pacific.

_Helix putris_ (_Succinea putris_, Lam.), so common in Europe, where it

reaches from Norway to Italy, is also said to occur in the United States

and in Newfoundland. As this animal inhabits constantly the borders of

pools and streams where there is much moisture, it is not impossible

that different water-fowl have been the agents of spreading some of its

minute eggs, which may have been entangled in their feathers. The

freshwater snail, _Lymneus palustris_, so abundant in English ponds,

ranges uninterruptedly from Europe to Cashmere, and thence to the

eastern parts of Asia. _Helix aspersa_, one of the commonest of our

larger land-shells, is found in St. Helena and other distant countries.

Some conchologists have conjectured that it was accidentally imported

into St. Helena in some ship; for it is an eatable species, and these

animals are capable of retaining life during long voyages, without air

or nourishment.[918]

Perhaps no species has a better claim to be called cosmopolite than one

of our British bivalves, _Saxicava rugosa_. It is spread over all the

north-polar seas, and ranges in one direction through Europe to Senegal,

occurring on both sides of the Atlantic; while in another it finds its

way into the North Pacific, and thence to the Indian Ocean. Nor do its

migrations cease till it reaches the Australian seas.

A British brachiopod, named _Terebratula caput-serpentis_, is common,

according to Professor E. Forbes, to both sides of the North Atlantic,

and to the South African and Chinese seas.

_Confined range of other species._--Mr. Lowe, in a memoir published in

the Cambridge Transactions in 1834, enumerates seventy-one species of

land Mollusca, collected by him in the islands of Madeira and Porto

Santo, sixty of which belonged to the genus Helix alone, including as

sub-genera Bulimus and Achatina, and excluding Vitrina and Clausilia;

forty-four of these are new. It is remarkable that very few of the

above-mentioned species are common to the neighbouring archipelago of

the Canaries; but it is a still more striking fact, that of the sixty

species of the three genera above mentioned, thirty-one are natives of

Porto Santo; whereas, in Madeira, which contains ten times the

superficies, were found but twenty-nine. Of these only four were common

to the two islands, which are separated by a distance of only twelve

leagues; and two even of these four (namely _Helix rhodostoma_ and _H.

ventrosa_) are species of general diffusion, common to Madeira, the

Canaries, and the south of Europe.[919]

The confined range of these mollusks may easily be explained, if we

admit that species have only one birth-place; and the only problem to be

solved would relate to the exceptions--to account for the dissemination

of some species throughout several islands, and the European continent.

May not the eggs, when washed into the sea by the undermining of cliffs,

or blown by a storm from the land, float uninjured to a distant shore?

_Their mode of diffusion._--Notwithstanding the proverbially slow motion

of snails and mollusks in general, and although many aquatic species

adhere constantly to the same rock for their whole lives, they are by no

means destitute of provision for disseminating themselves rapidly over a

wide area. "Some Mollusca," says Professor E. Forbes, "migrate in their

larva state, for all of them undergo a metamorphosis either in the egg

or out of the egg. The gasteropoda commence life under the form of a

small spiral shell, and an animal furnished with ciliated wings, or

lobes, like a pteropod, by means of which it can swim freely, and in

this form can migrate with ease through the sea."[920]

We are accustomed to associate in our minds the idea of the greatest

locomotive powers with the most mature and perfect state of each species

of invertebrate animal, especially when they undergo a series of

transformations; but in all the Mollusca the reverse is true. The young

fry of the cockle, for example (_Cardium_), possess, when young or in

the larva state, an apparatus which enables them both to swim and to be

carried along easily by a marine current. (See fig. 99.)

[Illustration: Fig. 99.

Tne young fry of a cockle (Cardium pygmaeum,) from Loven's Kongl.

Vetenskaps. Akadem. Handling, 1848.

A, The young just hatched, magnified 100 diameters.

B, the same farther advanced.

_a_, The ciliated organ of locomotion with its filamentous

appendage _b_.

_c_, The rudimentary intestine.

_d_, The rudimentary shell.

]

These small bodies here represented, which bear a considerable

resemblance to the fry of the univalve, or gasteropodous shells above

mentioned, are so minute at first as to be just visible to the naked

eye. They begin to move about from the moment they are hatched, by means

of the long cilia, _a_, _a_, placed on the edges of the locomotive disk

or velum. This disk shrinks up as they increase in size, and gradually

disappears, no trace of it being visible in the perfect animal.

Some species of shell-bearing Mollusca lay their eggs in a sponge-like

nidus, wherein the young remain enveloped for a time after their birth;

and this buoyant substance floats far and wide as readily as sea-weed.

The young of other viviparous tribes are often borne along entangled in

sea-weed. Sometimes they are so light, that, like grains of sand, they

can be easily moved by currents. Balani and Serpulae are sometimes found

adhering to floating cocoa-nuts, and even to fragments of pumice. In

rivers and lakes, on the other hand, aquatic univalves usually attach

their eggs to leaves and sticks which have fallen into the water, and

which are liable to be swept away during floods, from tributaries to the

main streams, and from thence to all parts of the same basins.

Particular species may thus migrate during one season from the head

waters of the Mississippi, or any other great river, to countries

bordering the sea, at the distance of many thousand miles.

An illustration of the mode of attachment of these eggs will be seen in

the annexed cut. (Fig. 100.)

The habit of some Testacea to adhere to floating wood is proved by their

fixing themselves to the bottoms of ships. By this mode of conveyance

_Mytilus polymorphus_, previously known only in the Danube and Wolga,

may have been brought to the Commercial Docks in the Thames, and to

Hamburgh, where the species is now domiciled. But Mr. Gray suggests that

as the animal is known to have the faculty of living for a very long

time out of water, it is more probable that it was brought in Russian

timber, than borne uninjured through the salt water at the bottom of a

vessel.[921]

A lobster (_Astacus marinus_) was lately taken alive covered with living

mussels (_Mytilus edulis_)[922]; and a large female crab (_Cancer

pagurus_), covered with oysters, and bearing also _Anomia ephippium_,

and Actiniae, was taken in April, 1832, off the English coast. The

oysters, seven in number, include individuals of six years' growth, and

the two largest are four inches long and three inches and a half broad.

Both the crab and the oysters were seen alive by Mr. Robert Brown.[923]

[Illustration: Fig. 100.

Eggs of Freshwater mollusks.

Fig. 1. Eggs of _Ampullaria ovata_ (a fluviatile species) fixed to

a small sprig which had fallen into the water.

Fig. 2. Eggs of _Planorbis albus_, attached to a dead leaf lying

under water.

Fig. 3. Eggs of the common Limneus (_L. vulgaris_), adhering to

a dead stick under water.

]

From this example we learn the manner in which oysters may be diffused

over every part of the sea where the crab wanders; and if they are at

length carried to a spot where there is nothing but fine mud, the

foundation of a new oyster-bank may be laid on the death of the crab. In

this instance the oysters survived the crab many days, and were killed

at last only by long exposure to the air.

_Geographical Distribution and Migrations of Zoophytes._

Zoophytes are very imperfectly known; but there can be little doubt that

each maritime region possesses species peculiar to itself. The

Madrepores, or lamelliferous Polyparia, are found in their fullest

development only in the tropical seas of Polynesia and the East and West

Indies; and this family is represented only by a few species in our

seas. The zoophytes of the Mediterranean, according to Ehrenberg, differ

almost entirely from those of the Red Sea, although only seventy miles

distant. Out of 120 species of Anthozoa, only two are common to both

seas.[924] Paron and Lesueur, after studying the Holothuriae, Medusae, and

other congeners of delicate and changeable forms, came to the conclusion

that each kind has its place of residence determined by the temperature

necessary to support its existence. Thus, for example, they found the

abode of _Pyrosoma Atlantica_ to be confined to one particular region of

the Atlantic Ocean.[925]

Let us now inquire how the transportation of zoophytes from one part of

the globe to another is effected. Many of them, as in the families

Flustra and Sertularia, attach themselves to sea-weed, and are

occasionally drifted along with it. Many fix themselves to the shells of

Mollusca, and are thus borne along by them to short distances. Others,

like some species of sea-pens, float about in the ocean, and are usually

believed to possess powers of spontaneous motion. But the most frequent

mode of transportation consists in the buoyancy of their eggs, or

certain small vesicles, which are detached, and are capable of becoming

the foundation of a new colony. These gems, as they are called, have, in

many instances, a locomotive power of their own, by which they proceed

in a determinate direction for several days after separation from the

parent. They are propelled by means of numerous short threads or

_ciliae_, which are in constant and rapid vibration; and, when thus

supported in the water, they may be borne along by currents to a great

distance.

That some zoophytes adhere to floating bodies, is proved by their being

found attached to the bottoms of ships, like certain Testacea before

alluded to.

_Geographical Distribution and Migrations of Insects._

Before I conclude this sketch of the manner in which the habitable parts

of the earth are shared out among particular assemblages of organic

beings, I must offer a few remarks on insects, which, by their numbers

and the variety of their powers and instincts, exert a prodigious

influence in the economy of animate nature. As a large portion of these

minute creatures are strictly dependent for their subsistence on certain

species of vegetables, the entomological provinces must coincide in

considerable degree with the botanical.

All the insects, says Latreille, brought from the eastern parts of Asia

and China, whatever be their latitude and temperature, are distinct from

those of Europe and of Africa. The insects of the United States,

although often approaching very close to our own, are, with very few

exceptions, specifically distinguishable by some characters. In South

America, the equinoctial lands of New Granada and Peru on the one side,

and of Guiana on the other, contain for the most part distinct groups;

the Andes forming the division, and interposing a narrow line of severe

cold between climates otherwise very similar.[926]

_Migratory instincts._--Nearly all the insects of the United States and

Canada, differ specifically from the European; while those of Greenland

appear to be in a great measure identical with our own. Some insects are

very local; while a few, on the contrary, are common to remote

countries, between which the torrid zone and the ocean intervene. Thus

our painted lady butterfly (_Vanessa cardui_) re-appears at the Cape of

Good Hope and in New Holland and Japan with scarcely a varying

streak.[927] The same species is said to be one of the few insects which

are universally dispersed over the earth, being found in Europe, Asia,

Africa, and America; and its wide range is the more interesting, because

it seems explained by its migratory instinct, seconded, no doubt, by a

capacity, enjoyed by few species, of enduring a great diversity of

temperature.

A vast swarm of this species, forming a column from ten to fifteen feet

broad, was, a few years since, observed in the Canton de Vaud; they

traversed the country with great rapidity from north to south, all

flying onwards in regular order, close together, and not turning from

their course on the approach of other objects. Professor Bonelli, of

Turin, observed, in March of the same year, a similar swarm of the same

species, also directing their flight from north to south, in Piedmont,

in such immense numbers that at night the flowers were literally covered

with them. They had been traced from Coni, Raconi, Susa, &c. A similar

flight at the end of the last century is recorded by M. Louch in the

Memoirs of the Academy of Turin. The fact is the more worthy of notice,

because the caterpillars of this butterfly are not gregarious, but

solitary from the moment that they are hatched; and this instinct

remains dormant, while generation after generation passes away, till it

suddenly displays itself in full energy when their numbers happen to be

in excess.

Not only peculiar species, but certain types, distinguish particular

countries; and there are groups, observes Kirby, which represent each

other in distant regions, whether in their form, their functions, or in

both. Thus the honey and wax of Europe, Asia, and Africa, are in each

case prepared by bees congenerous with our common hive-bee (_Apis_,

Latr.); while, in America, this genus is nowhere indigenous, but is

replaced by Melipona, Trigona, and Euglossa; and in New Holland by

a still different but undescribed type.[928] The European bee (_Apis

mellifica_), although not a native of the new world, is now established

both in North and South America. It was introduced into the United

States by some of the early settlers, and has since overspread the vast

forests of the interior, building hives in the decayed trunks of trees.

"The Indians," says Irving, "consider them as the harbinger of the white

man, as the buffalo is of the red man, and say that in proportion as the

bee advances the Indian and the buffalo retire. It is said," continues

the same writer, "that the wild bee is seldom to be met with at any

great distance from the frontier, and that they have always been the

heralds of civilization, preceding it as it advanced from the Atlantic

borders. Some of the ancient settlers of the west even pretend to give

the very year when the honey-bee first crossed the Mississippi."[929]

The same species is now also naturalized in Van Diemen's Land and New

Zealand.

