CAUSES OF EARTHQUAKES AND VOLCANOES--_continued_.
Review of the proofs of internal heat--Theory of an unoxidated
metallic nucleus--Whether the decomposition of water may be a source
of volcanic heat--Geysers of Iceland--Causes of
earthquakes--Wavelike motion--Expansive power of liquid
gases--Connection between the state of the atmosphere and
earthquakes--Permanent upheaval and subsidence of land--Expansion of
rocks by heat--The balance of dry land how preserved--Subsidence in
excess--Conclusion.
When we reflect that the largest mountains are but insignificant
protuberances upon the surface of the earth, and that these mountains
are nevertheless composed of different parts which have been formed in
succession, we may well feel surprise that the central fluidity of the
planet should have been called in to account for volcanic phenomena. To
suppose the entire globe to be in a state of igneous fusion, with the
exception of a solid shell, not more than from thirty to one hundred
miles thick, and to imagine that the central heat of this fluid spheroid
exceeds by more than two hundred times that of liquid lava, is to
introduce a force altogether disproportionate to the effects which it is
required to explain.
The ordinary repose of the surface implies, on the contrary, an
inertness in the internal mass which is truly wonderful. When we
consider the combustible nature of the elements of the earth, so far as
they are known to us,--the facility with which their compounds may be
decomposed and made to enter into new combinations,--the quantity of
heat which they evolve during these processes; when we recollect the
expansive power of steam, and that water itself is composed of two gases
which, by their union, produce intense heat; when we call to mind the
number of explosive and detonating compounds which have been already
discovered, we may be allowed to share the astonishment of Pliny, that a
single day should pass without a general conflagration:--"Excedit
profecto omnia miracula, ullum diem fuisse quo non cuncta
conflagrarent."[759]
The signs of internal heat observable on the surface of the earth do not
necessarily indicate the permanent existence of subterranean heated
masses, whether fluid or solid, by any means so vast as our continents
and seas; yet how insignificant would these appear if distributed
through an external shell of the globe one or two hundred miles in
depth! The principal facts in proof of the accumulation of heat below
the surface may be summed up in a few words. Several volcanoes are
constantly in eruption, as Stromboli and Nicaragua; others are known to
have been active for periods of 60, or even 150 years, as those of
Sangay in Quito, Popocatepetl in Mexico, and the volcano of the Isle of
Bourbon. Many craters emit hot vapors in the intervals between
eruptions, and solfataras evolve incessantly the same gases as
volcanoes. Steam of high temperature has continued for more than twenty
centuries to issue from the "stufas," as the Italians call them; thermal
springs abound not only in regions of earthquakes, but are found in
almost all countries, however distant from active vents; and, lastly,
the temperature in the mines of various parts of the world is found to
increase in proportion as we descend.
The diagram (fig. 93) in the next page, may convey some idea of the
proportion which our continents and the ocean bear to the radius of the
earth.[760] If all the land were about as high as the Himalaya
mountains, and the ocean everywhere as deep as the Pacific, the whole of
both might be contained within a space expressed by the thickness of the
line _a b_; and masses of nearly equal volume might be placed in the
space marked by the line _c d_, in the interior. Seas of lava,
therefore, of the size of the Mediterranean, or even of the Atlantic,
would be as nothing if distributed through such an outer shell of the
globe as is represented by the shaded portion of the figure _a b c d_.
If throughout that space we imagine electro-chemical causes to be
continually in operation, even of very feeble power, they might give
rise to heat which, if accumulated at certain points, might melt or
render red-hot entire mountains, or sustain the temperature of stufas
and hot springs for ages.
_Theory of an unoxidated metallic nucleus._--When Sir H. Davy first
discovered the metallic basis of the earths and alkalies, he threw out
the idea that those metals might abound in an unoxidized state in the
subterranean regions to which water must occasionally penetrate.
Whenever this happened, gaseous matter would be set free, the metals
would combine with the oxygen of the water, and sufficient heat might be
evolved to melt the surrounding rocks. This hypothesis, although
afterwards abandoned by its author, was at first very favorably received
both by the chemist and the geologist: for silica, alumina, lime, soda,
and oxide of iron,--substances of which lavas are principally
composed,--would all result from the contact of the inflammable metals
alluded to with water. But whence this abundant store of unsaturated
metals in the interior? It was assumed that, in the beginning of things,
the nucleus of the earth was mainly composed of inflammable metals, and
that oxidation went on with intense energy at first; till at length,
when a superficial crust of oxides had been formed, the chemical action
became more and more languid.
[Illustration: Fig. 93.
Centre of the earth.]
This speculation, like all others respecting the primitive state of the
earth's nucleus, rests unavoidably on arbitrary assumptions. But we may
fairly inquire whether any existing causes may have the power of
deoxidating the earthy and alkaline compounds formed from time to time
by the action of water upon the metallic bases. If so, and if the
original crust or nucleus of the planet contained distributed through it
here and there some partial stores of potassium, sodium, and other
metallic bases, these might be oxidated and again deoxidated, so as to
sustain for ages a permanent chemical action. Yet even then we should be
unable to explain why such a continuous circle of operations, after
having been kept up for thousands of years in one district, should
entirely cease, and why another region, which had enjoyed a respite from
volcanic action for one or many geological periods, should become a
theatre for the development of subterranean heat.
It is well known to chemists, that the metallization of oxides, the most
difficult to reduce, may be effected by hydrogen brought into contact
with them at a red heat; and it is more than probable that the
production of potassium itself, in the common gun-barrel process, is due
to the power of nascent hydrogen derived from the water which the
hydrated oxide contains. According to the recent experiments, also, of
Faraday, it would appear that every case of metallic reduction by
voltaic agency, from saline solutions, in which water is present, is due
to the secondary action of hydrogen upon the oxide; both of these being
determined to the negative pole and then reacting upon one another.
It is admitted that intense heat would be produced by the occasional
contact of water with the metallic bases; and it is certain that, during
the process of saturation, vast volumes of hydrogen must be evolved. The
hydrogen, thus generated, might permeate the crust of the earth in
different directions, and become stored up for ages in fissures and
caverns, sometimes in a liquid form, under the necessary pressure.
Whenever, at any subsequent period, in consequence of the changes
effected by earthquakes in the shell of the earth, this gas happened to
come in contact with metallic oxides at a high temperature, the
reduction of these oxides might be the result.
No theory seems at first more startling than that which represents water
as affording an inexhaustible supply of fuel to the volcanic fires; yet
is it by no means visionary. It is a fact that must not be overlooked,
that while a great number of volcanoes are entirely submarine, the
remainder occur for the most part in islands or maritime tracts. There
are a few exceptions; but some of these, observes Dr. Daubeny, are near
inland salt lakes, as in Central Tartary; while others form part of a
train of volcanoes, the extremities of which are near the sea.
Sir H. Davy suggested that, when the sea is distant, as in the case of
some of the South American volcanoes, they may still be supplied with
water from subterranean lakes; since, according to Humboldt, large
quantities of fish are often thrown out during eruptions.[761] Mr. Dana
also, in his valuable and original observations on the volcanoes of the
Sandwich Islands, reminds us of the prodigious volume of atmospheric
water which must be absorbed into the interior of such large and lofty
domes, composed as they are entirely of porous lava. To this source
alone he refers the production of the steam by which the melted matter
is propelled upwards, even to the summit of cones three miles in
height.[762]
When treating of springs and overflowing wells, I have stated that
porous rocks are percolated by fresh water to great depths, and that
sea-water probably penetrates in the same manner through the rocks which
form the bed of the ocean. But, besides this universal circulation in
regions not far from the surface, it must be supposed that, wherever
earthquakes prevail, much larger bodies of water will be forced by the
pressure of the ocean into fissures at great depths, or swallowed up in
chasms; in the same manner as on the land, towns, houses, cattle, and
trees are sometimes engulfed. It will be remembered, that these chasms
often close again after houses have fallen into them; and for the same
reason, when water has penetrated to a mass of melted lava, the steam
into which it is converted may often rush out at a different aperture
from that by which the water entered.
The gases, it is said, exhaled from volcanoes, together with steam, are
such as would result from the decomposition of salt water, and the fumes
which escape from the Vesuvian lava have been observed to deposit common
salt.[763] The emission of free muriatic acid gas in great quantities is
also thought by many to favor the theory of the decomposition of the
salt contained in sea-water. It has been objected, however, that M.
Boussingault did not meet with this gas in his examination of the
elastic fluids evolved from the volcanoes of equatorial America; which
only give out aqueous vapor (in very large quantity), carbonic acid gas,
sulphurous acid gas, and sometimes fumes of sulphur.[764] In reply, Dr.
Daubeny has remarked, that muriatic acid may have ceased to be
disengaged, because the volcanic action has become languid in equatorial
America, and sea-water may no longer obtain admission.
M. Gay Lussac, while he avows his opinion that the decomposition of
water contributes largely to volcanic action, called attention,
nevertheless, to the supposed fact, that hydrogen had not been detected
in a separate form among the gaseous products of volcanoes; nor can it,
he says, be present; for, in that case, it would be inflamed in the air
by the red-hot stones thrown out during an eruption. Dr. Davy, in his
account of Graham Island, says, "I watched when the lightning was most
vivid, and the eruption of the greatest degree of violence, to see if
there was any inflammation occasioned by this natural electric
spark--any indication of the presence of inflammable gas; but in
vain."[765]
May not the hydrogen, Gay Lussac inquires, be combined with chlorine,
and produce muriatic acid? for this gas has been observed to be evolved
from Vesuvius--and the chlorine may have been derived from sea salt;
which was, in fact, extracted by simple washing from the Vesuvian lava
of 1822, in the proportion of nine per cent.[766] But it was answered,
that Sir H. Davy's experiments had shown, that hydrogen is not
combustible when mixed with muriatic acid gas; so that if muriatic gas
was evolved in large quantities, the hydrogen might be present without
inflammation.[767] M. Abich, on the other hand, assures us, "that
although it be true that vapor illuminated by incandescent lava has
often been mistaken for flame," yet he clearly detected in the eruption
of Vesuvius in 1834 the flame of hydrogen.[768]
M. Gay Lussac, in the memoir just alluded to, expressed doubt as to the
presence of sulphurous acid; but the abundant disengagement of this gas
during eruptions has been since ascertained: and thus all difficulty in
regard to the general absence of hydrogen in an inflammable state is
removed; for, as Dr. Daubeny suggests, the hydrogen of decomposed water
may unite with sulphur to form sulphuretted hydrogen gas, and this gas
will then be mingled with the sulphurous acid as it rises to the crater.
It is shown by experiment, that these gases mutually decompose each
other when mixed where steam is present; the hydrogen of the one
immediately uniting with the oxygen of the other to form water, while
the excess of sulphurous acid alone escapes into the atmosphere. Sulphur
is at the same time precipitated.