As almost all insects are winged, they can readily spread themselves

wherever their progress is not opposed by uncongenial climates, or by

seas, mountains, and other physical impediments; and these barriers they

can sometimes surmount by abandoning themselves to violent winds, which,

as I before stated, when speaking of the dispersion of seeds (p. 618.),

may in a few hours carry them to very considerable distances. On the

Andes some sphinxes and flies have been observed by Humboldt, at the

height of 19,180 feet above the sea, and which appeared to him to have

been involuntarily carried into these regions by ascending currents of

air.[930]

White mentions a remarkable shower of aphides which seem to have

emigrated, with an east wind, from the great hop plantations of Kent and

Sussex, and blackened the shrubs and vegetables where they alighted at

Selbourne, spreading at the same time in great clouds all along the vale

from Farnham to Alton. These aphides are sometimes accompanied by vast

numbers of the common lady-bird (_Coccinella septempunctata_), which

feed upon them.[931]

It is remarkable, says Kirby, that many of the insects which are

occasionally observed to emigrate, as, for instance, the Libellulae,

Coccinellae, Carabi, Cicadae, &c. are not usually social insects; but seem

to congregate, like swallows, merely for the purpose of emigration.[932]

Here, therefore, we have an example of an instinct developing itself on

certain rare emergencies, causing unsocial species to become gregarious

and to venture sometimes even to cross the ocean.

The armies of locusts which darken the air in Africa, and traverse the

globe from Turkey to our southern counties in England, are well known to

all. When the western gales sweep over the Pampas they bear along with

them myriads of insects of various kinds. As a proof of the manner in

which species may be thus diffused, I may mention that when the Creole

frigate was lying in the outer roads off Buenos Ayres, in 1819, at the

distance of six miles from the land, her decks and rigging were suddenly

covered by thousands of flies and grains of sand. The sides of the

vessel had just received a fresh coat of paint, to which the insects

adhered in such numbers as to spot and disfigure the vessel, and to

render it necessary partially to renew the paint.[933] Captain W. H.

Smyth was obliged to repaint his vessel, the Adventure, in the

Mediterranean, from the same cause. He was on his way from Malta to

Tripoli, when a southern wind blowing from the coast of Africa, then one

hundred miles distant, drove such myriads of flies upon the fresh paint,

that not the smallest point was left unoccupied by insects.

To the southward of the river Plate, off Cape St. Antonio, and at the

distance of fifty miles from land, several large dragon-flies alighted

on the Adventure frigate, during Captain King's late expedition to the

Straits of Magellan. If the wind abates when insects are thus crossing

the sea, the most delicate species are not necessarily drowned; for many

can repose without sinking on the water. The slender long-legged tipulae

have been seen standing on the surface of the sea, when driven out far

from our coast, and took wing immediately on being approached.[934]

Exotic beetles are sometimes thrown on our shore, which revive after

having been long drenched in salt water; and the periodical appearance

of some conspicuous butterflies amongst us, after being unseen some for

five others for fifty years, has been ascribed, not without probability,

to the agency of the winds.

Inundations of rivers, observes Kirby, if they happen at any season

except in the depths of winter, always carry down a number of insects,

floating on the surface of bits of stick, weeds, &c.; so that when the

waters subside, the entomologist may generally reap a plentiful harvest.

In the dissemination, moreover, of these minute beings, as in that of

plants, the larger animals play their part. Insects are, in numberless

instances, borne along in the coats of animals, or the feathers of

birds; and the eggs of some species are capable, like seeds, of

resisting the digestive powers of the stomach, and after they are

swallowed with herbage, may be ejected again unharmed in the dung.

_Geographical Distribution and Diffusion of Man._

I have reserved for the last some observations on the range and

diffusion of the human species over the earth, and the influence of man

in spreading other animals and plants, especially the terrestrial.

Many naturalists have amused themselves in speculating on the probable

birth-place of mankind, the point from which, if we assume the whole

human race to have descended from a single pair, the tide of emigration

must originally have proceeded. It has been always a favorite

conjecture, that this birth-place was situated within or near the

tropics, where perpetual summer reigns, and where fruits, herbs, and

roots are plentifully supplied throughout the year. The climate of these

regions, it has been said, is suited to a being born without any

covering, and who had not yet acquired the arts of building habitations

or providing clothes.

_Progress of Human Population._--"The hunter state," it has been argued,

"which Montesquieu placed the first, was probably only the second stage

to which mankind arrived; since so many arts must have been invented to

catch a salmon, or a deer, that society could no longer have been in its

infancy when they came into use."[935] When regions where the

spontaneous fruits of the earth abound became overpeopled, men would

naturally diffuse themselves over the neighboring parts of the temperate

zone; but a considerable time would probably elapse before this event

took place; and it is possible, as a writer before cited observes, that

in the interval before the multiplication of their numbers and their

increasing wants had compelled them to emigrate, some arts to take

animals were invented, but far inferior to what we see practised at this

day among savages. As their habitations gradually advanced into the

temperate zone, the new difficulties they had to encounter would call

forth by degrees the spirit of invention, and the probability of such

inventions always rises with the number of people involved in the same

necessity.[936]

A distinguished modern writer, who coincides for the most part in the

views above mentioned, has introduced one of the persons in his second

dialogue, as objecting to the theory of the human race having gradually

advanced from a savage to a civilized state, on the ground that "the

first man must have inevitably been destroyed by the elements or

devoured by savage beasts, so infinitely his superiors in physical

force."[937] He then contends against the difficulty here started by

various arguments, all of which were, perhaps, superfluous; for if a

philosopher is pleased to indulge in conjectures on this subject, why

should he not assign, as the original seat of man, some one of those

large islands within the tropics, which are as free from large beasts of

prey as Van Diemen's Land or Australia? Here man may have remained for a

period, peculiar to a single island, just as some of the large

anthropomorphous species are now limited to one island within the

tropics. In such a situation, the new-born race might have lived in

security, though far more helpless than the New Holland savages, and

might have found abundance of vegetable food. Colonies may afterwards

have been sent forth from this mother country, and then the peopling of

the earth may have proceeded according to the hypothesis before alluded

to.

To form a probable conjecture respecting the country from whence the

early civilization of India was derived, has been found almost as

difficult as to determine the original birth-place of the human race.

That the dawn of oriental civilization did not arise within the limits

of the tropics, is the conclusion to which Baron William von Humboldt

has come after much patient research into "the diversities of the

structure of language and their influence on the mental development of

the human race." According to him the ancient Zend country from whence

the spread of knowledge and the arts has been traced in a south-easterly

direction, lay to the north-west of the upper Indus.[938]

As to the time of the first appearance of man upon the earth, if we are

to judge from the discordance of opinion amongst celebrated

chronologers, not even a rude approximation has yet been made towards

determining a point of so much interest. The problem seems hitherto to

have baffled the curiosity of the antiquary, if possible, more

completely than the fixing on a geographical site for the original

habitation of the ancestors of the human race. The Chevalier Bunsen, in

his elaborate and philosophical work on Ancient Egypt,[939] has

satisfied not a few of the learned, by an appeal to monumental

inscriptions still extant, that the successive dynasties of kings may be

traced back without a break, to Menes, and that the date of his reign

would correspond with the year 3640 B. C. He supposes at the same time,

what is most reasonable, that the Egyptian people must have existed for

a long period (probably at least for five centuries), in their earlier

and less settled state, before they reached the point of civilization at

which Menes consolidated them into a great and united empire. This would

carry us back to upwards of 4000 years B. C., or to an epoch coincident

with that commonly set down for the creation of the world in accordance

with computations founded on the combined ages of the successive

antediluvian patriarchs. It follows that the same epoch of Menes is

anterior by a great many centuries to the most ancient of the dates

usually fixed upon for the Mosaic deluge. The fact that no record or

tradition of any great and overwhelming flood has been detected in the

mythology, or monumental annals of the Egyptians, will suggest many

reflections to a geologist who has weighed well the evidence we possess

of a variety of partial deluges which have happened in districts not

free like Egypt, for the last 3000 years, from earthquakes and other

causes of great aqueous catastrophes. The tales and legends of

calamitous floods preserved in Greece, Asia Minor, the southern shores

of the Baltic, China, Peru, and Chili, have, as we have seen, been all

of them handed down to us by the inhabitants of regions in which the

operation of natural causes in modern times, and the recurrence of a

succession of disastrous floods, afford us data for interpreting the

meaning of the obscure traditions of an illiterate age.[940]

In his learned treatise on ancient chronology, Dr. Hales has selected,

from a much greater number, a list of no less than 120 authors, all of

whom give a different period for the epoch of the creation of the world,

the extreme range of difference between them amounting to no less than

3268 years. It appears that even amongst authorities, who in England are

generally regarded, as orthodox, there is a variance, not of years or of

one or two centuries, but of upwards of a millennium, according as they

have preferred to follow the Hebrew, or the Samaritan, or the Greek

versions of the Mosaic writings. Can we then wonder that they who

decipher the monuments of Egypt, or the geologist who interprets the

earth's autobiography, should arrive at views respecting the date of an

ancient empire, or the age of our planet, irreconcilable with every one

of these numerous and conflicting chronologies? The want of agreement

amongst the learned in regard to the probable date of the deluge of Noah

is a source of far greater perplexity and confusion than our extreme

uncertainty as to the epoch of the creation,--the deluge being a

comparatively modern event, from which the repeopling of the earth and

the history of the present races of mankind is made to begin.

Naturalists have long felt that to render probable the received opinion

that all the leading varieties of the human family have originally

sprung from a single pair, (a doctrine against which there appears to me

to be no sound objection,) a much greater lapse of time is required for

the slow and gradual formation of races, (such as the Caucasian,

Mongolian, and Negro,) than is embraced in any of the popular systems of

chronology. The existence of two of those marked varieties above

mentioned can be traced back 3000 years before the present time, or to

the painting of pictures, preserved in the tombs or on the walls of

buried temples in Egypt. In these we behold the Negro and Caucasian

physiognomies portrayed as faithfully and in as strong contrast as if

the likenesses of those races had been taken yesterday. When we consider

therefore the extreme slowness of the changes, which climate and other

modifying causes have produced in modern times, we must allow for a vast

series of antecedent ages, in the course of which the long-continued

influence of similar external circumstances gave rise to peculiarities,

probably increased in many successive generations, until they were fixed

by hereditary transmission. The characteristic forms and features thus

acquired by certain tribes, may have been afterwards diffused by

migration from a few centres over wide continental spaces. The theory,

therefore, that all the races of man have come from one common stock

receives support from every investigation which forces us to expand our

ideas of the duration of past time, or which multiplies the number of

years that have passed away since the origin of man. Hitherto, geology

has neither enlarged nor circumscribed the "human period;" but simply

proved that in the history of animated nature it is comparatively

modern, or the last of a long series of antecedent epochs, in each of

which the earth has been successively peopled by distinct species of

animals and plants.

In an early stage of society the necessity of hunting acts as a

principle of repulsion, causing men to spread with the greatest rapidity

over a country, until the whole is covered with scattered settlements.

It has been calculated that eight hundred acres of hunting-ground

produce only as much food as half an acre of arable land. When the game

has been in a great measure exhausted, and a state of pasturage

succeeds, the several hunter tribes, being already scattered, may

multiply in a short time into the greatest number which the pastoral

state is capable of sustaining. The necessity, says Brand, thus imposed

upon the two savage states, of dispersing themselves far and wide over

the country, affords a reason why, at a very early period, the worst

parts of the earth may have become inhabited.

But this reason, it may be said, is only applicable in as far as regards

the peopling of a continuous continent; whereas the smallest islands,

however remote from continents, have almost always been found inhabited

by man. St. Helena, it is true, afforded an exception; for when that

island was discovered in 1501, it was only inhabited by sea-fowl, and

occasionally by seals and turtles, and was covered with a forest of

trees and shrubs, all of species peculiar to it, with one or two

exceptions, and which seem to have been expressly created for this

remote and insulated spot.[941]

The islands also of Mauritius, Bourbon, Pitcairns, and Juan Fernandez,

and those of the Galapagos archipelago, one of which is seventy miles

long, were inhabited when first discovered, and, what is more remarkable

than all, the Falkland Islands, which together are 120 miles in length

by 60 in breadth, and abounding in food fit for the support of man.

_Drifting of canoes to vast distances._--But very few of the numerous

coral islets and volcanoes of the vast Pacific, capable of sustaining a

few families of men, have been found untenanted; and we have, therefore,

to inquire whence and by what means, if all the members of the great

human family have had one common source, could those savages have

migrated. Cook, Forster, and others, have remarked that parties of

savages in their canoes must have often lost their way, and must have

been driven on distant shores, where they were forced to remain,

deprived both of the means and of the requisite intelligence for

returning to their own country. Thus Captain Cook found on the island of

Wateoo three inhabitants of Otaheite, who had been drifted thither in a

canoe, although the distance between the two isles is 550 miles. In

1696, two canoes, containing thirty persons, who had left Ancorso, were

thrown by contrary winds and storms on the island of Samar, one of the

Philippines, at a distance of 800 miles. In 1721, two canoes, one of

which contained twenty-four, and the other six persons, men, women, and

children, were drifted from an island called Farroilep to the island of

Guaham, one of the Marians, a distance of 200 miles.[942]

Kotzebue, when investigating the Coral Isles of Radack, at the eastern

extremity of the Caroline Isles, became acquainted with a person of the

name of Kadu, who was a native of Ulea, an isle 1500 miles distant, from

which he had been drifted with a party. Kadu and three of his

countrymen one day left Ulea in a sailing boat, when a violent storm

arose, and drove them out of their course: they drifted about the open

sea for eight months, according to their reckoning by the moon, making a

knot on a cord at every new moon. Being expert fishermen, they subsisted

entirely on the produce of the sea; and when the rain fell, laid in as

much fresh water as they had vessels to contain it. "Kadu," says

Kotzebue, "who was the best diver, frequently went down to the bottom of

the sea, where it is well known that the water is not so salt, with a

cocoa-nut shell, with only a small opening."[943] When these unfortunate

men reached the isles of Radack, every hope and almost every feeling had

died within them; their sail had long been destroyed, their canoe had

long been the sport of winds and waves, and they were picked up by the

inhabitants of Aur in a state of insensibility; but by the hospitable

care of those islanders they soon recovered, and were restored to

perfect health.[944]

Captain Beechey, in his voyage to the Pacific, fell in with some natives

of the Coral Islands, who had in a similar manner been carried to a

great distance from their native country. They had embarked, to the

number of 150 souls, in three double canoes, from Anaa, or Chain Island,

situated about three hundred miles to the eastward of Otaheite. They

were overtaken by the monsoon, which dispersed the canoes; and after

driving them about the ocean, left them becalmed, so that a great number

of persons perished. Two of the canoes were never heard of; but the

other was drifted from one uninhabited island to another, at each of

which the voyagers obtained a few provisions; and at length, after

having wandered for a distance of 600 miles, they were found and carried

to their home in the Blossom.[945]

Mr. Crawfurd informs me that there are several well-authenticated

accounts of canoes having been drifted from Sumatra to Madagascar, and

by such causes a portion of the Malayan language, with some useful

plants, have been transferred to that island, which is principally

peopled by negroes.

The space traversed in some of these instances was so great, that

similar accidents might suffice to transport canoes from various parts

of Africa to the shores of South America, or from Spain to the Azores,

and thence to North America; so that man, even in a rude state of

society, is liable to be scattered involuntarily by the winds and waves

over the globe, in a manner singularly analogous to that in which many

plants and animals are diffused. We ought not, then, to wonder, that

during the ages required for some tribes of the human race to attain

that advanced stage of civilization which empowers the navigator to

cross the ocean in all directions with security, the whole earth should

have become the abode of rude tribes of hunters and fishers. Were the

whole of mankind now cut off, with the exception of one family,

inhabiting the old or new continent, or Australia, or even some coral

islet of the Pacific, we might expect their descendants, though they

should never become more enlightened than the South Sea Islanders or the

Esquimaux, to spread in the course of ages over the whole earth,

diffused partly by the tendency of population to increase, in a limited

district, beyond the means of subsistence, and partly by the accidental

drifting of canoes by tides and currents to distant shores.

_Involuntary Influence of Man in diffusing Animals and Plants._

Many of the general remarks which have been made respecting the

influence of man in spreading or in checking the diffusion of plants

apply equally to his relations with the animal kingdom. On a future

occasion I shall be led to speak of the instrumentality of our species

in naturalizing useful animals and plants in new regions, when

explaining my views of the effects which the spreading and increase of

certain species exert in the extirpation of others. At present I shall

confine myself to a few remarks on the involuntary aid which man lends

to the dissemination of species.

In the mammiferous class our influence is chiefly displayed in

increasing the number of quadrupeds which are serviceable to us, and in

exterminating or reducing the number of those which are noxious.

Sometimes, however, we unintentionally promote the multiplication of

inimical species, as when we introduced the rat, which was not

indigenous in the new world, into all parts of America. They have been

conveyed over in ships, and now infest a great multitude of islands and

parts of that continent. In like manner the Norway rat (_Mus decumanus_)

has been imported into England, where it plunders our property in ships

and houses.

Among birds, the house sparrow may be cited as a species known to have

extended its range with the tillage of the soil. During the last century

it has spread gradually over Asiatic Russia towards the north and east,

always following the progress of cultivation. It made its first

appearance on the Irtisch in Tobolsk, soon after the Russians had

ploughed the land. It came in 1735 up the Obi to Beresow, and four years

after to Naryn, about fifteen degrees of longitude farther east. In

1710, it had been seen in the higher parts of the coast of the Lena, in

the government of Irkutzk. In all these places it is now common, but is

not yet found in the uncultivated regions of Kamtschatka.[946]

The great viper (_Fer de lance_), a species no less venomous than the

rattlesnake, which now ravages Martinique and St. Lucia, was

accidentally introduced by man, and exists in no other part of the West

Indies.

Many parasitic insects which attack our persons, and some of which are

supposed to be peculiar to our species, have been carried into all parts

of the earth, and have as high a claim as man to a _universal_

geographical distribution.

A great variety of insects have been transported in ships from one

country to another, especially in warmer latitudes. The European

house-fly has been introduced in this way into all the South Sea

Islands. Notwithstanding the coldness of our climate in England we have

been unable to prevent the cockroach (_Blatta orientalis_) from entering

and diffusing itself in our ovens and kneading troughs, and availing

itself of the artificial warmth which we afford. It is well known also,

that beetles, and many other kinds of ligniperdous insects, have been

introduced into Great Britain in timber; especially several North

American species. "The commercial relations," says Malte-Brun[947],

"between France and India have transported from the latter country the

aphis, which destroys the apple tree, and two sorts of Neuroptera, the

_Lucifuga_ and _Flavicola_, mostly confined to Provence and the

neighbourhood of Bourdeaux, where they devour the timber in the houses

and naval arsenals."

Among mollusks we may mention the _Teredo navalis_, which is a native of

equatorial seas, but which, by adhering to the bottom of ships, was

transported to Holland, where it has been most destructive to vessels

and piles. The same species has also become naturalized in England, and

other countries enjoying an extensive commerce. _Bulimus undatus_, a

land species of considerable size, native of Jamaica and other West

Indian islands, has been imported, adhering to tropical timber, into

Liverpool; and, as I learn from Mr. Broderip, is now naturalized in the

woods near that town.

In all these and innumerable other instances we may regard the

involuntary agency of man as strictly analogous to that of the inferior

animals. Like them, we unconsciously contribute to extend or limit the

geographical range and numbers of certain species, in obedience to

general rules in the economy of nature, which are for the most part

beyond our control.

CHAPTER XL.

THEORIES RESPECTING THE ORIGINAL INTRODUCTION OF SPECIES.

Proposal of an hypothesis on this subject--Supposed centres or foci

of creation--Why distinct provinces of animals and plants have not

become more blended together--Brocchi's speculations on the loss of

species--Stations of plants and animals--Causes on which they

depend--Stations of plants how affected by animals--Equilibrium in

the number of species how preserved--Peculiar efficacy of insects in

this task--Rapidity with which certain insects multiply or decrease

in numbers--Effect of omnivorous animals in preserving the

equilibrium of species--Reciprocal influence of aquatic and

terrestrial species on each other.

_Theory of Linnaeus._--It would be superfluous to examine the various

attempts which were made to explain the phenomena of the distribution of

species alluded to in the preceding chapters, in the infancy of the

sciences of botany, zoology, and physical geography. The theories or

rather conjectures then indulged now stand refuted by a simple statement

of facts; and if Linnaeus were living he would be the first to renounce

the notions which he promulgated. For he imagined the habitable world to

have been for a certain time limited to one small tract, the only

portion of the earth's surface that was as yet laid bare by the

subsidence of the primaeval ocean. In this fertile spot he supposed the

originals of all the species of plants which exist on this globe to have

been congregated together with the first ancestors of all animals and of

the human race. "In qua commode habitaverint animalia omnia, et

vegetabilia laete germinaverint." In order to accommodate the various

habitudes of so many creatures, and to provide a diversity of climate

suited to their several natures, the tract in which the creation took

place was supposed to have been situated in some warm region of the

earth, but to have contained a lofty mountain range, on the heights and

in the declivities of which were to be found all temperatures and every

climate, from that of the torrid to that of the frozen zone.[948]

That there never was a universal ocean since the planet was inhabited,

or, rather, since the oldest groups of strata yet known to contain

organic remains were formed, is proved by the presence of terrestrial

plants or by indications of shores in all the older formations; and if

this conclusion was not established, yet no geologist could deny that,

since the first small portion of the earth was laid dry, there have been

many entire changes in the species of plants and animals inhabiting the

land.

But, without dwelling on the above and other refuted theories, let us

inquire whether some hypothesis cannot be substituted as simple as that

of Linnaeus, to which the phenomena now ascertained in regard to the

distribution both of aquatic and terrestrial species may be referred.

The following may, perhaps, be reconcileable with known facts:--Each

species may have had its origin in a single pair, or individual, where

an individual was sufficient, and species may have been created in

succession at such times and in such places as to enable them to

multiply and endure for an appointed period, and occupy an appointed

space on the globe.

In order to explain this theory, let us suppose every living thing to be

destroyed in the western hemisphere, both on the land and in the ocean,

and permission to be given to man to people this great desert, by

transporting into it animals and plants from the eastern hemisphere, a

strict prohibition being enforced against introducing two original

stocks of the same species.

Now it is easy to show that the result of such a mode of colonizing

would correspond exactly, so far as regards the grouping of animals and

plants, with that now observed throughout the globe. In the first place,

it would be necessary for naturalists, before they imported species into

particular localities, to study attentively the climate and other

physical conditions of each spot. It would be no less requisite to

introduce the different species in succession, so that each plant and

animal might have time and opportunity to multiply before the species

destined to prey upon it was admitted. Many herbs and shrubs, for

example, must spread far and wide before the sheep, the deer, and the

goat could be allowed to enter, lest they should devour and annihilate

the original stocks of many plants, and then perish themselves for want

of food. The above-mentioned herbivorous animals in their turn must be

permitted to make considerable progress before the entrance of the first

pair of wolves or lions. Insects must be allowed to swarm before the

swallow could be permitted to skim through the air, and feast on

thousands at one repast.

It is evident that, however equally in this case our original stocks

were distributed over the whole surface of land and water, there would

nevertheless arise distinct botanical and zoological provinces, for

there are a great many natural barriers which oppose common obstacles to

the advance of a variety of species. Thus, for example, almost all the

animals and plants naturalized by us, towards the extremity of South

America, would be unable to spread beyond a certain limit, towards the

east, west, and south; because they would be stopped by the ocean, and a

few of them only would succeed in reaching the cooler latitudes of the

northern hemisphere, because they would be incapable of bearing the heat

of the tropics, through which they must pass. In the course of ages,

undoubtedly, exceptions would arise, and some species might become

common to the temperate and polar regions, or both sides of the equator;

for I have before shown that the powers of diffusion conferred on some

classes are very great. But we might confidently predict that these

exceptions would never become so numerous as to invalidate the general

rule.

Some of the plants and animals transplanted by us to the coast of Chili

and Peru would never be able to cross the Andes, so as to reach the

eastern plains; nor, for a similar reason, would those first established

in the Pampas, or the valleys of the Amazon and the Orinoco, ever arrive

at the shores of the Pacific.

In the ocean an analogous state of things would prevail; for there,

also, climate would exert a great influence in limiting the range of

species, and the land would stop the migrations of aquatic tribes as

effectually as the sea arrests the dispersion of the terrestrial. As

certain birds, insects, and the seeds of plants, can never cross the

direction of prevailing winds, so currents form natural barriers to the

dissemination of many oceanic races. A line of shoals may be as

impassable to deep-water species, as are the Alps and the Andes to

plants and animals peculiar to plains; while deep abysses may prove

insuperable obstacles to the migrations of the inhabitants of shallow

waters.

_Supposed centres, or foci, of creation._--It is worthy of observation,

that one effect of the introduction of single pairs of each species must

be the confined range of certain groups in spots, which, like small

islands, or solitary inland lakes, have few means of interchanging their

inhabitants with adjoining regions. Now this congregating in a small

space of many peculiar species, would give an appearance of _centres_ or

_foci_ of creation, as they have been termed, as if they were favourite

points where the creative energy has been in greater action than in

others, and where the numbers of peculiar organic beings have

consequently become more considerable.