This explanation is sufficient; but it may also be observed that the
flame of hydrogen would rarely be visible during an eruption; as that
gas, when inflamed in a pure state, burns with a very faint blue flame,
which even in the night could hardly be perceptible by the side of
red-hot and incandescent cinders. Its immediate, conversion into water
when inflamed in the atmosphere, might also account for its not
appearing in a separate form.
Dr. Daubeny is of opinion that water containing atmospheric air may
descend from the surface of the earth to the volcanic foci, and that the
same process of combustion by which water is decomposed may deprive such
subterranean air of its oxygen. In this manner he explains the great
quantities of nitrogen evolved from volcanic vents and thermal waters,
and the fact that air disengaged from the earth in volcanic regions is
either wholly or in part deprived of its oxygen.
Sir H. Davy, in his memoir on the "Phenomena of Volcanoes," remarks,
that there was every reason to suppose in Vesuvius the existence of a
descending current of air; and he imagined that subterranean cavities
which threw out large volumes of steam during the eruption, might
afterwards, in the quiet state of the volcano, become filled with
atmospheric air.[769] The presence of ammoniacal salts in volcanic
emanations, and of ammonia (which is in part composed of nitrogen) in
lava, favors greatly the notion of air as well as water being deoxidated
in the interior of the earth.[770]
It has been alleged by Professor Bischoff that the slight specific
gravity of the metals of the alkalies is fatal to Davy's hypothesis, for
if the mean density of the earth, as determined by astronomers, surpass
that of all kinds of rocks, these metals cannot exist, at least not in
great quantities in the interior of the earth.[771] But Dr. Daubeny has
shown, that if we take the united specific gravity of potassium, sodium,
silicon, iron, and all the materials which, when united with oxygen,
constitute ordinary lava, and then compare their weight with lava of
equal bulk, the difference is not very material, the specific gravity of
the lava only exceeding by about one-fourth that of the unoxidized
metals. Besides, at great depths, the metallic bases of the earths and
alkalies may very probably be rendered heavier by pressure.[772] Nor is
it fair to embarrass the chemical theory of volcanoes with a doctrine so
purely gratuitous, as that which supposes the entire nucleus of the
planet to have been at first composed of unoxidated metals.
Professor Bunsen of Marburg, after analyzing the gases which escape from
the volcanic fumeroles and solfataras of Iceland, and after calculating
the quantity of hydrogen evolved between two eruptions, affirms, in
contradiction of opinions previously entertained, that the hydrogen
bears a perfect relation in quantity to the magnitude of the streams of
lava, assuming the fusion of these last to have been the result of the
heat evolved during the oxidation of alkaline and earthy metals, and
this to have been brought about by the decomposition of water. Yet after
having thus succeeded in removing the principal objection once so
triumphantly urged against Davy's hypothesis, Bunsen concludes by
declaring that the hydrogen evolved in volcanic regions cannot have been
generated by the decomposition of water coming in contact with alkaline
and earthy metallic bases. For, says the Professor, this process
presupposes the prevalence of a temperature in which carbonic acid
cannot exist in contact with hydrogen without suffering a partial
reduction to carbonic oxide; "and not a trace of carbonic oxide is ever
found in volcanic exhalations."[773] At the same time it will be seen,
by consulting the able memoirs of the Marburg chemist, that he supposes
many energetic kinds of chemical action to be continually going on in
the interior of the earth, capable of causing the disengagement of
hydrogen; and there can be no doubt that this gas may be a source of
innumerable new changes, capable of producing the local development of
internal heat.
_Cause of volcanic eruptions._--The most probable causes of a volcanic
outburst at the surface have been in a great degree anticipated in the
preceding speculations on the liquefaction of rocks and the generation
of gases. When a minute hole is bored in a tube filled with gas
condensed into a liquid, the whole becomes instantly aeriform, or, as
some writers have expressed it, "flashes into vapor," and often bursts
the tube. Such an experiment may represent the mode in which gaseous
matter may rush through a rent in the rocks, and continue to escape for
days or weeks through a small orifice, with an explosive power
sufficient to reduce every substance which opposes its passage into
small fragments or even dust. Lava may be propelled upwards at the same
time, and ejected in the form of scoriae. In some places, where the fluid
lava lies at the bottom of a deep fissure, communicating on the one hand
with the surface, and on the other with a cavern in which a considerable
body of vapor has been formed, there may be an efflux of lava, followed
by the escape of gas. Eruptions often commence and close with the
discharge of vapor; and, when this is the case, the next outburst may be
expected to take place by the same vent, for the concluding evolution of
elastic fluids will keep open the duct, and leave it unobstructed.
The breaking out of lava from the side or base of a lofty cone, rather
than from the summit, may be attributed to the hydrostatic pressure to
which the flanks of the mountain are exposed, when the column of lava
has risen to a great height. Or if, before it has reached the top, there
should happen to be any stoppage in the main duct, the upward pressure
of the ascending column of gas and lava may burst a lateral opening.
In the case however of Mount Loa, in the Sandwich Islands, there appears
to be a singular want of connection or sympathy between the eruptions of
the central and the great lateral vent. The great volcanic cone alluded
to rises to the height of 13,760 feet above the level of the sea, having
a crater at its summit, from which powerful streams of lava have flowed
in recent times, and having another still larger crater, called Kilauea,
on its southeastern slope, about 4000 feet above the sea. This lateral
cavity resembles a huge quarry cut in the mountain's side, being about
1000 feet deep when in its ordinary state. It is seven miles and a half
in circuit, and scattered over its bottom, at different levels, are
lakes and pools of lava, always in a state of ebullition. The liquid in
one of these will sometimes sink 100 or 150 feet, while it is
overflowing in another at a higher elevation, there being, it should
seem, no communication between them. In like manner, lava overflows in
the summit crater of Mount Loa, nearly 14,000 feet high, while the great
lateral cauldron just alluded to (of Kilauea) continues as tranquil as
usual, affording no relief to any part of the gases or melted matter
which are forcing their way upwards in the centre of the mountain.
"How," asks Mr. Dana, "if there were any subterranean channel connecting
the two great vents, could this want of sympathy exist? How, according
to the laws of hydrostatic pressure, can a column of fluid stand 10,000
feet higher in one leg of the siphon than in the other?" The eruptions,
he observes, are not paroxysmal; on the contrary, the lava rises slowly
and gradually to the summit of the lofty cone, and then escapes there
without any commotion manifesting itself in Kilauea, a gulf always open
on the flanks of the same mountain. One conclusion, he says, is certain,
namely, _that volcanoes are no safety valves_ as they have been called;
for here two independent and apparently isolated centres of volcanic
activity, only sixteen miles distant from each other, are sustained in
one and the same cone.[774]
Without pretending to solve this enigma, I cannot refrain from
remarking, that the supposed independence of several orifices of
eruption in one crater like Kilauea, when adduced in confirmation of the
doctrine of two distinct sources of volcanic action underneath one
mountain, proves too much. No one can doubt, that the pools of lava in
Kilauea have been derived from some common reservoir, and have resulted
from a combination of causes commonly called volcanic, which are at work
in the interior at some unknown distance below. These causes have given
rise in Mount Loa to eruptions from many points, but principally from
one centre, so that a vast dome of ejected matter has been piled up. The
subsidiary crater has evidently never given much relief to the
imprisoned, heated, and liquefied matter, for Kilauea does not form a
lateral protuberance interfering with the general shape or uniform
outline of Mount Loa.
_Geysers of Iceland._--As aqueous vapor constitutes the most abundant of
the aeriform products of volcanoes in eruption, it may be well to
consider attentively a case in which steam is exclusively the moving
power--that of the Geysers of Iceland. These intermittent hot springs
occur in a district situated in the southwestern division of Iceland,
where nearly one hundred of them are said to break out within a circle
of two miles. That the water is of atmospheric origin, derived from rain
and melted snow, is proved, says Professor Bunsen, by the nitrogen which
rises from them either pure or mixed with other gases. The springs rise
through a thick current of lava, which may perhaps have flowed from
Mount Hecla, the summit of that volcano being seen from the spot at the
distance of more than thirty miles. In this district the rushing of
water is sometimes heard in chasms beneath the surface; for here, as on
Etna, rivers flow in subterranean channels through the porous and
cavernous lavas. It has more than once happened, after earthquakes, that
some of the boiling fountains have increased or diminished in violence
and volume, or entirely ceased, or that new ones have made their
appearance--changes which may be explained by the opening of new rents
and the closing of pre-existing fissures.
Few of the Geysers play longer than five or six minutes at a time,
although sometimes half an hour. The intervals between their eruptions
are for the most part very irregular. The Great Geyser rises out of a
spacious basin at the summit of a circular mound composed of siliceous
incrustations deposited from the spray of its waters. The diameter of
this basin, in one direction, is fifty-six feet, and forty-six in
another. (See fig. 94.) In the centre is a pipe seventy-eight feet in
perpendicular depth, and from eight to ten feet in diameter, but
gradually widening, as it rises into the basin. The inside of the basin
is whitish, consisting of a siliceous crust, and perfectly smooth, as
are likewise two small channels on the sides of the mound, down which
the water escapes when the bowl is filled to the margin. The circular
basin is sometimes empty, as represented in the following sketch; but is
usually filled with beautifully transparent water in a state of
ebullition. During the rise of the boiling water in the pipe, especially
when the ebullition is most violent, and when the water is thrown up in
jets, subterranean noises are heard, like the distant firing of cannon,
and the earth is slightly shaken. The sound then increases and the
motion becomes more violent, till at length a column of water is thrown
up, with loud explosions, to the height of one or two hundred feet.
After playing for a time like an artificial fountain, and giving off
great clouds of vapor, the pipe or tube is emptied; and a column of
steam, rushing up with amazing force and a thundering noise, terminates
the eruption.
[Illustration: Fig. 94.
View of the Crater of the Great Geyser in Iceland.[775]]
If stones are thrown into the crater, they are instantly ejected; and
such is the explosive force, that very hard rocks are sometimes shivered
by it into small pieces. Henderson found that by throwing a great
quantity of large stones into the pipe of Strockr, one of the Geysers,
he could bring on an eruption in a few minutes.[776] The fragments of
stone, as well as the boiling water, were thrown in that case to a much
greater height than usual. After the water had been ejected, a column of
steam continued to rush up with a deafening roar for nearly an hour; but
the Geyser, as if exhausted by this effort, did not send out a fresh
eruption when its usual interval of rest had elapsed. The account given
by Sir George Mackenzie of a Geyser which he saw in eruption in 1810
(see fig. 95), agrees perfectly with the above description by Henderson.