I do not mean to call in question the soundness of the inferences of

some botanists, as to the former existence of certain limited spots

whence species of plants have been propagated, radiating, as it were, in

all directions from a common centre. On the contrary, I conceive these

phenomena to be the necessary consequences of the plan of nature before

suggested, operating during the successive mutations of the surface,

some of which the geologist can prove to have taken place subsequently

to the period when many species now existing were created. In order to

exemplify how this arrangement of plants may have been produced, let us

imagine that, about three centuries before the discovery of St. Helena

(itself of submarine volcanic origin), a multitude of new islands had

been thrown up in the surrounding sea, and that these had each become

clothed with plants emigrating from St. Helena, in the same manner as

the wild plants of Campania have diffused themselves over Monte Nuovo.

Whenever the first botanist investigated the new archipelago, he would,

in all probability, find a different assemblage of plants in each of the

islands of recent formation; but in St. Helena itself, he would meet

with individuals of every species, belonging to all parts of the

archipelago, and some, in addition, peculiar to itself, viz., those

which had not been able to obtain a passage into any one of the

surrounding new-formed lands. In this case it might be truly said that

the original island was the primitive focus, or centre, of a certain

type of vegetation; whereas, in the surrounding islands, there would be

a smaller number of species, yet all belonging to the same group.

But this peculiar distribution of plants would not warrant the

conclusion that, in the space occupied by St. Helena, there had been a

greater exertion of creative power than in the spaces of equal area

occupied by the new adjacent lands; because, within the period in which

St. Helena had acquired its peculiar vegetation, each of the spots

supposed to be subsequently converted into land may have been the

birth-place of a great number of _marine_ animals and plants, which may

have had time to scatter themselves far and wide over the southern

Atlantic.

_Why distinct provinces not more blended._--Perhaps it may be objected

to some parts of the foregoing train of reasoning, that during the lapse

of past ages, especially during many partial revolutions of the globe of

comparatively modern date, different zoological and botanical provinces

ought to have become more confounded and blended together--that the

distribution of species approaches too nearly to what might have been

expected, if animals and plants had been introduced into the globe when

its physical geography had already assumed the features which it now

wears; whereas we know that, in certain districts, considerable

geographical changes have taken place since species identical with those

now in being were created.

_Brocchi's speculations on loss of species._--These and many kindred

topics cannot be fully discussed until we have considered, not merely

the general laws which may regulate the first introduction of species,

but those which may limit their _duration_ on the earth. Brocchi

remarked, when hazarding some interesting conjectures respecting "the

loss of species," that a modern naturalist had no small assurance, who

declared "that individuals alone were capable of destruction, and that

species were so perpetuated that nature could not annihilate them, so

long as the planet lasted, or at least that nothing less than the shock

of a comet, or some similar disaster, could put an end to their

existence."[949] The Italian geologist, on the contrary, had satisfied

himself that many species of Testacea, which formerly inhabited the

Mediterranean, had become extinct, although a great number of others,

which had been the contemporaries of those lost races, still survived.

He came to the opinion that about half the species which peopled the

waters when the Subapennine strata were deposited had gone out of

existence; and in this inference he does not appear to have been far

wrong.

But, instead of seeking a solution of this problem; like some other

geologists of his time, in a violent and general catastrophe, Brocchi

endeavoured to imagine some regular and constant law by which species

might be made to disappear from the earth gradually and in succession.

The death, he suggested, of a species might depend, like that of

individuals, on certain peculiarities of constitution conferred upon

them at their birth; and as the longevity of the one depends on a

certain force of vitality, which, after a period, grows weaker and

weaker, so the duration of the other may be governed by the quantity of

prolific power bestowed upon the species which, after a season, may

decline in energy, so that the fecundity and multiplication of

individuals may be gradually lessened from century to century, "until

that fatal term arrives when the embryo, incapable of extending and

developing itself, abandons, almost at the instant of its formation, the

slender principle of life by which it was scarcely animated,--and so all

dies with it."

Now we may coincide in opinion with the Italian naturalist, as to the

gradual extinction of species one after another, by the operation of

regular and constant causes, without admitting an inherent principle of

deterioration in their physiological attributes. We might concede, "that

many species are on the decline, and that the day is not far distant

when they will cease to exist;" yet deem it consistent with what we know

of the nature of organic beings, to believe that the last individuals of

each species retain their prolific powers in their full intensity.

Brocchi has himself speculated on the share which a change of climate

may have had in rendering the Mediterranean unfit for the habitation of

certain Testacea, which still continued to thrive in the Indian Ocean,

and of others which were now only represented by analogous forms within

the tropics. He must also have been aware that other extrinsic causes,

such as the progress of human population, or the increase of some one of

the inferior animals, might gradually lead to the extirpation of a

particular species, although its fecundity might remain to the last

unimpaired. If, therefore, amid the vicissitudes of the animate and

inanimate world, there are known causes capable of bringing about the

decline and extirpation of species, it became him thoroughly to

investigate the full extent to which these might operate, before he

speculated on any cause of so purely hypothetical a kind as "the

diminution of the prolific virtue."

If it could have been shown that some wild plant had insensibly dwindled

away and died out, as sometimes happens to cultivated varieties

propagated by cuttings, even though climate, soil, and every other

circumstance, should continue identically the same--if any animal had

perished while the physical condition of the earth, and the number and

force of its foes, with every other extrinsic cause, remain unaltered,

then might we have some ground for suspecting that the infirmities of

age creep on as naturally on species as upon individuals. But, in the

absence of such observations, let us turn to another class of facts, and

examine attentively the circumstances which determine the _stations_ of

particular animals and plants, and perhaps we shall discover, in the

vicissitudes to which these stations are exposed, a cause fully adequate

to explain the phenomena under consideration.

_Stations of plants and animals._--Stations comprehend all the

circumstances, whether relating to the animate or inanimate world, which

determine whether a given plant or animal can exist in a given place; so

that if it be shown that stations can become essentially modified by the

influence of known causes, it will follow that species, as well as

individuals, are mortal.

Every naturalist is familiar with the fact, that although in a

particular country, such as Great Britain, there may be more than three

thousand species of plants, ten thousand insects, and a great variety in

each of the other classes; yet there will not be more than a hundred,

perhaps not half that number, inhabiting any given locality. There may

be no want of space in the supposed tract: it may be a large mountain,

or an extensive moor, or a great river plain, containing room enough for

individuals of every species in our island; yet the spot will be

occupied by a few to the exclusion of many, and these few are enabled,

throughout long periods, to maintain their ground successfully against

every intruder, notwithstanding the facilities which species enjoy, by

virtue of their power of diffusion, of invading adjacent territories.

The principal causes which enable a certain assemblage of plants thus to

maintain their ground against all others depend, as is well known, on

the relations between the physiological nature of each species, and the

climate, exposure, soil, and other physical conditions of the locality.

Some plants live only on rocks, others in meadows, a third class in

marshes. Of the latter, some delight in a fresh-water morass,--others in

salt marshes, where their roots may copiously absorb saline particles.

Some prefer an alpine region in a warm latitude, where, during the heat

of summer, they are constantly irrigated by the cool waters of melting

snows. To others loose sand, so fatal to the generality of species,

affords the most proper station. The _Carex arenaria_ and the _Elymus

arenarius_ acquire their full vigor on a sandy dune, obtaining an

ascendancy over the very plants which in a stiff clay would immediately

stifle them.

Where the soil of a district is of so peculiar a nature that it is

extremely favorable to certain species, and agrees ill with every other,

the former get exclusive possession of the ground, and, as in the case

of heaths, live in societies. In like manner the bog moss (_Sphagnum_)

is fully developed in peaty swamps, and becomes, like the heath, in the

language of botanists, a social plant. Such monopolies, however, are not

common, for they are checked by various causes. Not only are many

species endowed with equal powers to obtain and keep possession of

similar stations, but each plant, for reasons not fully explained by the

physiologist, has the property of rendering the soil where it has grown

less fitted for the support of other individuals of its own species, or

even other species of the same family. Yet the same spot, so far from

being impoverished, is improved, for plants of _another_ family. Oaks,

for example, render the soil more fertile for the fir tribe, and firs

prepare the soil for oaks. Every agriculturist feels the force of this

law of the organic world, and regulates accordingly the rotation of his

crops.

_Equilibrium in the number of species, how preserved._--"All the plants

of a given country," says De Candolle, in his usual spirited style, "are

at war one with another. The first which establish themselves by chance

in a particular spot tend, by the mere occupancy of space, to exclude

other species--the greater choke the smaller; the longest livers replace

those which last for a shorter period; the more prolific gradually make

themselves masters of the ground, which species multiplying more slowly

would otherwise fill."

In this continual strife it is not always the resources of the plant

itself which enable it to maintain or extend its ground. Its success

depends, in a great measure, on the number of its foes or allies among

the animals and plants inhabiting the same region. Thus, for example, a

herb which loves the shade may multiply, if some tree with spreading

boughs and dense foliage flourish in the neighborhood. Another, which,

if unassisted, would be overpowered by the rank growth of some hardy

competitor, is secure because its leaves are unpalatable to cattle;

which, on the other hand, annually crop down its antagonist, and rarely

suffer it to ripen its seed.

Oftentimes we see some herb which has flowered in the midst of a thorny

shrub, when all the other individuals of the same species, in the open

fields around, are eaten down, and cannot bring their seed to maturity.

In this case, the shrub has lent his armor of spines and prickles to

protect the defenceless herb against the mouths of the cattle, and thus

a few individuals which occupied, perhaps, the most unfavorable station

in regard to exposure, soil, and other circumstances, may, nevertheless,

by the aid of an ally, become the principal source whereby the winds are

supplied with seeds which perpetuate the species throughout the

surrounding tract. Thus, in the New Forest in Hampshire, the young oaks

which are not consumed by the deer, or uprooted by the swine, are

indebted to the holly for their escape.

In the above examples we see one plant shielding another from the

attacks of animals; but instances are, perhaps, still more numerous,

where some animal defends a plant against the enmity of some other

subject of the vegetable kingdom.

Scarcely any beast, observes a Swedish naturalist, will touch the

nettle, but fifty different kinds of insects are fed by it.[950] Some of

these seize upon the root, others upon the stem; some eat the leaves,

others devour the seeds and flowers; but for this multitude of enemies,

the nettle (_Urtica dioica_), which is now found in all the four

quarters of the globe, would annihilate a great number of plants.

Linnaeus tells us, in his "Tour in Scania," that goats were turned into

an island which abounded with the _Agrostis arundinacea_, where they

perished by famine; but horses which followed them grew fat on the same

plant. The goat, also, he says, thrives on the meadow-sweet and

water-hemlock, plants which are injurious to cattle.[951]

_Agency of insects._--Every plant, observes Wilcke, has its proper

insect allotted to it to curb its luxuriancy, and to prevent it from

multiplying to the exclusion of others. "Thus grass in meadows sometimes

flourishes so as to exclude all other plants; here the Phalaena graminis

(_Bombyx gram._), with her numerous progeny, finds a well-spread table;

they multiply in immense numbers, and the farmer, for some years,

laments the failure of his crop; but the grass being consumed, the moths

die with hunger, or remove to another place. Now the quantity of grass

being greatly diminished, the other plants, which were before choked by

it, spring up, and the ground becomes variegated with a multitude of

different species of flowers. Had not nature given a commission to this

minister for that purpose, the grass would destroy a great number of

species of vegetables, of which the equilibrium is now kept up."[952]

In the above passage allusion is made to the ravages committed in 1740,

and the two following years, in many provinces of Sweden, by a most

destructive insect. The same moth is said never to touch the foxtail

grass, so that it may be classed as a most active ally and benefactor of

that species, and as peculiarly instrumental in preserving it in its

present abundance.[953] A discovery of Rolander, cited in the treatise

of Wilcke above mentioned, affords a good illustration of the checks and

counter-checks which nature has appointed to preserve the balance of

power among species. "The _Phalaena strobilella_ has the fir cone

assigned to it to deposit its eggs upon; the young caterpillars coming

out of the shell consume the cone and superfluous seed; but, lest the

destruction should be too general, the _Ichneumon strobilellae_ lays its

eggs in the caterpillar, inserting its long tail in the openings of the

cone till it touches the included insect, for its body is too large to

enter. Thus it fixes its minute egg upon the caterpillar, which being

hatched, destroys it."[954]

Entomologists enumerate many parallel cases where insects, appropriated

to certain plants, are kept down by other insects, and these again by

parasites expressly appointed to prey on them.[955] Few perhaps are in

the habit of duly appreciating the extent to which insects are active in

preserving the balance of species among plants, and thus regulating

indirectly the relative numbers of many of the higher orders of

terrestrial animals.

The peculiarity of their agency consists in their power of suddenly

multiplying their numbers to a degree which could only be accomplished

in a considerable lapse of time in any of the larger animals, and then

as instantaneously relapsing, without the intervention of any violent

disturbing cause, into their former insignificance.