The steam and water rose for half an hour to the height of 70 feet, and
the white column remained perpendicular notwithstanding a brisk gale of
wind which was blowing against it. Stones thrown into the pipe were
projected to a greater height than the water. To leeward of the vapor a
heavy shower of rain was seen to fall.[777]
[Illustration: Fig. 95.
Eruption of the New Geyser in 1810. (Mackenzie.)]
Among the different theories proposed to account for these phenomena, I
shall first mention one suggested by Sir. J. Herschel. An imitation of
these jets, he says, may be produced on a small scale, by heating red
hot the stem of a tobacco pipe, filling the bowl with water, and so
inclining the pipe as to let the water run through the stem. Its escape,
instead of taking place in a continued stream, is then performed by a
succession of violent explosions, at first of steam alone, then of water
mixed with steam; and, as the pipe cools, almost wholly of water. At
every such paroxysmal escape of the water, a portion is driven back,
accompanied with steam, into the bowl. The intervals between the
explosions depend on the heat, length, and inclination of the pipe;
their continuance, on its thickness and conducting power.[778] The
application of this experiment to the Geysers merely requires that a
subterranean stream, flowing through the pores and crevices of lava,
should suddenly reach a fissure in which the rock is red hot or nearly
so. Steam would immediately be formed, which, rushing up the fissure,
might force up water along with it to the surface, while, at the same
time, part of the steam might drive back the water of the supply for a
certain distance towards its source. And when, after the space of some
minutes, the steam was all condensed, the water would return, and a
repetition of the phenomena take place.
[Illustration: Fig. 96.
Supposed reservoir and pipe of a Geyser in Iceland.[779]]
There is, however, another mode of explaining the action of the Geyser,
perhaps more probable than that above described. Suppose water
percolating from the surface of the earth to penetrate into the
subterranean cavity A D (fig. 96) by the fissures F F, while, at the
same time, steam at an extremely high temperature, such as is commonly
given out from the rents of lava currents during congelation, emanates
from the fissures C. A portion of the steam is at first condensed into
water, while the temperature of the water is raised by the latent heat
thus evolved, till, at last, the lower part of the cavity is filled with
boiling water and the upper with steam under high pressure. The
expansive force of the steam becomes, at length, so great, that the
water is forced up the fissure or pipe E B, and runs over the rim of the
basin. When the pressure is thus diminished, the steam in the upper part
of the cavity A expands, until all the water D is driven into the pipe;
and when this happens, the steam, being the lighter of the two fluids,
rushes up through the water with great velocity. If the pipe be choked
up artificially, even for a few minutes, a great increase of heat must
take place; for it is prevented from escaping in a latent form in steam;
so that the water is made to boil more violently, and this brings on an
eruption.
Professor Bunsen, before cited, adopts this theory to account for the
play of the "Little Geyser," but says it will not explain the phenomena
of the Great one. He considers this, like the others, to be a thermal
spring, having a narrow funnel-shaped tube in the upper part of its
course, where the walls of the channel have become coated over with
siliceous incrustations. At the mouth of this tube the water has a
temperature, corresponding to the pressure of the atmosphere, of about
212 degrees Fahr., but at a certain depth below it is much hotter. This
the professor succeeded in proving by experiment; a thermometer
suspended by a string in the pipe rising to 266 degrees Fahr., or no
less than 48 degrees above the boiling point. After the column of water
has been expelled, what remains in the basin and pipe is found to be
much cooled.
Previously to these experiments of Bunsen and Descloizeaux, made in
Iceland in 1846, it would scarcely have been supposed possible that the
lower part of a free and open column of water could be raised so much in
temperature without causing a circulation of ascending and descending
currents, followed by an almost immediate equalization of heat. Such
circulation is no doubt impeded greatly by the sides of the well not
being vertical, and by numerous contractions of its diameter, but the
phenomenon may be chiefly due to another cause. According to recent
experiments on the cohesion of liquids by Mr. Donny of Ghent, it appears
that when water is freed from all admixture of air, its temperature can
be raised, even under ordinary atmospheric pressure, to 275 degrees
Fahr., so much does the cohesion of its molecules increase[780] when
they are not separated by particles of air. As water long boiled becomes
more and more deprived of air, it is probably very free from such
intermixture at the bottom of the Geysers.
Among other results of the experiments of Bunsen and his companion, they
convinced themselves that the column of fluid filling the tube is
constantly receiving accessions of hot water from below, while it
becomes cooler above by evaporation on the broad surface of the basin.
They also came to a conclusion of no small interest, as bearing on the
probable mechanism of ordinary volcanic eruptions, namely that the tube
itself is the main seat or focus of mechanical force. This was proved by
letting down stones suspended by strings to various depths. Those which
were sunk to considerable distances from the surface were not cast up
again, whereas those nearer the mouth of the tube were ejected to great
heights. Other experiments also were made tending to demonstrate the
singular fact, that there is often scarce any motion below, when a
violent rush of steam and water is taking place above. It seems that
when a lofty column of water possesses a temperature increasing with the
depth, any slight ebullition or disturbance of equilibrium in the upper
portion may first force up water into the basin, and then cause it to
flow over the edge. A lower portion, thus suddenly relieved of part of
its pressure, expands and is converted into vapor more rapidly than the
first, owing to its greater heat. This allows the next subjacent
stratum, which is much hotter, to rise and flash into a gaseous form;
and this process goes on till the ebullition has descended from the
middle to near the bottom of the funnel.[781]
In speculating, therefore, on the mechanism of an ordinary volcanic
eruption, we may suppose that large subterranean cavities exist at the
depth of some miles below the surface of the earth, in which melted lava
accumulates; and when water containing the usual mixture of air
penetrates into these, the steam thus generated may press upon the lava
and force it up the duct of a volcano, in the same manner as a column of
water is driven up the pipe of a Geyser. In other cases we may suppose a
continuous column of liquid lava mixed with _red-hot water_ (for water
may exist in that state, as Professor Bunsen reminds us, under
pressure), and this column may have a temperature regularly increasing
downwards. A disturbance of equilibrium may first bring on an eruption
near the surface, by the expansion and conversion into gas of entangled
water and other constituents of what we call lava, so as to occasion a
diminution of pressure. More steam would then be liberated, carrying up
with it jets of melted rock, which being hurled up into the air may fall
in showers of ashes on the surrounding country, and at length, by the
arrival of lava and water more and more heated at the orifice of the
duct or the crater of the volcano, expansive power may be acquired
sufficient to expel a massive current of lava. After the eruption has
ceased, a period of tranquillity succeeds, during which fresh accessions
of heat are communicated from below, and additional masses of rock fused
by degrees, while at the same time atmospheric or sea water is
descending from the surface. At length the conditions required for a new
outburst are obtained, and another cycle of similar changes is renewed.
_Causes of earthquakes--wave-like motion._--I shall now proceed to
examine the manner in which the heat of the interior may give rise to
earthquakes. One of the most common phenomena attending subterranean
movements, is the undulatory motion of the ground. And this, says
Michell, will seem less extraordinary, if we call to mind the extreme
elasticity of the earth and the compressibility of even the most solid
materials. Large districts, he suggests, may rest on fluid lava; and,
when this is disturbed, its motions may be propagated through the
incumbent rocks. He also adds the following ingenious speculation:--"As
a small quantity of vapor almost instantly generated at some
considerable depth below the surface of the earth will produce a
vibratory motion, so a very large quantity (whether it be generated
almost instantly, or in any small portion of time) will produce a
wave-like motion. The manner in which this wave-like motion will be
propagated may, in some measure, be represented by the following
experiment:--Suppose a large cloth, or carpet (spread upon a floor), to
be raised at one edge, and then suddenly brought down again to the
floor; the air under it, being by this means propelled, will pass along
till it escapes at the opposite side, raising the cloth in a wave all
the way as it goes. In like manner, a large quantity of vapor may be
conceived to raise the earth in a wave, as it passes along between the
strata, which it may easily separate in a horizontal direction, there
being little or no cohesion between one stratum and another. The part of
the earth that is first raised being bent from its natural form, will
endeavor to restore itself by its elasticity; and the parts next to it
being to have their weight supported by the vapor, which will insinuate
itself under them, will be raised in their turn, till it either finds
some vent, or is again condensed by the cold into water, and by that
means prevented from proceeding any farther."[782] In a memoir published
in 1843, on the structure of the Appalachian chain, by the Professors
Rogers,[783] the following hypothesis is proposed as "simpler and more
in accordance with dynamical considerations, and the recorded
observations on earthquakes."--"In place," say they, "of supposing it
possible for a body of vapor or gaseous matter to pass horizontally
between the strata, or even between the crust and the fluid lava upon
which it floats, and with which it must be closely entangled, we are
inclined to attribute the movement to an _actual pulsation_, engendered
in the _molten matter itself_, by a linear disruption under enormous
tension, giving vent explosively to elastic vapors, escaping either to
the surface, or into cavernous spaces beneath. According to this
supposition, the movement of the subterranean vapors would be _towards_,
and not from, the disrupted belt, and the oscillation of the crust would
originate in the tremendous and sudden disturbance of the previous
pressure on the surface of the lava mass below, brought about by the
instantaneous and violent rending of the overlying strata."
This theory requires us to admit that the crust of the earth is so
flexible, that it can assume the form, and follow the motion of an
undulation in the fluid below. Even if we grant this, says Mr. Mallet,
another more serious objection presents itself, viz. the great velocity
attributed to the transit of the wave in the subterranean sea of lava.
We are called upon to admit that the speed of the wave below equals that
of the true earthquake shock at the surface, which is so immense, that
it is not inferior to the velocity of sound in the same solids. But the
undulation in the fluid below must follow the laws of a tidal wave, or
of the great sea-wave already spoken of. "Its velocity, like that of the
tidal wave of our seas, will be a function of its length and of the
depth of the fluid, diminished in this case by certain considerations as
to the density and degree of viscidity of the liquid; and although it
would be at present impossible, for want of data, to calculate the exact
velocity with which this subterraneous lava-wave could move, it may be
certainly affirmed that its velocity would be immeasurably short of the
observed or theoretic velocity of the great earth-wave, or true shock in
earthquakes."[784]
_Liquid gases._--The rending and upheaving of continental masses are
operations which are not difficult to explain, when we are once
convinced that heat, of sufficient power, not only to melt but to reduce
to a gaseous form a great variety of substances, is accumulated in
certain parts of the interior. We see that elastic fluids are capable of
projecting solid masses to immense heights in the air; and the volcano
of Cotopaxi has been known to throw out, to the distance of eight or
nine miles, a mass of rock about one hundred cubic yards in volume. When
we observe these aeriform fluids rushing out from particular vents for
months, or even years, continuously, what power may we not expect them
to exert in other places, where they happen to be confined under an
enormous weight of rock?