If, for the sake of employing, on different but rare occasions, a power

of many hundred horses, we were under the necessity of feeding all these

animals at great cost in the intervals when their services were not

required, we should greatly admire the invention of a machine, such as

the steam-engine, which was capable at any moment of exerting the same

degree of strength without any consumption of food during periods of

inaction. The same kind of admiration is strongly excited when we

contemplate the powers of insect life, in the creation of which the

Author of nature has been so prodigal. A scanty number of minute

individuals, to be detected only by careful research, are ready in a few

days, weeks, or months, to give birth to myriads, which may repress any

degree of monopoly in another species, or remove nuisances, such as dead

carcases, which might taint the air. But no sooner has the destroying

commission been executed than the gigantic power becomes dormant--each

of the mighty host soon reaches the term of its transient existence, and

the season arrives when the whole species passes naturally into the egg,

and thence into the larva and pupa state. In this defenceless condition

it may be destroyed either by the elements, or by the augmentation of

some of its numerous foes which may prey upon it in the early stages of

its transformation: or it often happens that in the following year the

season proves unfavorable to the hatching of the eggs or the development

of the pupae.

Thus the swarming myriads depart which may have covered the vegetation

like the aphides, or darkened the air like locusts. In almost every

season there are some species which in this manner put forth their

strength, and then, like Milton's spirits, which thronged the spacious

hall, "reduce to smallest forms their shapes immense"--

----So thick the aary crowd

Swarm'd and were straiten'd; till the signal given,

Behold a wonder! they but how who seem'd

In bigness to surpass earth's giant sons,

Now less than smallest dwarfs.

A few examples will illustrate the mode in which this force operates. It

is well known that, among the countless species of the insect creation,

some feed on animal, others on vegetable matter; and upon considering a

catalogue of eight thousand British Insects and Arachnidae, Mr. Kirby

found that these two divisions were nearly a counterpoise to each other,

the carnivorous being somewhat preponderant. There are also distinct

species, some appointed to consume living, others dead or putrid animal

and vegetable substances. One female, of _Musca carnaria_, will give

birth to twenty thousand young; and the larvae of many flesh-flies devour

so much food in twenty-four hours, and grow so quickly, as to increase

their weight two hundred-fold! In five days after being hatched they

arrive at their full growth and size, so that there was ground, says

Kirby, for the assertion of Linnaeus, that three flies of _M. vomitoria_

could devour a dead horse as quickly as a lion[956]; and another Swedish

naturalist remarks, that so great are the powers of propagation of a

single species even of the smallest insects, that each can commit, when

required, more ravages than the elephant.[957]

Next to locusts, the aphides, perhaps, exert the greatest power over the

vegetable world, and, like them, are so sometimes so numerous as to

darken the air. The multiplication of these little creatures is without

parallel, and almost every plant has its peculiar species. Reaumur has

proved that in five generations one aphis may be the progenitor of

5,904,900,000 descendants; and it is supposed that in one year there may

be twenty generations.[958] Mr. Curtis observes that, as among

caterpillars we find some that are constantly and unalterably attached

to one or more particular species of plants, and others that feed

indiscriminately on most sorts of herbage, so it is precisely with the

aphides: some are particular, others more general feeders; and as they

resemble other insects in this respect, so they do also in being more

abundant in some years than in others.[959] In 1793 they were the chief,

and in 1798 the sole, cause of the failure of the hops. In 1794, a

season, almost unparalleled for drought, the hop was perfectly free from

them; while peas and beans, especially the former, suffered very much

from their depredations.

The ravages of the caterpillars of some of our smaller moths afford a

good illustration of the temporary increase of a species. The oak-trees

of a considerable wood have been stripped of their leaves as bare as in

winter by the caterpillars of a small green moth (_Tortrix viridana_),

which has been observed the year following not to abound.[960] The

silver Y moth (_Plusia gamma_), although one of our common species, is

not dreaded by us for its devastations; but legions of their

caterpillars have at times created alarm in France, as in 1735. Reaumur

observes that the female moth lays about four hundred eggs; so that if

twenty caterpillars were distributed in a garden, and all lived through

the winter and became moths in the succeeding May, the eggs laid by

these, if half of them were female and all fertile, would in the next

generation produce 800,000 caterpillars.[961] A modern writer,

therefore, justly observes that, did not Providence put causes in

operation to keep them in due bounds, the caterpillars of this moth

alone, leaving out of consideration the two thousand other British

species, might soon destroy more than half of our vegetation.[962]

In the latter part of the last century an ant most destructive to the

sugar-cane (_Formica saccharivora_), appeared in such infinite hosts in

the island of Granada, as to put a stop to the cultivation of that

vegetable. Their numbers were incredible. The plantations and roads were

filled with them; many domestic quadrupeds, together with rats, mice,

and reptiles, and even birds, perished in consequence of this plague. It

was not till 1780 that they were at length annihilated by torrents of

rain, which accompanied a dreadful hurricane.[963]

_Devastations caused by locusts._--We may conclude by mentioning some

instances of the devastations of locusts in various countries. Among

other parts of Africa, Cyrenaica has been at different periods infested

by myriads of these creatures, which have consumed nearly every green

thing. The effect of the havoc committed by them may be estimated

by the famine they occasioned. St. Augustin mentions a plague of this

kind in Africa, which destroyed no less than 800,000 men in the kingdom

of Massinissa alone, and many more upon the territories bordering upon

the sea. It is also related, that in the year 591 an infinite army of

locusts migrated from Africa into Italy; and, after grievously ravaging

the country, were cast into the sea, when there arose a pestilence from

their stench, which carried off nearly a million of men and beasts.

In the Venetian territory, also, in 1748, more than thirty thousand

persons are said to have perished in a famine occasioned by this

scourge; and other instances are recorded of their devastations in

France, Spain, Italy, Germany, &c. In different parts of Russia also,

Hungary, and Poland, in Arabia and India, and other countries, their

visitations have been periodically experienced. Although they have a

preference for certain plants, yet, when these are consumed, they will

attack almost all the remainder. In the accounts of the invasion of

locusts, the statements which appear most marvellous relate to the

prodigious mass of matter which encumbers the sea wherever they are

blown into it, and the pestilence arising from its putrefaction. Their

dead bodies are said to have been, in some places, heaped one upon

another, to the depth of four feet, in Russia, Poland and Lithuania; and

when, in Southern Africa, they were driven into the sea, by a north-west

wind, they formed, says Barrow, along the shore, for fifty miles, a bank

three or four feet high.[964] But when we consider that forests are

stripped of their foliage, and the earth of its green garment for

thousands of square miles, it may well be supposed that the volume of

animal matter produced may equal that of great herds of quadrupeds and

flights of large birds suddenly precipitated into the sea.

The occurrence of such events, at certain intervals, in hot countries,

like the severe winters and damp summers returning after a series of

years in the temperate zone, may affect the proportional numbers of

almost all classes of animals and plants, and probably prove fatal to

the existence of many which would otherwise thrive there; while, on the

contrary, the same occurrences can scarcely fail to be favorable to

certain species which, if deprived of such aid, might not maintain their

ground.

Although it may usually be remarked that the extraordinary increase of

some one species is immediately followed and checked by the

multiplication of another, yet this does not always happen; partly

because many species feed in common on the same kinds of food, and

partly because many kinds of food are often consumed indifferently by

one and the same species. In the former case, where a variety of

different animals have precisely the same taste, as, for example, when

many insectivorous birds and reptiles devour alike some particular fly

or beetle, the unusual numbers of these insects may cause only a slight

and almost imperceptible augmentation of each of these species of bird

and reptile. In the other instances, where one animal preys on others

of almost every class, as for example, where our English buzzards devour

not only small quadrupeds, as rabbits and field-mice, but also birds,

frogs, lizards, and insects, the profusion of any one of these last may

cause all such general feeders to subsist more exclusively upon the

species thus in excess, by which means the balance may be restored.

_Agency of omnivorous animals._--The number of species which are nearly

omnivorous is considerable; and although every animal has, perhaps, a

predilection for some one description of food rather than another, yet

some are not even confined to one of the great kingdoms of the organic

world. Thus, when the raccoon of the West Indies can procure neither

fowls, fish, snails, nor insects, it will attack the sugar-canes, and

devour various kinds of grain. The civets, when animal food is scarce,

maintain themselves on fruits and roots.

Numerous birds, which feed indiscriminately on insects and plants, are

perhaps more instrumental than any other of the terrestrial tribes in

preserving a constant equilibrium between the relative numbers of

different classes of animals and vegetables. If the insects become very

numerous and devour the plants, these birds will immediately derive a

larger portion of their subsistence from insects, just as the Arabians,

Syrians, and Hottentots feed on locusts, when the locusts devour their

crops.

_Reciprocal influence of aquatic and terrestrial species._--The intimate

relation of the inhabitants of the water to those of the land, and the

influence exerted by each on the relative number of species, must not be

overlooked amongst the complicated causes which determine the existence

of animals and plants in certain regions. A large portion of the

amphibious quadrupeds and reptiles prey partly on aquatic plants and

animals, and in part on terrestrial; and a deficiency of one kind of

prey causes them to have immediate recourse to the other. The voracity

of certain insects, as the dragon-fly, for example, is confined to the

water during one stage of their transformations, and in their perfect

state to the air. Innumerable water-birds, both of rivers and seas,

derive in like manner their food indifferently from either element; so

that the abundance or scarcity of prey in one induces them either to

forsake or more constantly to haunt the other. Thus an intimate

connection between the state of the animate creation in a lake or river,

and in the adjoining dry land, is maintained; or between a continent,

with its lakes and rivers, and the ocean. It is well known that many

birds migrate, during stormy seasons, from the sea-shore into the

interior, in search of food; while others, on the contrary, urged by

like wants, forsake their inland haunts, and live on substances rejected

by the tide.

The migration of fish into rivers during the spawning season supplies

another link of the same kind. Suppose the salmon to be reduced in

numbers by some marine foes, as by seals and grampuses, the consequence

must often be, that in the course of a few years the otters at the

distance of several hundred miles inland will be lessened in number from

the scarcity of fish. On the other hand, if there be a dearth of food

for the young fry of the salmon in rivers and estuaries, so that few

return to the sea, the sand eels and other marine species, which are

usually kept down by the salmon, will swarm in greater profusion.

It is unnecessary to accumulate a greater number of illustrations in

order to prove that the stations of different plants and animals depend

on a great complication of circumstances,--on an immense variety of

relations in the state of the inanimate worlds. Every plant requires a

certain climate, soil, and other conditions, and often the aid of many

animals, in order to maintain its ground. Many animals feed on certain

plants, being often restricted to a small number, and sometimes to one

only; other members of the animal kingdom feed on plant-eating species,

and thus become dependent on the conditions of the _stations_ not only

of their prey, but of the plants consumed by them.

Having duly reflected on the nature and extent of these mutual relations

in the different parts of the organic and inorganic worlds, we may next

proceed to examine the results which may be anticipated from the

fluctuations now continually in progress in the state of the earth's

surface, and in the geographical distribution of its living productions.

CHAPTER XLI.

EXTINCTION OF SPECIES.--CHANGES IN THE STATIONS OF ANIMALS.

Extension of the range of one species alters the condition of many

others--The first appearance of a new species causes the chief

disturbance--Changes known to have resulted from the advance of

human population--Whether man increases the productive powers of the

earth--Indigenous quadrupeds and birds extirpated in Great

Britain--Extinction of the dodo--Rapid propagation of domestic

quadrupeds in America--Power of exterminating species no prerogative

of man--Concluding remarks.

We have seen that the stations of animals and plants depend not merely

on the influence of external agents in the inanimate world, and the

relations of that influence to the structure and habits of each species,

but also on the state of the contemporary living beings which inhabit

the same part of the globe. In other words, the possibility of the

existence of a certain species in a given place, or of its thriving more

or less therein, is determined not merely by temperature, humidity,

soil, elevation, and other circumstances of the like kind; but also by

the existence or non-existence, the abundance or scarcity, of a

particular assemblage of other plants and animals in the same region.

If it be shown that both these classes of circumstances, whether

relating to the animate or inanimate creation, are perpetually changing,

it will follow that species are subject to incessant vicissitudes; and

if the result of these mutations, in the course of ages, be so great as

materially to affect the general condition of _stations_, it will follow

that the successive destruction of species must now be part of the

regular and constant order of nature.

_Extension of the range of one species alters the condition of the

others._--It will be desirable, first, to consider the effects which

every extension of the numbers or geographical range of one species must

produce on the condition of others inhabiting the same regions. When the

necessary consequences of such extensions have been fully explained, the

reader will be prepared to appreciate the important influence which

slight modifications in the physical geography of the globe may exert on

the condition of organic beings.