The experiments of Faraday and others have shown, within the last twelve
years, that many of the gases, including all those which are most
copiously disengaged from volcanic vents, as the carbonic, sulphurous,
and muriatic acids, may be condensed into liquids by pressure. At
temperatures of from 30 degrees to 50 degrees F., the pressure required
for this purpose varies from fifteen to fifty atmospheres; and this
amount of pressure we may regard as very insignificant in the operations
of nature. A column of Vesuvian lava that would reach from the lip of
the crater to the level of the sea, must be equal to about three hundred
atmospheres; so that, at depths which may be termed moderate in the
interior of the crust of the earth, the gases may be condensed into
liquids, even at very high temperatures. The method employed to reduce
some of these gases to a liquid state is, to confine the materials, from
the mutual action of which they are evolved, in tubes hermetically
sealed, so that the accumulated pressure of the vapor, as it rises and
expands, may force some part of it to assume the liquid state. A similar
process may, and indeed must, frequently take place in subterranean
caverns and fissures, or even in the pores and cells of many rocks; by
which means, a much greater store of expansive power may be _packed_
into a small space than could happen if these vapors had not the
property of becoming liquid. For, although the gas occupies much less
room in a liquid state, yet it exerts exactly the same pressure upon the
sides of the containing cavity as if it remained in the form of vapor.
If a tube, whether of glass or other materials, filled with condensed
gas, have its temperature slightly raised, it will often burst; for a
slight increment of heat causes the elasticity of the gas to increase in
a very high ratio. We have only to suppose certain rocks, permeated by
these liquid gases (as porous strata are sometimes filled with water),
to have their temperature raised some hundred degrees, and we obtain a
power capable of lifting superincumbent masses of almost any conceivable
thickness; while, if the depth at which the gas is confined be great,
there is no reason to suppose that any other appearances would be
witnessed by the inhabitants of the surface than vibratory movements and
rents, from which no vapor might escape. In making their way through
fissures a very few miles only in length, or in forcing a passage
through soft yielding strata, the vapors may be cooled and absorbed by
water. For water has a strong affinity to several of the gases, and will
absorb large quantities, with a very slight increase of volume. In this
manner, the heat or the volume of springs may be augmented, and their
mineral properties made to vary.
_Connection between the state of the atmosphere and earthquakes._--The
inhabitants of Stromboli, who are mostly fishermen, are said to make use
of that volcano as a weather-glass, the eruptions being comparatively
feeble when the sky is serene, but increasing in turbulence during
tempestuous weather, so that in winter the island often seems to shake
from its foundations. Mr. P. Scrope, after calling attention to these
and other analogous facts, first started the idea (as long ago as the
year 1825) that the diminished pressure of the atmosphere, the
concomitant of stormy weather, may modify the intensity of the volcanic
action. He suggests that where liquid lava communicates with the
surface, as in the crater of Stromboli, it may rise or fall in the vent
on the same principle as mercury in a barometer; because the ebullition
or expansive power of the steam contained in the lava would be checked
by every increase, and augmented by every diminution of weight. In like
manner, if a bed of liquid lava be confined at an immense depth below
the surface, its expansive force may be counteracted partly by the
weight of the incumbent rocks, and also in part by atmospheric pressure
acting contemporaneously on a vast superficial area. In that case, if
the upheaving force increase gradually in energy, it will at length be
restrained by only the slightest degree of superiority in the antagonist
or repressive power, and then the equilibrium may be suddenly destroyed
by any cause, such as an ascending draught of air, which is capable of
depressing the barometer. In this manner we may account for the
remarkable coincidence so frequently observed between the state of the
weather and subterranean commotions, although it must be admitted that
earthquakes and volcanic eruptions react in their turn upon the
atmosphere, so that disturbances of the latter are generally the
consequences rather than the forerunners of volcanic disturbances.[785]
From an elaborate catalogue of the earthquakes experienced in Europe and
Syria during the last fifteen centuries, M. Alexis Perrey has deduced
the conclusion that the number which happen in the winter season
preponderates over those which occur in any one of the other seasons of
the year, there being, however, some exceptions to this rule, as in the
Pyrenees. Curious and valuable as are these data, M. d'Archiac justly
remarks, in commenting upon them, that they are not as yet sufficiently
extensive or accordant in different regions, to entitle us to deduce any
general conclusions from them respecting the laws of subterranean
movements throughout the globe.[786]
_Permanent elevation and subsidence._--It is easy to conceive that the
shattered rocks may assume an arched form during a convulsion, so that
the country above may remain permanently upheaved. In other cases gas
may drive before it masses of liquid lava, which may thus be injected
into newly opened fissures. The gas having then obtained more room, by
the forcing up of the incumbent rocks, may remain at rest; while the
lava congealing in the rents may afford a solid foundation for the newly
raised district.
Experiments have recently been made in America, by Colonel Totten, to
ascertain the ratio according to which some of the stones commonly used
in architecture expand with given increments of heat.[787] It was found
impossible, in a country where the annual variation of temperature was
more than 90 degrees F., to make a coping of stones, five feet in
length, in which the joints should fit so tightly as not to admit water
between the stone and the cement; the annual contraction and expansion
of the stones causing, at the junctions, small crevices, the width of
which varied with the nature of the rock. It was ascertained that
fine-grained granite expanded with 1 degrees F. at the rate of
.000004825; while crystalline marble .000005668; and red sandstone
.000009532, or about twice as much as granite.
Now, according to this law of expansion, a mass of sandstone a mile in
thickness, which should have its temperature raised 200 degrees F.,
would lift a superimposed layer of rock to the height of ten feet above
its former level. But, suppose a part of the earth's crust, one hundred
miles in thickness and equally expansive, to have its temperature raised
600 degrees or 800 degrees, this might produce an elevation of between
two and three thousand feet. The cooling of the same mass might
afterwards cause the overlying rocks to sink down again and resume their
original position. By such agency we might explain the gradual rise of
Scandinavia or the subsidence of Greenland, if this last phenomenon
should also be established as a fact on farther inquiry.
It is also possible that as the clay in Wedgwood's pyrometer contracts,
by giving off its water, and then, by incipient vitrification; so, large
masses of argillaceous strata on the earth's interior may shrink, when
subjected to heat and chemical changes, and allow the incumbent rocks to
subside gradually.
Moreover, if we suppose that lava cooling slowly at great depths may be
converted into various granitic rocks, we obtain another source of
depression; for, according to the experiments of Deville and the
calculations of Bischoff, the contraction of granite when passing from a
melted or plastic to a solid and crystalline state must be more than ten
per cent.[788] The sudden subsidence of land may also be occasioned by
subterranean caverns giving way, when gases are condensed, or when they
escape through newly-formed crevices. The subtraction, moreover, of
matter from certain parts of the interior, by the flowing of lava and of
mineral springs, must, in the course of ages, cause vacuities below, so
that the undermined surface may at length fall in.
_The balance of dry land, how preserved._--In the present state of our
knowledge, we cannot pretend to estimate the average number of
earthquakes which may happen in the course of a single year. As the area
of the ocean is nearly three times that of the land, it is probable that
about three submarine earthquakes may occur for one exclusively
continental; and when we consider the great frequency of slight
movements in certain districts, we can hardly suppose that a day, if,
indeed, an hour, ever passes without one or more shocks being
experienced in some part of the globe. We have also seen that in Sweden,
and other countries, changes in the relative level of sea and land may
take place without commotion, and these perhaps produce the most
important geographical and geological changes; for the position of land
may be altered to a greater amount by an elevation or depression of one
inch over a vast area, than by the sinking of a more limited tract, such
as the forest of Aripao, to the depth of many fathoms at once.[789]
It must be evident, from the historical details above given, that the
force of subterranean movement, whether intermittent or continuous,
whether with or without disturbance, does not operate at random, but is
developed in certain regions only; and although the alterations produced
during the time required for the occurrence of a few volcanic eruptions
may be inconsiderable, we can hardly doubt that, during the ages
necessary for the formation of large volcanic cones, composed of
thousands of lava currents, shoals might be converted into lofty
mountains, and low lands into deep seas.
In a former chapter (p. 198), I have stated that aqueous and igneous
agents may be regarded as antagonist forces; the aqueous laboring
incessantly to reduce the inequalities of the earth's surface to a
level, while the igneous are equally active in renewing the unevenness
of the surface. By some geologists it has been thought that the
levelling power of running water was opposed rather to the _elevating_
force of earthquakes than to their action generally. This opinion is,
however, untenable; for the sinking down of the bed of the ocean is one
of the means by which the gradual submersion of land is prevented. The
depth of the sea cannot be increased at any one point without a
universal fall of the waters, nor can any partial deposition of sediment
occur without the displacement of a quantity of water of equal volume,
which will raise the sea, though in an imperceptible degree, even to the
antipodes. The preservation, therefore, of the dry land may sometimes be
effected by the subsidence of part of the earth's crust (that part,
namely, which is covered by the ocean), and in like manner an upheaving
movement must often tend to destroy land; for if it render the bed of
the sea more shallow, it will displace a certain quantity of water, and
thus tend to submerge low tracts.
Astronomers having proved (see above, p. 129) that there has been no
change in the diameter of the earth during the last two thousand years,
we may assume it as probable, that the dimensions of the planet remain
uniform. If, then, we inquire in what manner the force of earthquakes
must be regulated, in order to restore perpetually the inequalities of
the surface which the levelling power of water tends to efface, it will
be found, that the amount of depression must exceed that of elevation.
It would be otherwise if the action of volcanoes and mineral springs
were suspended; for then the forcing outwards of the earth's envelope
ought to be no more than equal to its sinking in.
To understand this proposition more clearly, it must be borne in mind,
that the deposits of rivers and currents probably add as much to the
height of lands which are rising, as they take from those which have
risen. Suppose a large river to bring down sediment to a part of the
ocean two thousand feet deep, and that the depth of this part is
gradually reduced by the accumulation of sediment till only a shoal
remains, covered by water at high tides; if now an upheaving force
should uplift this shoal to the height of 2000 feet, the result would be
a mountain 2000 feet high. But had the movement raised the same part of
the bottom of the sea before the sediment of the river had filled it up;
then, instead of changing a shoal into a mountain 2000 feet high, it
would only have converted a deep sea into a shoal.
It appears, then, that the operations of the earthquake are often such
as to cause the levelling power of water to counteract itself; and,
although the idea may appear paradoxical, we may be sure, wherever we
find hills and mountains composed of stratified deposits, that such
inequalities of the surface would have had no existence if water, at
some former period, had not been laboring to reduce the earth's surface
to one level.