In the first place, it is clear that when any region is stocked with as

great a variety of animals and plants as the productive powers of that

region will enable it to support, the addition of any new species, or

the _permanent_ numerical increase of one previously established, must

always be attended either by the local extermination or the numerical

decrease of some other species.

There may undoubtedly be considerable fluctuations from year to year,

and the equilibrium may be again restored without any permanent

alteration; for, in particular seasons, a greater supply of heat,

humidity, or other causes, may augment the total quantity of vegetable

produce, in which case all the animals subsisting on vegetable food, and

others which prey on them, may multiply without any one species giving

way: but whilst the aggregate quantity of vegetable produce remains

unaltered, the progressive increase of one animal or plant implies the

decline of another.

All agriculturists and gardeners are familiar with the fact that when

weeds intrude themselves into the space appropriated to cultivated

species, the latter are starved in their growth, or stifled. If we

abandon for a short time a field or garden, a host of indigenous plants,

The darnel, hemlock, and rank fumitory,

pour in and obtain the mastery, extirpating the exotics, or putting an

end to the monopoly of some native plants.

If we inclose a park, and stock it with as many deer as the herbage will

support, we cannot add sheep without lessening the number of the deer;

nor can other herbivorous species be subsequently introduced, unless the

individuals of each species in the park become fewer in proportion.

So, if there be an island where leopards are the only beasts of prey,

and the lion, tiger, and hyaena afterwards enter, the leopards, if they

stand their ground, will be reduced in number. If the locusts then

arrive and swarm greatly, they may deprive a large number of

plant-eating animals of their food, and thereby cause a famine, not only

among them, but among the beasts of prey: certain species perhaps, which

had the weakest footing in the island, may thus be annihilated.

We have seen how many distinct geographical provinces there are of

aquatic and terrestrial species, and how great are the powers of

migration conferred on different classes, whereby the inhabitants of one

region may be enabled from time to time to invade another, and do

actually so migrate and diffuse themselves over new countries. Now,

although our knowledge of the history of the animate creation dates from

so recent a period, that we can scarcely trace the advance or decline of

any animal or plant, except in those cases where the influence of man

has intervened; yet we can easily conceive what must happen when some

new colony of wild animals or plants enters a region for the first time,

and succeeds in establishing itself.

_Supposed effects of the first entrance of the polar bear into

Iceland._--Let us consider how great are the devastations committed at

certain periods by the Greenland bears, when they are drifted to the

shores of Iceland in considerable numbers on the ice. These periodical

invasions are formidable even to man; so that when the bears arrive, the

inhabitants collect together, and go in pursuit of them with

fire-arms--each native who slays one being rewarded by the King of

Denmark. The Danes of old, when they landed in their marauding

expeditions upon our coast, hardly excited more alarm, nor did our

islanders muster more promptly for the defence of their lives and

property against the common enemy, than the modern Icelanders against

these formidable brutes. It often happens, says Henderson, that the

natives are pursued by the bear when he has been long at sea, and when

his natural ferocity has been heightened by the keenness of hunger; if

unarmed, it is frequently by stratagem only that they make their

escape.[965]

Let us cast our thoughts back to the period when the first polar bears

reached Iceland, before it was colonized by the Norwegians in 874: we

may imagine the breaking up of an immense barrier of ice like that

which, in 1816 and the following year, disappeared from the east coast

of Greenland, which it had surrounded for four centuries. By the aid of

such means of transportation a great number of these quadrupeds might

effect a landing at the same time, and the havoc which they would make

among the species previously settled in the island would be terrific.

The deer, foxes, seals, and even birds, on which these animals sometimes

prey, would be soon thinned down.

But this would be a part only, and probably an insignificant portion, of

the aggregate amount of change brought about by the new invader. The

plants on which the deer fed, being less consumed in consequence of the

lessened numbers of that herbivorous species, would soon supply more

food to several insects, and probably to some terrestrial testacea, so

that the latter would gain ground. The increase of these would furnish

other insects and birds with food, so that the numbers of these last

would be augmented. The diminution of the seals would afford a respite

to some fish which they had persecuted; and these fish, in their turn,

would then multiply and press upon their peculiar prey. Many

water-fowls, the eggs and young of which are devoured by foxes, would

increase when the foxes were thinned down by the bears; and the fish on

which the water-fowls subsisted would then, in their turn, be less

numerous. Thus the numerical proportions of a great number of the

inhabitants, both of the land and sea, might be permanently altered by

the settling of one new species in the region; and the changes caused

indirectly would ramify through all classes of the living creation, and

be almost endless.

An actual illustration of what we have here only proposed

hypothetically, is in some degree afforded by the selection of small

islands by the eider duck for its residence during the season of

incubation, its nest being seldom if ever found on the shores of the

main land, or even of a large island. The Icelanders are so well aware

of this, that they have expended a great deal of labor in forming

artificial islands, by separating from the main land certain

promontories, joined to it by narrow isthmuses. This insular position is

necessary to guard against the destruction of the eggs and young birds,

by foxes, dogs, and other animals. One year, says Hooker, it happened

that, in the small island of Vidoe, adjoining the coast of Iceland, a

fox got over _upon the ice_, and caused great alarm, as an immense

number of ducks were then sitting on their eggs or young ones. It was

long before he was taken, which was at last, however, effected by

bringing another fox to the island, and fastening it by a string near

the haunt of the former, by which he was allured within shot of the

hunter.[966]

_The first appearance of a new species causes the chief

disturbance._--It is usually the first appearance of an animal or plant,

in a region to which it was previously a stranger, that gives rise to

the chief alteration; since, after a time, an equilibrium is again

established. But it must require ages before such a new adjustment of

the relative forces of so many conflicting agents can be definitely

settled. The causes in simultaneous action are so numerous, that they

admit of an almost infinite number of combinations; and it is necessary

that all these should have occurred once before the total amount of

change, capable of flowing from any new disturbing force, can be

estimated.

Thus, for example, suppose that once in two centuries a frost of unusual

intensity, or a volcanic eruption of great violence accompanied by

floods from the melting of glaciers, should occur in Iceland; or an

epidemic disease, fatal to the larger number of individuals of some one

species, and not affecting others,--these, and a variety of other

contingencies, all of which may occur at once, or at periods separated

by different intervals of time, ought to happen before it would be

possible for us to declare what ultimate alteration the presence of any

new comer, such as the bear before mentioned, might occasion in the

animal population of the isle.

Every new condition in the state of the organic or inorganic creation, a

new animal or plant, an additional snow-clad mountain, any permanent

change, however slight in comparison to the whole, gives rise to a new

order of things, and may make a material change in regard to some one or

more species. Yet a swarm of locusts, or a frost of extreme intensity,

or an epidemic disease, may pass away without any great apparent

derangement; no species may be lost, and all may soon recover their

former relative numbers, because the same scourges may have visited the

region again and again, at preceding periods. Every plant that was

incapable of resisting such a degree of cold, every animal which was

exposed to be entirely cut off by an epidemic or by famine caused by the

consumption of vegetation by the locusts, may have perished already, so

that the subsequent recurrence of similar catastrophes is attended only

by a temporary change.

_Changes caused by Man_

We are best acquainted with the mutations brought about by the progress

of human population, and the growth of plants and animals favored by

man. To these, therefore, we should in the first instance turn our

attention. If we conclude, from the concurrent testimony of history and

of the evidence yielded by geological data, that man is, comparatively

speaking, of very modern origin, we must at once perceive how great a

revolution in the state of the animate world the increase of the human

race, considered merely as consumers of a certain quantity of organic

matter, must necessarily cause.

_Whether man increases the productive powers of the earth._--It may

perhaps, be said, that man has, in some degree, compensated for the

appropriation to himself of so much food, by artificially improving the

natural productiveness of soils, by irrigation, manure, and a judicious

intermixture of mineral ingredients conveyed from different localities.

But it admits of reasonable doubt whether, upon the whole, we fertilize

or impoverish the lands which we occupy. This assertion may seem

startling to many; because they are so much in the habit of regarding

the sterility or productiveness of land in relation to the wants of man,

and not as regards the organic world generally. It is difficult, at

first, to conceive, if a morass is converted into arable land, and made

to yield a crop of grain, even of moderate abundance, that we have not

improved the capabilities of the habitable surface--that we have not

empowered it to support a larger quantity of organic life. In such

cases, however, a tract, before of no utility to man, may be reclaimed,

and become of high agricultural importance, though it may, nevertheless,

yield a scantier vegetation. If a lake be drained, and turned into a

meadow, the space will provide sustenance to man, and many terrestrial

animals serviceable to him, but not, perhaps, so much food as it

previously yielded to the aquatic races.

If the pestiferous Pontine marshes were drained, and covered with corn,

like the plains of the Po, they might, perhaps, feed a smaller number of

animals than they do now; for these morasses are filled with herds of

buffaloes and swine, and they swarm with birds, reptiles, and insects.

The felling of dense and lofty forests, which covered, even within the

records of history, a considerable space on the globe, now tenanted by

civilized man, must generally have lessened the amount of vegetable food

throughout the space where these woods grew. We must also take into our

account the area covered by towns, and a still larger surface occupied

by roads.

If we force the soil to bear extraordinary crops one year, we are,

perhaps, compelled to let it lie fallow the next. But nothing so much

counterbalances the fertilizing effects of human art as the extensive

cultivation of foreign herbs and shrubs, which, although they are often

more nutritious to man, seldom thrive with the same rank luxuriance as

the native plants of a district. Man is, in truth, continually striving

to diminish the natural diversity of the _stations_ of animals and

plants in every country, and to reduce them all to a small number fitted

for species of economical use. He may succeed perfectly in attaining his

object, even though the vegetation be comparatively meagre, and the

total amount of animal life be greatly lessened.

Spix and Martius have given a lively description of the incredible

number of insects which lay waste the crops in Brazil, besides swarms of

monkeys, flocks of parrots, and other birds, as well as the paca,

agouti, and wild swine. They describe the torment which the planter and

the naturalist suffer from the musquitoes, and the devastation of the

ants and blattae; they speak of the dangers to which they were exposed

from the jaguar, the poisonous serpents, crocodiles, scorpions,

centipedes, and spiders. But with the increasing population and

cultivation of the country, say these naturalists, these evils will

gradually diminish; when the inhabitants have cut down the woods,

drained the marshes, made roads in all directions, and founded villages

and towns, man will, by degrees, triumph over the rank vegetation and

the noxious animals, and all the elements will second and amply

recompense his activity.[967]

The number of human beings now peopling the earth is supposed to amount

to eight hundred millions, so that we may easily understand how great a

number of beasts of prey, birds, and animals of every class, this

prodigious population must have displaced, independently of the still

more important consequences which have followed from the derangement

brought about by man in the relative numerical strength of particular

species.

_Indigenous quadrupeds and birds extirpated in Great Britain._--Let us

make some inquiries into the extent of the influence which the progress

of society has exerted during the last seven or eight centuries, in

altering the distribution of indigenous British animals. Dr. Fleming has

prosecuted this inquiry with his usual zeal and ability; and in a memoir

on the subject has enumerated the best-authenticated examples of the

decrease or extirpation of certain species during a period when our

population has made the most rapid advances. I shall offer a brief

outline of his results.[968]

The stag, as well as the fallow deer and the roe, were formerly so

abundant in our island, that, according to Lesley, from five hundred to

a thousand were sometimes slain at a hunting match; but the native races

would already have been extinguished, had they not been carefully

preserved in certain forests. The otter, the marten, and the polecat,

were also in sufficient numbers to be pursued for the sake of their fur;

but they have now been reduced within very narrow bounds. The wild cat

and fox have also been sacrificed throughout the greater part of the

country, for the security of the poultry-yard or the fold. Badgers have

been expelled from nearly every district, which at former periods they

inhabited.