But, besides the transfer of matter by running water from the continents
to the ocean, there is a constant transportation from below upwards, by
mineral springs and volcanic vents. As mountain masses are, in the
course of ages, created by the pouring forth of successive streams of
lava, so stratified rocks, of great extent, originate from the
deposition of carbonate of lime, and other mineral ingredients, with
which springs are impregnated. The surface of the land, and portions of
the bottom of the sea, being thus raised, the external accessions due to
these operations would cause the dimensions of the planet to enlarge
continually, if the amount of depression of the earth's crust were no
more than equal to the elevation. In order, therefore, that the mean
diameter of the earth should remain uniform, and the unevenness of the
surface be preserved, it is necessary that the amount of subsidence
should be in excess. And such a predominance of depression is far from
improbable, on mechanical principles, since every upheaving movement
must be expected either to produce caverns in the mass below, or to
cause some diminution of its density. Vacuities must, also, arise from
the subtraction of the matter poured out from volcanoes and mineral
springs, or from the contraction of argillaceous masses by subterranean
heat; and the foundations having been thus weakened, the earth's crust,
shaken and rent by reiterated convulsions, must, in the course of time,
fall in.
If we embrace these views, important geological consequences will
follow; since, if there be, upon the whole, more subsidence than
elevation, the average depth to which former surfaces have sunk beneath
their original level must exceed the height which ancient marine strata
have attained above the sea. If, for example, marine strata, about the
age of our chalk and greensand, have been lifted up in Europe to an
extreme height of more than eleven thousand feet, and a mean elevation
of some hundreds, we may conclude that certain parts of the surface,
which existed when those strata were deposited, have sunk to an extreme
depth of _more than_ eleven thousand feet below their original level,
and to a mean depth of _more than_ a few hundreds.
In regard to faults, also, we must infer, according to the hypothesis
now proposed, that a greater number have arisen from the sinking down
than from the elevation of rocks.
To conclude: it seems to be rendered probable, by the views above
explained, that the constant repair of the land, and the subserviency of
our planet to the support of terrestrial as well as aquatic species, are
secured by the elevating and depressing power of causes acting in the
interior of the earth; which, although so often the source of death and
terror to the inhabitants of the globe--visiting in succession every
zone, and filling the earth with monuments of ruin and disorder--are
nevertheless the agents of a conservative principle above all others
essential to the stability of the system.
BOOK III.
CHAPTER XXXIII.
CHANGES OF THE ORGANIC WORLD NOW IN PROGRESS.
Division of the subject--Examination of the question, Whether
species have a real existence in nature?--Importance of this
question in geology--Sketch of Lamarck's arguments in favor of the
transmutation of species, and his conjectures respecting the origin
of existing animals and plants--His theory of the transformation of
the orang-outang into the human species.
The last book, from chapters fourteen to thirty-three inclusive, was
occupied with the consideration of the changes brought about on the
earth's surface, within the period of human observation, by inorganic
agents; such, for example, as rivers, marine currents, volcanoes, and
earthquakes. But there is another class of phenomena relating to the
organic world, which have an equal claim on our attention, if we desire
to obtain possession of all the preparatory knowledge respecting the
existing course of nature, which may be available in the interpretation
of geological monuments. It appeared from our preliminary sketch of the
progress of the science, that the most lively interest was excited among
its earlier cultivators, by the discovery of the remains of animals and
plants in the interior of mountains frequently remote from the sea. Much
controversy arose respecting the nature of these remains, the causes
which may have brought them into so singular a position, and the want of
a specific agreement between them and known animals and plants. To
qualify ourselves to form just views on these curious questions, we must
first study the present condition of the animate creation on the globe.
This branch of our inquiry naturally divides itself into two parts:
first, we may examine the vicissitudes to which species are subject;
secondly, the processes by which certain individuals of these species
occasionally become fossil. The first of these divisions will lead us,
among other topics, to inquire, first, whether species have a real and
permanent existence in nature? or whether they are capable, as some
naturalists pretend, of being indefinitely modified in the course of a
long series of generations? Secondly, whether, if species have a real
existence, the individuals composing them have been derived originally
from many similar stocks, or each from one only, the descendants of
which have spread themselves gradually from a particular point over the
habitable lands and waters? Thirdly, how far the duration of each
species of animal and plant is limited by its dependence on certain
fluctuating and temporary conditions in the state of the animate and
inanimate world? Fourthly, whether there be proofs of the successive
extermination of species in the ordinary course of nature, and whether
there be any reason for conjecturing that new animals and plants are
created from time to time, to supply their place?
_Whether species have a real existence in nature._--Before we can
advance a step in our proposed inquiry, we must be able to define
precisely the meaning which we attach to the term species. This is even
more necessary in geology than in the ordinary studies of the
naturalist; for they who deny that such a thing as a species exists,
concede nevertheless that a botanist or zoologist may reason as if the
specific character were constant, because they confine their
observations to a brief period of time. Just as the geographer, in
constructing his maps from century to century, may proceed as if the
apparent places of the fixed stars remained absolutely the same, and as
if no alteration were brought about by the precession of the equinoxes;
so, it is said, in the organic world, the stability of a species may be
taken as absolute, if we do not extend our views beyond the narrow
period of human history; but let a sufficient number of centuries
elapse, to allow of important revolutions in climate, physical
geography, and other circumstances, and the characters, say they, of the
descendants of common parents may deviate indefinitely from their
original type.
Now, if these doctrines be tenable, we are at once presented with a
principle of incessant change in the organic world; and no degree of
dissimilarity in the plants and animals which may formerly have existed,
and are found fossil, would entitle us to conclude that they may not
have been the prototypes and progenitors of the species now living.
Accordingly M. Geoffroy St. Hilaire has declared his opinion, that there
has been an uninterrupted succession in the animal kingdom, effected by
means of generation, from the earliest ages of the world up to the
present day, and that the ancient animals whose remains have been
preserved in the strata, however different, may nevertheless have been
the ancestors of those now in being. This notion is not very generally
received, but we are not warranted in assuming the contrary, without
fully explaining the data and reasoning by which it may be refuted.
I shall begin by stating as concisely as possible all the facts and
ingenious arguments by which the theory has been supported; and for this
purpose I cannot do better than offer the reader a rapid sketch of
Lamarck's statement of the proofs which he regards as confirmatory of
the doctrine, and which he has derived partly from the works of his
predecessors and in part from original investigations.
His proofs and inferences will be best considered in the order in which
they appear to have influenced his mind, and I shall then point out some
of the results to which he was led while boldly following out his
principles to their legitimate consequences.
_Lamarck's arguments in favor of the transmutation of species._--The
name of species, observes Lamarck, has been usually applied to "every
collection of similar individuals produced by other individuals like
themselves."[790] This definition, he admits, is correct; because every
living individual bears a very close resemblance to those from which it
springs. But this is not all which is usually implied by the term
species; for the majority of naturalists agree with Linnaeus in supposing
that all the individuals propagated from one stock have certain
distinguishing characters in common, which will never vary, and which
have remained the same since the creation of each species.
In order to shake this opinion, Lamarck enters upon the following line
of argument:--The more we advance in the knowledge of the different
organized bodies which cover the surface of the globe, the more our
embarrassment increases, to determine what ought to be regarded as a
species, and still more how to limit and distinguish genera. In
proportion as our collections are enriched, we see almost every void
filled up, and all our lines of separation effaced! We are reduced to
arbitrary determinations, and are sometimes fain to seize upon the
slight differences of mere varieties, in order to form characters for
what we choose to call a species; and sometimes we are induced to
pronounce individuals but slightly differing, and which others regard as
true species, to be varieties.
The greater the abundance of natural objects assembled together, the
more do we discover proofs that every thing passes by insensible shades
into something else; that even the more remarkable differences are
evanescent, and that nature has, for the most part, left us nothing at
our disposal for establishing distinctions, save trifling, and, in some
respects, puerile particularities.
We find that many genera amongst animals and plants are of such an
extent, in consequence of the number of species referred to them, that
the study and determination of these last has become almost
impracticable. When the species are arranged in a series, and placed
near to each other, with due regard to their natural affinities, they
each differ in so minute a degree from those next adjoining, that they
almost melt into each other, and are in a manner confounded together. If
we see isolated species, we may presume the absence of some more closely
connected, and which have not yet been discovered. Already are there
genera, and even entire orders--nay, whole classes, which present an
approximation to the state of things here indicated.
If, when species have been thus placed in a regular series, we select
one, and then, making a leap over several intermediate ones, we take a
second, at some distance from the first, these two will, on comparison,
be seen to be very dissimilar; and it is in this manner that every
naturalist begins to study the objects which are at his own door. He
then finds it an easy task to establish generic and specific
distinctions; and it is only when his experience is enlarged, and when
he has made himself master of the intermediate links, that his
difficulties and ambiguities begin. But while we are thus compelled to
resort to trifling and minute characters in our attempt to separate the
species, we find a striking disparity between individuals which we know
to have descended from a common stock; and these newly acquired
peculiarities are regularly transmitted from one generation to another,
constituting what are called _races_.
From a great number of facts, continues the author, we learn that in
proportion as the individuals of one of our species change their
situation, climate, and manner of living, they change also, by little
and little, the consistence and proportions of their parts, their form,
their faculties, and even their organization, in such a manner that
every thing in them comes at last to participate in the mutations to
which they have been exposed. Even in the same climate, a great
difference of situation and exposure causes individuals to vary; but if
these individuals continue to live and to be reproduced under the same
difference of circumstances, distinctions are brought about in them
which become in some degree essential to their existence. In a word, at
the end of many successive generations, these individuals, which
originally belonged to another species, are transformed into a new and
distinct species.[791]
Thus, for example, if the seeds of a grass, or any other plant which
grows naturally in a moist meadow, be accidentally transported, first to
the slope of some neighboring hill, where the soil, although at a
greater elevation, is damp enough to allow the plant to live; and if,
after having lived there, and having been several times regenerated, it
reaches by degrees the drier and almost arid soil of a mountain
declivity, it will then, if it succeeds in growing, and perpetuates
itself for a series of generations, be so changed that botanists who
meet with it will regard it as a particular species.[792] The
unfavorable climate in this case, deficiency of nourishment, exposure to
the winds, and other causes, give rise to a stunted and dwarfish race,
with some organ more developed than others, and having proportions often
quite peculiar.
What nature brings about in a great lapse of time, we occasion suddenly
by changing the circumstances in which a species has been accustomed to
live. All are aware that vegetables taken from their birthplace, and
cultivated in gardens, undergo changes which render them no longer
recognizable as the same plants. Many which were naturally hairy become
smooth, or nearly so; a great number of such as were creepers and
trailed along the ground, rear their stalks and grow erect. Others lose
their thorns or asperities; others, again, from the ligneous state which
their stem possessed in hot climates, where they were indigenous, pass
to the herbaceous; and, among them, some which were perennials become
mere annuals. So well do botanists know the effects of such changes of
circumstances, that they are averse to describe species from garden
specimens, unless they are sure that they have been cultivated for a
very short period.