Besides these, which have been driven out from their favorite haunts,

and everywhere reduced in number, there are some which have been wholly

extirpated; such as the ancient breed of indigenous horses, and the wild

boar; of the wild oxen a few remains are still preserved in some of the

old English parks. The beaver, which is eagerly sought after for its

fur, had become scarce at the close of the ninth century; and, by the

twelfth century, was only to be met with, according to Giraldus de

Barri, in one river in Wales, and another in Scotland. The wolf, once so

much dreaded by our ancestors, is said to have maintained its ground in

Ireland so late as the beginning of the eighteenth century (1710),

though it had been extirpated in Scotland thirty years before, and in

England at a much earlier period. The bear, which, in Wales, was

regarded as a beast of the chase equal to the hare or the boar[969],

only perished, as a native of Scotland, in the year 1057.[970]

Many native birds of prey have also been the subjects of unremitting

persecution. The eagles, larger hawks, and ravens, have disappeared from

the more cultivated districts. The haunts of the mallard, the snipe, the

redshank, and the bittern, have been drained equally with the summer

dwellings of the lapwing and the curlew. But these species still linger

in some portion of the British isles; whereas the larger capercailzies

or wood grouse, formerly natives of the pine-forests of Ireland and

Scotland, have been destroyed within the last sixty years. The egret and

the crane, which appear to have been formerly very common in Scotland,

are now only occasional visitants.[971]

The bustard (_Otis tarda_), observes Graves, in his British

Ornithology[972], "was formerly seen in the downs and heaths of various

parts of our island, in flocks of forty or fifty birds; whereas it is

now a circumstance of rare occurrence to meet with a single individual."

Bewick also remarks, "that they were formerly more common in this island

than at present; they are now found only in the open counties of the

south and east--in the plains of Wiltshire, Dorsetshire, and some parts

of Yorkshire."[973] In the few years that have elapsed since Bewick

wrote, this bird has entirely disappeared from Wiltshire and

Dorsetshire.

These changes, it may be observed, are derived from very imperfect

memorials, and relate only to the larger and more conspicuous animals

inhabiting a small spot on the globe; but they cannot fail to exalt our

conception of the enormous revolutions which, in the course of several

thousand years, the whole human species must have effected.

_Extinction of the dodo._--The kangaroo and the emu are retreating

rapidly before the progress of colonization in Australia; and it

scarcely admits of doubt, that the general cultivation of that country

must lead to the extirpation of both. The most striking example of the

loss, even within the last two centuries, of a remarkable species, is

that of the dodo--a bird first seen by the Dutch, when they landed on

the Isle of France, at that time uninhabited, immediately after the

discovery of the passage to the East Indies by the Cape of Good Hope. It

was of a large size, and singular form; its wings short, like those of

an ostrich, and wholly incapable of sustaining its heavy body, even for

a short flight. In its general appearance it differed from the ostrich,

cassowary, or any known bird.[974]

Many naturalists gave figures of the dodo after the commencement of the

seventeenth century; and there is a painting of it in the British

Museum, which is said to have been taken from a living individual.

Beneath the painting is a leg, in a fine state of preservation, which

ornithologists are agreed cannot belong to any other known bird. In the

museum at Oxford, also, there is a foot and a head in an imperfect

state.

In spite of the most active search, during the last century, no

information respecting the dodo was obtained, and some authors have gone

so far as to pretend that it never existed; but a great mass of

satisfactory evidence in favor of its recent existence has now been

collected by Mr. Broderip,[975] and by Mr. Strickland and Dr. Melville.

Mr. Strickland, agreeing with Professor Reinhardt, of Copenhagen, in

referring the dodo to the Columbidae, calls it a "vulture-like

frugivorous pigeon." It appears, also, that another short-winged bird of

the same order, called "The Solitaire," inhabited the small island of

Rodrigues, 300 miles east of the Mauritius, and has been exterminated by

man, as have one or two different but allied birds of the Isle of

Bourbon.[976]

_Rapid propagation of domestic quadrupeds over the American

continent._--Next to the direct agency of man, his indirect influence in

multiplying the numbers of large herbivorous quadrupeds of domesticated

races may be regarded as one of the most obvious causes of the

extermination of species. On this, and on several other grounds, the

introduction of the horse, ox, and other mammalia, into America, and

their rapid propagation over that continent within the last three

centuries, is a fact of great importance in natural history. The

extraordinary herds of wild cattle and horses which overran the plains

of South America sprung from a very few pairs first carried over by the

Spaniards; and they prove that the wide geographical range of large

species in great continents does not necessarily imply that they have

existed there from remote periods.

Humboldt observes, in his Travels, on the authority of Azzara, that it

is believed there exist, in the Pampas of Buenos Ayres, twelve million

cows and three million horses, without comprising, in this enumeration,

the cattle that have no acknowledged proprietor. In the Llanos of

Caraccas, the rich hateros, or proprietors of pastoral farms, are

entirely ignorant of the number of cattle they possess. The young are

branded with a mark peculiar to each herd, and some of the most wealthy

owners mark as many as fourteen thousand a year.[977] In the northern

plains, from the Orinoco to the lake of Maraycabo, M. Depons reckoned

that 1,200,000 oxen, 180,000 horses, and 90,000 mules, wandered at

large.[978] In some parts of the valley of the Mississippi, especially

in the country of the Osage Indians, wild horses are immensely numerous.

The establishment of black cattle in America dates from Columbus's

second voyage to St. Domingo. They there multiplied rapidly; and that

island presently became a kind of nursery from which these animals were

successively transported to various parts of the continental coast, and

from thence into the interior. Notwithstanding these numerous

exportations, in twenty-seven years after the discovery of the island,

herds of four thousand head, as we learn from Oviedo, were not uncommon,

and there were even some that amounted to eight thousand. In 1587, the

number of hides exported from St. Domingo alone, according to Acosta's

report, was 35,444; and in the same year there were exported 64,350 from

the ports of New Spain. This was in the sixty-fifth year after the

taking of Mexico, previous to which event the Spaniards, who came into

that country, had not been able to engage in anything else than

war.[979] Every one is aware that these animals are now established

throughout the American continent from Canada to the Straits of

Magellan.

The ass has thriven very generally in the New World; and we learn from

Ulloa, that in Quito they ran wild, and multiplied in amazing numbers,

so as to become a nuisance. They grazed together in herds, and when

attacked defended themselves with their mouths. If a horse happened to

stray into the places where they fed, they all fell upon him, and did

not cease biting and kicking till they left him dead.[980]

The first hogs were carried to America by Columbus, and established in

the Island of St. Domingo the year following its discovery, in November,

1493. In succeeding years they were introduced into other places where

the Spaniards settled; and, in the space of half a century, they were

found established in the New World, from the latitude of 25 degrees

north, to the 40th degree of south latitude. Sheep, also, and goats have

multiplied enormously in the New World, as have also the cat and the

rat; which last, as before stated, has been imported unintentionally in

ships. The dogs introduced by man which have at different periods become

wild in America, hunted in packs, like the wolf and the jackall,

destroying not only hogs, but the calves and foals of the wild cattle

and horses.

Ulloa in his voyage, and Buffon on the authority of old writers, relate

a fact which illustrates very clearly the principle before explained, of

the check which the increase of one animal necessarily offers to that of

another. The Spaniards had introduced goats into the Island of Juan

Fernandez, where they became so prolific as to furnish the pirates who

infested those seas with provisions. In order to cut off this resource

from the buccaneers, a number of dogs were turned loose into the island;

and so numerous did they become in their turn, that they destroyed the

goats in every accessible part, after which the number of the wild dogs

again decreased.[981]

_Increase of rein-deer imported into Iceland._--As an example of the

rapidity with which a large tract may become peopled by the offspring of

a single pair of quadrupeds, it may be mentioned that in the year 1773

thirteen rein-deer were exported from Norway, only three of which

reached Iceland. These were turned loose into the mountains of

Guldbringe Syssel, where they multiplied so greatly, in the course of

forty years, that it was not uncommon to meet with herds, consisting of

from forty to one hundred, in various districts.

The rein-deer, observes a modern writer, is in Lapland a loser by his

connexion with man, but Iceland will be this creature's paradise. There

is, in the interior, a tract which Sir. G. Mackenzie computes at not

less than forty thousand square miles, without a single human

habitation, and almost entirely unknown to the natives themselves. There

are no wolves: the Icelanders will keep out the bears; and the

reindeer, being almost unmolested by man, will have no enemy whatever,

unless it has brought with it its own tormenting gad-fly.[982]

Besides the quadrupeds before enumerated, our domestic fowls have also

succeeded in the West Indies and America, where they have the common

fowl, the goose, the duck, the peacock, the pigeon, and the guinea-fowl.

As these were often taken suddenly from the temperate to very hot

regions, they were not reared at first without much difficulty: but

after a few generations, they became familiarized to the climate, which,

in many cases, approached much nearer than that of Europe to the

temperature of their original native countries.

The fact of so many millions of wild and tame individuals of our

domestic species, almost all of them the largest quadrupeds and birds,

having been propagated throughout the new continent within the short

period that has elapsed since the discovery of America, while no

appreciable improvement can have been made in the productive powers of

that vast continent, affords abundant evidence of the extraordinary

changes which accompany the diffusion and progressive advancement of the

human race over the globe. That it should have remained for us to

witness such mighty revolutions is a proof, even if there was no other

evidence, that the entrance of man into the planet is, comparatively

speaking, of extremely modern date, and that the effects of his agency

are only beginning to be felt.

_Population which the globe is capable of supporting._--A modern writer

has estimated, that there are in America upwards of four million square

miles of useful soil, each capable of supporting 200 persons; and nearly

six million, each mile capable of supporting 490 persons.[983] If this

conjecture be true, it will follow, as that author observes, that if the

natural resources of America were fully developed, it would afford

sustenance to five times as great a number of inhabitants as the entire

mass of human beings existing at present upon the globe. The new

continent, he thinks, though less than half the size of the old,

contains an equal quantity of useful soil, and much more than an equal

amount of productive power. Be this as it may, we may safely conclude

that the amount of human population now existing constitutes but a small

proportion of that which the globe is capable of supporting, or which it

is destined to sustain at no distant period, by the rapid progress of

society, especially in America, Australia, and certain parts of the old

continent.

_Power of exterminating species no prerogative of man._--But if we

reflect that many millions of square miles of the most fertile land,

occupied originally by a boundless variety of animal and vegetable

forms, have been already brought under the dominion of man, and

compelled, in a great measure, to yield nourishment to him, and to a

limited number of plants and animals which he has caused to increase, we

must at once be convinced, that the annihilation of a multitude of

species has already been effected, and will continue to go on hereafter,

in certain regions, in a still more rapid ratio, as the colonies of

highly civilized nations spread themselves over unoccupied lands.

Yet, if we wield the sword of extermination as we advance, we have no

reason to repine at the havoc committed, nor to fancy, with the Scottish

poet, that "we violate the social union of nature;" or complain, with

the melancholy Jacques, that we

Are mere usurpers, tyrants, and what's worse,

To fright the animals and to kill them up

In their assign'd and native dwelling-place.

We have only to reflect, that in thus obtaining possession of the earth

by conquest, and defending our acquisitions by force, we exercise no

exclusive prerogative. Every species which has spread itself from a

small point over a wide area must, in like manner, have marked its

progress by the diminution or the entire extirpation of some other, and

must maintain its ground by a successful struggle against the

encroachments of other plants and animals. That minute parasitic plant,

called "the rust" in wheat, has, like the Hessian fly, the locust, and

the aphis, caused famines ere now amongst the "lords of the creation."

The most insignificant and diminutive species, whether in the animal or

vegetable kingdom, have each slaughtered their thousands, as they

disseminated themselves over the globe, as well as the lion, when first

it spread itself over the tropical regions of Africa.

_Concluding remarks._--Although we have as yet considered one class only

of the causes (the organic) by which species may become exterminated,

yet it cannot but appear evident that the continued action of these

alone, throughout myriads of future ages, must work an entire change in

the state of the organic creation, not merely on the continents and

islands, where the power of man is chiefly exerted, but in the great

ocean, where his control is almost unknown. The mind is prepared by the

contemplation of such future revolutions to look for the signs of

others, of an analogous nature, in the monuments of the past. Instead of

being astonished at the proofs there manifested of endless mutations in

the animate world, they will appear to one who has thought profoundly on

the fluctuations now in progress, to afford evidence in favor of the

uniformity of the system, unless, indeed, we are precluded from speaking

of _uniformity_ when we characterize a principle of endless variation.

CHAPTER XLII.

EXTINCTION OF SPECIES.--INFLUENCE OF INORGANIC CAUSES.

Powers of diffusion indispensable, that each species may maintain

its ground--How changes in physical geography affect the

distribution of species--Rate of the change of species due to this

cause cannot be uniform--Every change in the physical geography of

large regions tends to the extinction of species--Effects of a

general alteration of climate on the migration of species--Gradual

refrigeration would cause species in the northern and southern

hemispheres to become distinct--Elevation of temperature the

reverse--Effects on the condition of species which must result from

inorganic changes inconsistent with the theory of transmutation.

_Powers of diffusion indispensable, that each species may maintain its

ground._--Having shown in the last chapter, how considerably the

numerical increase or the extension of the geographical range of any one

species must derange the numbers and distribution of others, let us now

direct our attention to the influence which the inorganic causes

described in the second book are continually exerting on the habitations

of species.