"Is not the cultivated wheat" (_Triticum sativum_), asks Lamarck, "a
vegetable brought by man into the state in which we now see it? Let any
one tell me in what country a similar plant grows wild, unless where it
has escaped from cultivated fields? Where do we find in nature our
cabbages, lettuces, and other culinary vegetables, in the state in which
they appear in our gardens? Is it not the same in regard to a great
quantity of animals which domesticity has changed or considerably
modified?"[793] Our domestic fowls and pigeons are unlike any wild
birds. Our domestic ducks and geese have lost the faculty of raising
themselves into the higher regions of the air, and crossing extensive
countries in their flight, like the wild ducks and wild geese from which
they were originally derived. A bird which we breed in a cage cannot,
when restored to liberty, fly like others of the same species which have
been always free. This small alteration of circumstances, however, has
only diminished the power of flight, without modifying the form of any
part of the wings. But when individuals of the same race are retained in
captivity during a considerable length of time, the form even of their
parts is gradually made to differ, especially if climate, nourishment,
and other circumstances be also altered.
The numerous races of dogs which we have produced by domesticity are
nowhere to be found in a wild state. In nature we should seek in vain
for mastiffs, harriers, spaniels, greyhounds, and other races, between
which the differences are sometimes so great that they would be readily
admitted as specific between wild animals; "yet all these have sprung
originally from a single race, at first approaching very near to a wolf,
if, indeed, the wolf be not the true type which at some period or other
was domesticated by man."
Although important changes in the nature of the places which they
inhabit modify the organization of animals as well as vegetables; yet
the former, says Lamarck, require more time to complete a considerable
degree of transmutation; and, consequently, we are less sensible of such
occurrences. Next to a diversity of the medium in which animals or
plants may live, the circumstances which have most influence in
modifying their organs are differences in exposure, climate, the nature
of the soil, and other local particulars. These circumstances are as
varied as are the characters of the species, and, like them, pass by
insensible shades into each other, there being every intermediate
gradation between the opposite extremes. But each locality remains for a
very long time the same, and is altered so slowly that we can only
become conscious of the reality of the change by consulting geological
monuments, by which we learn that the order of things which how reigns
in each place has not always prevailed, and by inference anticipate that
it will not always continue the same.[794]
Every considerable alteration in the local circumstances in which each
race of animals exists causes a change in their wants, and these new
wants excite them to new actions and habits. These actions require the
more frequent employment of some parts before but slightly exercised,
and then greater development follows as a consequence of their more
frequent use. Other organs no longer in use are impoverished and
diminished in size, nay, are sometimes entirely annihilated, while in
their place new parts are insensibly produced for the discharge of new
functions.[795]
I must here interrupt the author's argument, by observing, that no
positive fact is cited to exemplify the substitution of some _entirely
new_ sense, faculty, or organ, in the room of some other suppressed as
useless. All the instances adduced go only to prove that the dimensions
and strength of members and the perfection of certain attributes may, in
a long succession of generations, be lessened and enfeebled by disuse;
or, on the contrary, be matured and augmented by active exertion; just
as we know that the power of scent is feeble in the greyhound, while its
swiftness of pace and its acuteness of sight are remarkable--that the
harrier and stag-hound, on the contrary, are comparatively slow in their
movements, but excel in the sense of smelling.
It was necessary to point out to the reader this important chasm in the
chain of evidence, because he might otherwise imagine that I had merely
omitted the illustrations for the sake of brevity; but the plain truth
is, that there were no examples to be found; and when Lamarck talks "of
the efforts of internal sentiment," "the influence of subtle fluids,"
and "acts of organization," as causes whereby animals and plants may
acquire _new organs_, he substitutes names for things; and, with a
disregard to the strict rules of induction, resorts to fictions, as
ideal as the "plastic virtue," and other phantoms of the geologists of
the middle ages.
It is evident that, if some well-authenticated facts could have been
adduced to establish one complete step in the process of transformation,
such as the appearance, in individuals descending from a common stock,
of a sense or organ entirely new, and a complete disappearance of some
other enjoyed by their progenitors, time alone might then be supposed
sufficient to bring about any amount of metamorphosis. The gratuitous
assumption, therefore, of a point so vital to the theory of
transmutation, was unpardonable on the part of its advocate.
But to proceed with the system: it being assumed as an undoubted fact,
that a change of external circumstances may cause one organ to become
entirely obsolete, and a new one to be developed, such as never before
belonged to the species, the following proposition is announced, which,
however staggering and absurd it may seem, is logically deduced from the
assumed premises. It is not the organs, or, in other words, the nature
and form of the parts of the body of an animal, which have given rise to
its habits, and its particular faculties; but, on the contrary, its
habits, its manner of living, and those of its progenitors, have in the
course of time determined the form of its body, the number and condition
of its organs--in short, the faculties which it enjoys. Thus otters,
beavers, waterfowl, turtles, and frogs, were not made web-footed in
order that they might swim; but their wants having attracted them to the
water in search of prey, they stretched out the toes of their feet to
strike the water and move rapidly along its surface. By the repeated
stretching of their toes, the skin which united them at the base
acquired a habit of extension, until, in the course of time, the broad
membranes which now connect their extremities were formed.
In like manner, the antelope and the gazelle were not endowed with light
agile forms, in order that they might escape by flight from carnivorous
animals; but, having been exposed to the danger of being devoured by
lions, tigers, and other beasts of prey, they were compelled to exert
themselves in running with great celerity; a habit which, in the course
of many generations, gave rise to the peculiar slenderness of their
legs, and the agility and elegance of their forms.
The camelopard was not gifted with a long flexible neck because it was
destined to live in the interior of Africa, where the soil was arid and
devoid of herbage; but, being reduced by the nature of that country to
support itself on the foliage of lofty trees, it contracted a habit of
stretching itself up to reach the high boughs, until its neck became so
elongated that it could raise its head to the height of twenty feet
above the ground.
Another line of argument is then entered upon, in farther corroboration
of the instability of species. In order, it is said, that individuals
should perpetuate themselves unaltered by generation, those belonging to
one species ought never to ally themselves to those of another; but such
sexual unions do take place, both among plants and animals; and although
the offspring of such irregular connections are usually sterile, yet
such is not always the case. Hybrids have sometimes proved prolific,
where the disparity between the species was not too great; and by this
means alone, says Lamarck, varieties may gradually be created by near
alliances, which would become races, and in the course of time would
constitute what we term species.[796]
But if the soundness of all these arguments and inferences be admitted,
we are next to inquire, what were the original types of form,
organization, and instinct, from which the diversities of character, as
now exhibited by animals and plants, have been derived? We know that
individuals which are mere varieties of the same species would, if their
pedigree could be traced back far enough, terminate in a single stock;
so, according to the train of reasoning before described, the species of
a genus, and even the genera of a great family, must have had a common
point of departure. What, then, was the single stem from which so many
varieties of form have ramified? Were there many of these, or are we to
refer the origin of the whole animate creation, as the Egyptian priests
did that of the universe, to a single egg?
In the absence of any positive data for framing a theory on so obscure a
subject, the following considerations were deemed of importance to guide
conjecture.
In the first place, if we examine the whole series of known animals,
from one extremity to the other, when they are arranged in the order of
their natural relations, we find that we may pass progressively, or, at
least, with very few interruptions, from beings of more simple to those
of a more compound structure; and, in proportion as the complexity of
their organization increases, the number and dignity of their faculties
increase also. Among plants, a similar approximation to a graduated
scale of being is apparent, Secondly, it appears, from geological
observations, that plants and animals of more simple organization
existed on the globe before the appearance of those of more compound
structure, and the latter were successively formed at more modern
periods; each new race being more fully developed than the most perfect
of the preceding era.
Of the truth of the last-mentioned geological theory, Lamarck seems to
have been fully persuaded; and he also shows that he was deeply
impressed with a belief prevalent amongst the older naturalists, that
the primeval ocean invested the whole planet long after it became the
habitation of living beings; and thus he was inclined to assert the
priority of the types of marine animals to those of the terrestrial, so
as to fancy, for example, that the testacea of the ocean existed first,
until some of them, by gradual evolution, were _improved_ into those
inhabiting the land.
These speculative views had already been, in a great degree, anticipated
by Demaillet in his Telliamed, and by several modern writers; so that
the tables were completely turned on the philosophers of antiquity, with
whom it was a received maxim, that created things were always most
perfect when they came first from the hands of their Maker; and that
there was a tendency to progressive deterioration in sublunary things
when left to themselves--
----omnia fatis
In pejus ruere, ac retro sublapsa referri.
So deeply was the faith of the ancient schools of philosophy imbued with
this doctrine, that, to check this universal proneness to degeneracy,
nothing less than the reintervention of the Deity was thought adequate;
and it was held, that thereby the order, excellence, and pristine energy
of the moral and physical world had been repeatedly restored.
But when the possibility of the indefinite modification of individuals
descending from common parents was once assumed, as also the geological
inference respecting the progressive development of organic life, it was
natural that the ancient dogma should be rejected, or rather reversed,
and that the most simple and imperfect forms and faculties should be
conceived to have been the originals whence all others were developed.
Accordingly, in conformity to these views, inert matter was supposed to
have been first endowed with life; until, in the course of ages,
sensation was superadded to mere vitality: sight, hearing, and the
other senses were afterwards acquired; then instinct and the mental
faculties; until, finally, by virtue of the tendency of things to
_progressive improvement_, the irrational was developed in the rational.
The reader, however, will immediately perceive that when all the higher
orders of plants and animals were thus supposed to be comparatively
modern, and to have been derived in a long series of generations from
those of more simple conformation, some farther hypothesis became
indispensable, in order to explain why, after an indefinite lapse of
ages, there were still so many beings of the simplest structure. Why
have the majority of existing creatures remained stationary throughout
this long succession of epochs, while others have made such prodigious
advances? Why are there such multitudes of infusoria and polyps, or of
confervae and other cryptogamic plants? Why, moreover, has the process of
development acted with such unequal and irregular force on those classes
of beings which have been greatly perfected, so that there are wide
chasms in the series; gaps so enormous, that Lamarck fairly admits we
can never expect to fill them up by future discoveries?
The following hypothesis was provided to meet these objections. Nature,
we are told, is not an intelligence, nor the Deity; but a delegated
power--a mere instrument--a piece of mechanism acting by necessity--an
order of things constituted by the Supreme Being, and subject to laws
which are the expressions of his will. This Nature is _obliged_ to
proceed gradually in all her operations; she cannot produce animals and
plants of all classes at once, but must always begin by the formation of
the most simple kinds, and out of them elaborate the more compound,
adding to them, successively, different systems of organs, and
multiplying more and more their number and energy.
This nature is daily engaged in the formation of the elementary
rudiments of animal and vegetable existence, which correspond to what
the ancients termed _spontaneous generation_. She is always beginning
anew, day by day, the work of creation, by forming monads, or "rough
draughts" (abauches), which are the only living things she gives birth
to _directly_.