So great is the instability of the earth's surface, that if nature were

not continually engaged in the task of sowing seeds and colonizing

animals, the depopulation of a certain portion of the habitable sea and

land would in a few years be considerable. Whenever a river transports

sediment into a lake or sea, so as materially to diminish its depth, the

aquatic animals and plants which delight in deep water are expelled: the

tract, however, is not allowed to remain useless; but is soon peopled by

species which require more light and heat, and thrive where the water is

shallow. Every addition made to the land by the encroachment of the

delta of a river banishes many subaqueous species from their native

abodes; but the new-formed plain is not permitted to lie unoccupied,

being instantly covered with terrestrial vegetation. The ocean devours

continuous lines of sea-coasts, and precipitates forests or rich pasture

land into the waves: but this space is not lost to the animate creation;

for shells and sea-weeds soon adhere to the new-made cliffs, and

numerous fish people the channel which the current has scooped out for

itself. No sooner has a volcanic island been thrown up than some lichens

begin to grow upon it, and it is sometimes clothed with verdure while

smoke and ashes are still occasionally thrown from the crater. The

cocoa, pandanus, and mangrove take root upon the coral reef before it

has fairly risen above the waves. The burning stream of lava that

descends from Etna rolls through the stately forest, and converts to

ashes every tree and herb which stands in its way; but the black strip

of land thus desolated is covered again in the course of time, with

oaks, pines, and chestnuts, as luxuriant as those which the fiery

torrent swept away.

Every flood and landslip, every wave which a hurricane or earthquake

throws upon the shore, every shower of volcanic dust and ashes which

buries a country far and wide to the depth of many feet, every advance

of the sand-flood, every conversion of salt water into fresh when rivers

alter their main channel of discharge, every permanent variation in the

rise or fall of tides in an estuary--these and countless other causes

displace, in the course of a few centuries, certain plants and animals

from stations which they previously occupied. If, therefore, the Author

of nature had not been prodigal of those numerous contrivances, before

alluded to, for spreading all classes of organic beings over the

earth--if he had not ordained that the fluctuations of the animate and

inanimate creation should be in perfect harmony with each other, it is

evident that considerable spaces, now the most habitable on the globe,

would soon be as devoid of life as are the Alpine snows, or the dark

abysses of the ocean, or the moving sands of the Sahara.

The powers, then, of migration and diffusion conferred on animals and

plants are indispensable to enable them to maintain their ground, and

would be necessary, even though it were never intended that a species

should gradually extend its geographical range. But a facility of

shifting their quarters being once given, it cannot fail to happen that

the inhabitants of one province should occasionally penetrate into some

other; since the strongest of those barriers which I before described as

separating distinct regions are all liable to be thrown down, one after

the other, during the vicissitudes of the earth's surface.

_How changes in physical Geography affect the distribution of

species._--The numbers and distribution of particular species are

affected in two ways, by changes in the physical geography of the

earth:--First, these changes promote or retard the migrations of

species; secondly, they alter the physical conditions of the localities

which species inhabit. If the ocean should gradually wear its way

through an isthmus, like that of Suez, it would open a passage for the

intermixture of the aquatic tribes of two seas previously disjoined, and

would, at the same time, close a free communication which the

terrestrial plants and animals of two continents had before enjoyed.

These would be, perhaps, the most important consequences, in regard to

the distribution of species, which would result from the breach made by

the sea in such a spot; but there would be others of a distinct nature,

such as the conversion of a certain tract of land, which formed the

isthmus, into sea. This space, previously occupied by terrestrial plants

and animals, would be immediately delivered over to the aquatic; a local

revolution which might have happened in innumerable other parts of the

globe, without being attended by any alteration in the blending together

of species of two distinct provinces.

_Rate of change of species cannot be uniform._--This observation leads

me to point out one of the most interesting conclusions to which we are

led by the contemplation of the vicissitudes of the inanimate world in

relation to those of the animate. It is clear that, if the agency of

inorganic causes be uniform, as I have supposed, they must operate very

irregularly on the state of organic beings, so that the rate according

to which these will change in particular regions will not be equal in

equal periods of time.

I am not about to advocate the doctrine of general catastrophes

recurring at certain intervals, as in the ancient Oriental cosmogonies,

nor do I doubt that, if very considerable periods of equal duration

could be compared one with another, the rate of change in the living, as

well as in the inorganic world, might be nearly uniform; but if we

regard each of the causes separately, which we know to be at present the

most instrumental in remodelling the state of the surface, we shall find

that we must expect each to be in action for thousands of years, without

producing any extensive alterations in the habitable surface, and then

to give rise, during a very brief period, to important revolutions.

_Illustration derived from subsidences._--I shall illustrate this

principle by a few of the most remarkable examples which present

themselves. In the course of the last century, as we have seen, a

considerable number of instances are recorded of the solid surface,

whether covered by water or not, having been permanently sunk or

upraised by subterranean movements. Most of these convulsions are only

accompanied by temporary fluctuations in the state of limited districts,

and a continued repetition of these events for thousands of years might

not produce any decided change in the state of many of those great

zoological or botanical provinces of which I have sketched the

boundaries.

When, for example, large parts of the ocean and even of inland seas are

a thousand fathoms or upwards in depth, it is a matter of no moment to

the animate creation that vast tracts should be heaved up many fathoms

at certain intervals, or should subside to the same amount. Neither can

any material revolution be produced in South America either in the

terrestrial or the marine plants or animals by a series of shocks on the

coast of Chili, each of which, like that of Penco, in 1751, should

uplift the coast about twenty-five feet. Nor if the ground sinks fifty

feet at a time, as in the harbor of Port Royal, in Jamaica, in 1692,

will such alterations of level work any general fluctuations in the

state of organic beings inhabiting the West Indian Islands, or the

Caribbean Sea.

It is only when the subterranean powers, by shifting gradually the

points where their principal force is developed, happen to strike upon

some particular region where a slight change of level immediately

affects the distribution of land and water, or the state of the climate,

or the barriers between distinct groups of species over extensive areas,

that the rate of fluctuation becomes accelerated, and may, in the course

of a few years or centuries, work mightier changes than had been

experienced in myriads of antecedent years.

Thus, for example, a repetition of subsidences causing the narrow

isthmus of Panama to sink down a few hundred feet, would, in a few

centuries, bring about a great revolution in the state of the animate

creation in the western hemisphere. Thousands of aquatic species would

pass, for the first time, from the Caribbean Sea into the Pacific; and

thousands of others, before peculiar to the Pacific Ocean, would make

their way into the Caribbean Sea, the Gulf of Mexico, and the Atlantic.

A considerable modification would probably be occasioned by the same

event in the direction or volume of the Gulf stream, and thereby the

temperature of the sea and the contiguous lands might be altered as far

as the influence of that current extends. A change of climate might thus

be produced in the ocean from Florida to Spitzbergen, and in many

countries of North America, Europe, and Greenland. Not merely the heat,

but the quantity of rain which falls, would be altered in certain

districts, so that many species would be excluded from tracts where they

before flourished: others would be reduced in number; and some would

thrive more and multiply. The seeds also and the fruits of plants would

no longer be drifted in precisely the same directions, nor the eggs of

aquatic animals; neither would species be any longer impeded in their

migrations towards particular stations before shut out from them by

their inability to cross the mighty current.

Let us take another example from a part of the globe which is at present

liable to suffer by earthquakes, namely, the low sandy tract which

intervenes between the sea of Azof and the Caspian. If there should

occur a sinking down to a trifling amount, and such ravines should be

formed as might be produced by a few earthquakes, not more considerable

than have fallen within our limited observation during the last 150

years, the waters of the Sea of Azof would pour rapidly into the

Caspian, which, according to the measurements lately made by the Academy

of St. Petersburg, is 84 feet below the level of the Black Sea.[984] The

Sea of Azof would immediately borrow from the Black Sea, that sea again

from the Mediterranean, and the Mediterranean from the Atlantic, so that

an inexhaustible current would pour down into the low tracts of Asia

bordering the Caspian, by which all the sandy salt steppes adjacent to

that sea would be inundated. An area of several thousand square leagues,

now below the level of the Mediterranean, would be converted from land

into sea.

_Illustration derived from the elevation of land._--Let us next imagine

a few cases of the elevation of land of small extent at certain critical

points, as, for example, in the shallowest part of the Straits of

Gibraltar, where the deepest soundings from the African to the European

side give only 220 fathoms. In proportion as this submarine barrier of

rock was upheaved, the whole channel would be contracted in width and

depth, and the volume of water which the current constantly flowing from

the Atlantic pours into the Mediterranean would be lessened. But the

loss of the inland sea by evaporation would remain the same; so that

being no longer able to draw on the ocean for a supply sufficient to

restore its equilibrium, it must sink, and leave dry a certain portion

of land around its borders. The current which now flows constantly out

of the Black Sea into the Mediterranean would then rush in more rapidly,

and the level of the Mediterranean would be thereby prevented from

falling so low; but the level of the Black Sea would, for the same

reason, sink; so that when, by a continued series of elevatory

movements, the Straits of Gibraltar had become completely closed up, we

might expect large and level sandy steppes to surround both the Black

Sea and Mediterranean, like those occurring at present on the skirts of

the Caspian and the Lake of Aral. The geographical range of hundreds of

aquatic species would be thereby circumscribed, and that of hundreds of

terrestrial plants and animals extended.

A line of submarine volcanos crossing the channel of some strait, and

gradually choking it up with ashes and lava, might produce a new barrier

as effectually as a series of earthquakes; especially if thermal

springs, charged with carbonate of lime, silica, and other mineral

ingredients, should promote the rapid multiplication of corals and

shells, and cement them together with solid matter precipitated during

the intervals between eruptions. Suppose in this manner a stoppage to be

caused of the Bahama channel between the bank of that name and the coast

of Florida. This insignificant revolution, confined to a mere spot in

the bottom of the ocean, would, by diverting the main current of the

Gulf stream, give rise to extensive changes in the climate and

distribution of animals and plants inhabiting the northern hemisphere.

_Illustration from the formation of new islands._--A repetition of

elevatory movements of earthquakes might continue over an area as

extensive as Europe, for thousands of ages, at the bottom of the ocean,

in certain regions, and produce no visible effects; whereas, if they

should operate in some shallow parts of the Pacific, amid the coral

archipelagos, they would soon give birth to a new continent. Hundreds of

volcanic islands may be thrown up, and become covered with vegetation,

without causing more than local fluctuations in the animate world; but

if a chain like the Aleutian archipelago, or the Kurile Isles, run for a

distance of many hundred miles, so as to form an almost uninterrupted

communication between two continents, or two distant islands, the

migrations of plants, birds, insects, and even of some quadrupeds, may

cause, in a short time, an extraordinary series of revolutions tending

to augment the range of some animals and plants, and to limit that of

others. A new archipelago might be formed in the Mediterranean, the Bay

of Biscay, and a thousand other places, and might produce less important

events than one rock which should rise up between Australia and Java, so

placed that winds and currents might cause an interchange of the plants,

insects, and birds.

_From the wearing through of an isthmus._--If we turn from the igneous

to the aqueous agents, we find the same tendency to an irregular rate of

change, naturally connected with the strictest uniformity in the energy

of those causes. When the sea, for example, gradually encroaches upon

both sides of a narrow isthmus, as that of Sleswick, separating the

North Sea from the Baltic, where, as before stated, the cliffs on both

the opposite coasts are wasting away[985], no material alteration

results for thousands of years, save only that there is a progressive

conversion of a small strip of land into water. A few feet only, or a

few yards, are annually removed; but if, at last, the partition should

be broken down, and the tides of the ocean should enter by a direct

passage into the inland sea, instead of going by a circuitous route

through the Cattegat, a body of salt water would sweep up as far as the

Gulfs of Bothnia and Finland, the waters of which are now brackish, or

almost fresh; and this revolution would be attended by the local

annihilation of many species.

Similar consequences must have resulted on a small scale, when the sea

opened its way through the Isthmus of Staveren in the thirteenth

century, forming a union between an inland lake and the ocean, and

opening, in the course of one century, a shallow strait, more than half

as wide as the narrowest part of that which divides England from France.

_Changes in physical geography which must occasion extinction of

species._--It will almost seem superfluous, after I have thus traced the

important modifications in the condition of living beings which flow

from changes of trifling extent, to argue that entire revolutions might

be brought about, if the climate and physical geography of the whole

globe were greatly altered. It has been stated, that species are in

general local, some being confined to extremely small spots, and

depending for their existence on a combination of causes, which, if they

are to be met with elsewhere, occur only in some very remote region.

Hence it must happen that, when the nature of these localities is

changed, the species will perish; for it will rarely happen that the

cause which alters the character of the district will afford new

facilities to the species to establish itself elsewhere.