There are distinct primary rudiments of plants and animals, and
_probably_ of each of the great divisions of the animal and vegetable
kingdoms.[797] These are gradually developed into the higher and more
perfect classes by the slow but unceasing agency of two influential
principles: first, _the tendency to progressive advancement_ in
organization, accompanied by greater dignity in instinct, intelligence,
&c.; secondly, _the force of external circumstances_, or of variations
in the physical condition of the earth, or the mutual relations of
plants and animals. For, as species spread themselves gradually over the
globe, they are exposed from time to time to variations in climate, and
to changes in the quantity and quality of their food; they meet with new
plants and animals which assist or retard their development, by
supplying them with nutriment, or destroying their foes. The nature,
also, of each locality, is in itself fluctuating; so that, even if the
relation of other animals and plants were invariable, the habits and
organization of species would be modified by the influence of local
revolutions.
Now, if the first of these principles, _the tendency to progressive
development_, were left to exert itself with perfect freedom, it would
give rise, says Lamarck, in the course of ages, to a graduated scale of
being, where the most insensible transition might be traced from the
simplest to the most compound structure, from the humblest to the most
exalted degree of intelligence. But, in consequence of the perpetual
interference of the _external causes_ before mentioned, this regular
order is greatly interfered with, and an approximation only to such a
state of things is exhibited by the animate creation, the progress of
some races being retarded by unfavorable, and that of others accelerated
by favorable, combinations of circumstances. Hence, all kinds of
anomalies interrupt the continuity of the plan; and chasms, into which
whole genera or families might be inserted, are seen to separate the
nearest existing portions of the series.
_Lamarck's theory of the transformation of the orang-outang into the
human species._--Such is the machinery of the Lamarckian system; but the
reader will hardly, perhaps, be able to form a perfect conception of so
complicated a piece of mechanism, unless it is exhibited in motion, so
that we may see in what manner it can work out, under the author's
guidance, all the extraordinary effects which we behold in the present
state of the animate creation. I have only space for exhibiting a small
part of the entire process by which a complete metamorphosis is
achieved, and shall therefore omit the mode by which, after a countless
succession of generations, a small gelatinous body is transformed into
an oak or an ape; passing on at once to the last grand step in the
progressive scheme, by which the orang-outang, having been already
evolved out of a monad, is made slowly to attain the attributes and
dignity of man.
One of the races of quadrumanous animals which had reached the highest
state of perfection, lost, by constraint of circumstances (concerning
the exact nature of which tradition is unfortunately silent), the habit
of climbing trees, and of hanging on by grasping the boughs with their
feet as with hands. The individuals of this race being obliged, for a
long series of generations, to use their feet exclusively for walking,
and ceasing to employ their hands as feet, were transformed into
bimanous animals, and what before were thumbs became mere toes, no
separation being required when their feet were used solely for walking.
Having acquired a habit of holding themselves upright, their legs and
feet assumed, insensibly, a conformation fitted to support them in an
erect attitude, till at last these animals could no longer go on
all-fours without much inconvenience.
The Angola orang (_Simia troglodytes_, Linn.) is the most perfect of
animals; much more so than the Indian orang (_Simia Satyrus_), which has
been called the orang-outang, although _both_ are _very inferior_ to man
in corporeal powers and intelligence. These animals frequently hold
themselves upright; but their organization has _not yet_ been
sufficiently modified to sustain them habitually in this attitude, so
that the standing posture is very uneasy to them. When the Indian orang
is compelled to take flight from pressing danger, he immediately falls
down upon all-fours, showing clearly that this was the original position
of the animal. Even in man, whose organization, in the course of a long
series of generations, has advanced so much farther, the upright posture
is fatiguing, and can be supported only for a limited time, and by aid
of the contraction of many muscles. If the vertebral column formed the
axis of the human body, and supported the head and all the other parts
in equilibrium, then might the upright position be a state of repose:
but, as the human head does not articulate in the centre of gravity, as
the chest, belly, and other parts press almost entirely forward with
their whole weight, and as the vertebral column reposes upon an oblique
base, a watchful activity is required to prevent the body from falling.
Children who have large heads and prominent bellies can hardly walk at
the end even of two years; and their frequent tumbles indicate the
natural tendency in man to resume the quadrupedal state.
Now, when so much progress had been made by the quadrumanous animals
before mentioned, that they could hold themselves habitually in an erect
attitude, and were accustomed to a wide range of vision, and ceased to
use their jaws for fighting and tearing, or for clipping herbs for food,
their snout became gradually shorter, their incisor teeth became
vertical, and the facial angle grew more open.
Among other ideas which the natural _tendency to perfection_ engendered,
the desire of ruling suggested itself, and this race succeeded at length
in getting the better of the other animals, and made themselves masters
of all those spots on the surface of the globe which best suited them.
They drove out the animals which approached nearest them in organization
and intelligence, and which were in a condition to dispute with them the
good things of this world, forcing them to take refuge in deserts,
woods, and wildernesses, where their multiplication was checked, and the
progressive development of their faculties retarded; while, in the mean
time, the dominant race spread itself in every direction, and lived in
large companies, where new wants were successively created, exciting
them to industry, and gradually perfecting their means and faculties.
In the supremacy and increased intelligence acquired by the ruling race,
we see an illustration of the natural tendency of the organic world to
grow more perfect; and, in their influence in repressing the advance of
others, an example of one of those disturbing causes before enumerated,
that _force of external circumstances_ which causes such wide chasms in
the regular series of animated being.
When the individuals of the dominant race became very numerous, their
ideas greatly increased in number, and they felt the necessity of
communicating them to each other, and of augmenting and varying the
signs proper for the communication of ideas. Meanwhile the inferior
quadrumanous animals, although most of them were gregarious, acquired no
new ideas, being persecuted and restless in the deserts, and obliged to
fly and conceal themselves, so that they conceived no new wants. Such
ideas as they already had remained unaltered, and they could dispense
with the communication of the greater part of these. To make themselves,
therefore, understood by their fellows, required merely a few movements
of the body or limbs--whistling, and the uttering of certain cries
varied by the inflexions of the voice.
On the contrary, the individuals of the ascendant race, animated with a
desire of interchanging their ideas, which became more and more
numerous, were prompted to multiply the means of communication, and were
no longer satisfied with mere pantomimic signs, nor even with all the
possible inflexions of the voice, but made continual efforts to acquire
the power of uttering articulate sounds, employing a few at first, but
afterwards varying and perfecting them according to the increase of
their wants. The habitual exercise of their throat, tongue, and lips,
insensibly modified the conformation of these organs, until they became
fitted for the faculty of speech.[798]
In effecting this mighty change, "the exigencies of the individuals were
the sole agents; they gave rise to efforts, and the organs proper for
articulating sounds were developed by their habitual employment." Hence,
in this peculiar race, the origin of the admirable faculty of speech;
hence also the diversity of languages, since the distance of places
where the individuals composing the race established themselves soon
favored the corruption of conventional signs.[799]
In conclusion, it may be proper to observe that the above sketch of the
Lamarckian theory is no exaggerated picture, and those passages which
have probably excited the greatest surprise in the mind of the reader
are literal translations from the original.
CHAPTER XXXIV.
TRANSMUTATION OF SPECIES--_Continued_.
Recapitulation of the arguments in favor of the theory of
transmutation of species--Their insufficiency--Causes of difficulty
in discriminating species--Some varieties possibly more distinct
than certain individuals of distinct species--Variability in a
species consistent with a belief that the limits of deviation are
fixed--No facts of transmutation authenticated--Varieties of the
Dog--the Dog and Wolf distinct species--Mummies of various animals
from Egypt identical in character with living individuals--Seeds and
plants from the Egyptian tombs--Modifications produced in plants by
agriculture and gardening.
The theory of the transmutation of species, considered in the last
chapter, has met with some degree of favor from many naturalists, from
their desire to dispense, as far as possible, with the repeated
intervention of a First Cause, as often as geological monuments attest
the successive appearance of new races of animals and plants, and the
extinction of those pre-existing. But, independently of a predisposition
to account, if possible, for a series of changes in the organic world by
the regular action of secondary causes, we have seen that in truth many
perplexing difficulties present themselves to one who attempts to
establish the nature and reality of the specific character. And if once
there appears ground of reasonable doubt, in regard to the constancy of
species, the amount of transformation which they are capable of
undergoing may seem to resolve itself into a mere question of the
quantity of time assigned to the past duration of animate existence.
Before entering upon the reasons which may be adduced for rejecting
Lamarck's hypothesis, I shall recapitulate, in a few words, the
phenomena, and the whole train of thought, by which I conceive it to
have been suggested, and which have gained for this and analogous
theories, both in ancient and modern times, a considerable number of
votaries.
In the first place, the various groups into which plants and animals may
be thrown seem almost invariably, to a beginner, to be so natural, that
he is usually convinced at first, as was Linnaeus to the last, "that
genera are as much founded in nature as the species which compose
them."[800] When by examining the numerous intermediate gradations the
student finds all lines of demarcation to be in most instances
obliterated, even where they at first appeared most distinct, he grows
more and more sceptical as to the real existence of genera, and finally
regards them as mere arbitrary and artificial signs, invented, like
those which serve to distinguish the heavenly constellations, for the
convenience of classification, and having as little pretensions to
reality.
Doubts are then engendered in his mind as to whether species may not
also be equally unreal. The student is probably first struck with the
phenomenon, that some individuals are made to deviate widely from the
ordinary type by the force of peculiar circumstances, and with the still
more extraordinary fact, that the newly acquired peculiarities are
faithfully transmitted to the offspring. How far, he asks, may such
variations extend in the course of indefinite periods of time, and
during great vicissitudes in the physical condition of the globe? His
growing incertitude is at first checked by the reflection that nature
has forbidden the intermixture of the descendants of distinct original
stocks, or has, at least, entailed sterility on their offspring, thereby
preventing their being confounded together, and pointing out that a
multitude of distinct types must have been created in the beginning, and
must have remained pure and uncorrupted to this day.
Relying on this general law, he endeavors to solve each difficult
problem by direct experiment, until he is again astounded by the
phenomenon of a prolific hybrid, and still more by an example of a
hybrid perpetuating itself throughout several generations in the
vegetable world. He then feels himself reduced to the dilemma of
choosing between two alternatives; either to reject the test, or to
declare that the two species, from the union of which the fruitful
progeny has sprung, were mere varieties. If he prefer the latter, he is
compelled to question the reality of the distinctness of all other
supposed species which differ no more than the parents of such prolific
hybrids; for although he may not be enabled immediately to procure, in
all such instances, a fruitful offspring; yet experiments show, that
after repeated failures, the union of two recognized species may at
last, under very favorable circumstances, give birth to a fertile
progeny. Such circumstances, therefore, the naturalist may conceive to
have occurred again and again, in the course of a great lapse of ages.
His first opinions are now fairly unsettled, and every stay at which he
has caught has given way one after another; he is in danger of falling
into any new and visionary doctrine which may be presented to him; for
he now regards every part of the animate creation as void of stability,
and in a state of continual flux. In this mood he encounters the
Geologist, who relates to him how there have been endless vicissitudes
in the shape and structure of organic beings in former ages--how the
approach to the present system of things has been gradual--that there
has been a progressive development of organization subservient to the
purposes of life, from the most simple to the most complex state--that
the appearance of man is the last phenomenon in a long succession of
events--and, finally, that a series of physical revolutions can be
traced in the inorganic world, coeval and co-extensive with those of
organic nature.
These views seem immediately to confirm all his preconceived doubts as
to the stability of the specific character, and he begins to think there
may exist an inseparable connection between a series of changes in the
inanimate world, and the capability of the species to be indefinitely
modified by the influence of external circumstances. Henceforth his
speculations know no definite bounds; he gives the rein to conjecture,
and fancies that the outward form, internal structure, instinctive
faculties, nay, that reason itself may have been gradually developed
from some of the simplest states of existence--that all animals, that
man himself, and the irrational beings, may have had one common origin;
that all may be parts of one continuous and progressive scheme of
development, from the most imperfect to the more complex; in fine, he
renounces his belief in the high genealogy of his species, and looks
forward, as if in compensation, to the future perfectibility of man in
his physical, intellectual, and moral attributes.
Let us now proceed to consider what is defective in evidence, and what
fallacious in reasoning, in the grounds of these strange conclusions.
Blumenbach judiciously observes, that "no general rule can be laid down
for determining the distinctness of species, as there is no particular
class of characters which can serve as a criterion. In each case we must
be guided by _analogy_ and _probability_." The multitude, in fact, and
complexity of the proofs to be weighed is so great, that we can only
hope to obtain presumptive evidence, and we must, therefore, be the more
careful to derive our general views as much as possible from those
observations where the chances of deception are least. We must be on our
guard not to tread in the footsteps of the naturalists of the middle
ages, who believed the doctrine of spontaneous generation to be
applicable to all those parts of the animal and vegetable kingdoms which
they least understood, in direct contradiction to the analogy of all the
parts best known to them; and who, when at length they found that
insects and cryptogamous plants were also propagated from eggs or seeds,
still persisted in retaining their old prejudices respecting the
infusory animalcules and other minute beings, the generation of which
had not then been demonstrated by the microscope to be governed by the
same laws.
Lamarck has, indeed, attempted to raise an argument in favor of his
system, out of the very confusion which has arisen in the study of some
orders of animals and plants, in consequence of the slight shades of
difference which separate the new species discovered within the last
half century. That the embarrassment of those who attempt to classify
and distinguish the new acquisitions, poured in such multitudes into our
museums, should increase with the augmentation of their number, is quite
natural; since to obviate this, it is not enough that our powers of
discrimination should keep pace with the increase of the objects, but we
ought to possess greater opportunities of studying each animal and plant
in all stages of its growth, and to know profoundly their history, their
habits, and physiological characters, throughout several generations;
for, in proportion as the series of known animals grows more complete
none can doubt there is a nearer approximation to a graduated scale of
being; and thus the most closely allied species will be found to possess
a greater number of characters in common.
_Causes of the difficulty of discriminating species._--But, in point of
fact, our new acquisitions consist, more and more as we advance, of
specimens brought from foreign and often very distant and barbarous
countries. A large proportion have never even been seen alive by
scientific inquirers. Instead of having specimens of the young, the
adult, and the aged individuals of each sex, and possessing means of
investigating the anatomical structure, the peculiar habits, and
instincts of each, what is usually the state of our information? A
single specimen, perhaps, of a dried plant, or a stuffed bird or
quadruped; a shell, without the soft parts of the animal; an insect in
one stage of its numerous transformations;--these are the scanty and
imperfect data which the naturalist possesses. Such information may
enable us to separate species which stand at a considerable distance
from each other; but we have no right to expect any thing but difficulty
and ambiguity, if we attempt, from such imperfect opportunities, to
obtain distinctive marks for defining the characters of species which
are closely related.
If Lamarck could introduce so much certainty and precision into the
classification of several thousand species of recent and fossil shells,
notwithstanding the extreme remoteness of the organization of these
animals from the type of those vertebrated species which are best known,
and in the absence of so many of the living inhabitants of shells, we
are led to form an exalted conception of the degree of exactness to
which specific distinctions are capable of being carried, rather than to
call in question their reality.
When our data are so defective, the most acute naturalist must expect to
be sometimes at fault, and, like the novice, to overlook essential
points of difference, passing unconsciously from one species to another,
until, like one who is borne along in a current, he is astonished on
looking back, at observing that he has reached a point so remote from
that whence he set out.
It is by no means improbable, that, when the series of species of
certain genera is very full, they may be found to differ less widely
from each other than do the mere varieties or races of certain species.
If such a fact could be established, it would, undoubtedly, diminish the
chance of our obtaining certainty in our results; but it would by no
means overthrow our confidence in the reality of species.
_Some mere varieties possibly more distinct than certain individuals of
distinct species._--It is almost necessary, indeed, to suppose that
varieties will differ in some cases more decidedly than some species, if
we admit that there is a graduated scale of being, and assume that the
following laws prevail in the economy of the animate creation:--first,
that the organization of individuals is capable of being modified to a
limited extent, by the force of external causes; secondly, that these
modifications are, to a certain extent, transmissible to their
offspring; thirdly, that there are fixed limits, beyond which the
descendants from common parents can never deviate from a certain type;
fourthly, that each species springs from one original stock, and can
never be permanently confounded by intermixing with the progeny of any
other stock; fifthly, that each species shall endure for a considerable
period of time. Now, let us assume, for the present, these rules
hypothetically, and see what consequences may naturally be expected to
result from them.
We must suppose that when the Author of Nature creates an animal or
plant, all the possible circumstances in which its descendants are
destined to live are foreseen, and that an organization is conferred
upon it which will enable the species to perpetuate itself and survive
under all the varying circumstances to which it must be inevitably
exposed. Now, the range of variation of circumstances will differ
essentially in almost every case. Let us take, for example, any one of
the most influential conditions of existence, such as temperature. In
some extensive districts near the equator, the thermometer might never
vary, throughout several thousand centuries, for more than 20 degrees
Fahrenheit; so that if a plant or animal be provided with an
organization fitting it to endure such a range, it may continue on the
globe for that immense period, although every individual might be liable
at once to be cut off by the least possible excess of heat or cold
beyond the determinate degree. But if a species be placed in one of the
temperate zones, and have a constitution conferred on it capable of
supporting a similar range of temperature only, it will inevitably
perish before a single year has passed away.
Humboldt has shown that, at Cumana, within the tropics, there is a
difference of only 4 degrees Fahr. between the temperature of the
warmest and coldest months; whereas, in the temperate zones, the annual
variation amounts to about 60 degrees, and the extreme range of the
thermometer in Canada is not less than 90 degrees.
The same remark might be applied to any other condition, as food, for
example; it may be foreseen that the supply will be regular throughout
indefinite periods in one part of the world, and in another very
precarious and fluctuating both in kind and quantity. Different
qualifications may be required for enabling species to live for a
considerable time under circumstances so changeable. If, then,
temperature and food be among those external causes which, according to
certain laws of animal and vegetable physiology, modify the
organization, form, or faculties, of individuals, we instantly perceive
that the degrees of variability from a common standard must differ
widely in the two cases above supposed; since there is a necessity of
accommodating a species in one case to a much greater latitude of
circumstances than in the other.
If it be a law, for instance, that scanty sustenance should check those
individuals in their growth which are enabled to accommodate themselves
to privations of this kind, and that a parent, prevented in this manner
from attaining the size proper to its species, should produce a dwarfish
offspring, a stunted race will arise, as is remarkably exemplified in
some varieties of the horse and dog. The difference of stature in some
races of dogs, when compared to others, is as one to five in linear
dimensions, making a difference of a hundred-fold in volume.[801] Now,
there is a good reason to believe that species in general are by no
means susceptible of existing under a diversity of circumstances, which
may give rise to such a disparity in size, and, consequently, there will
be a multitude of distinct species, of which no two adult individuals
can ever depart so widely from a certain standard of dimensions as the
mere varieties of certain other species--the dog, for instance. Now, we
have only to suppose that what is true of size, may also hold in regard
to color and many other attributes; and it will at once follow, that the
degree of possible discordance between varieties of the same species
may, in certain cases, exceed the utmost disparity which can arise
between two individuals of many distinct species.
The same remarks may hold true in regard to instincts; for, if it be
foreseen that one species will have to encounter a great variety of
foes, it may be necessary to arm it with great cunning and
circumspection, or with courage or other qualities capable of developing
themselves on certain occasions; such, for example, as those migratory
instincts which are so remarkably exhibited at particular periods, after
they have remained dormant for many generations. The history and habits
of one variety of such a species may often differ more considerably from
some other than those of many distinct species which have no such
latitude of accommodation to circumstances.
_Extent of known variability in species._--Lamarck has somewhat
mis-stated the idea commonly entertained of a species; for it is not
true that naturalists in general assume that the organisation of an
animal or plant remains absolutely constant, and that it can never vary
in any of its parts.[802] All must be aware that circumstances influence
the habits, and that the habits may alter the state of the parts and
organs; but the difference of opinion relates to the extent to which
these modifications of the habits and organs of a particular species may
be carried.
Now, let us first inquire what positive facts can be adduced in the
history of known species, to establish a great and permanent amount of
change in the form, structure, or instinct of individuals descending
from some common stock. The best authenticated examples of the extent to
which species can be made to vary may be looked for in the history of
domesticated animals and cultivated plants. It usually happens, that
those species, both of the animal and vegetable kingdom, which have the
greatest pliability of organisation, those which are most capable of
accommodating themselves to a great variety of new circumstances, are
most serviceable to man. These only can be carried by him into different
climates, and can have their properties or instincts variously
diversified by differences of nourishment and habits. If the resources
of a species be so limited, and its habits and faculties be of such a
confined and local character, that it can only flourish in a few
particular spots, it can rarely be of great utility.
We may consider, therefore, that in the domestication of animals and the
cultivation of plants, mankind have first selected those species which
have the most flexible frames and constitutions, and have then been
engaged for ages in conducting a series of experiments, with much
patience and at great cost, to ascertain what may be the greatest
possible deviation from a common type which can be elicited in these
extreme cases.
_Varieties of the dog--no transmutation._--The modifications produced in
the different races of dogs exhibit the influence of man in the most
striking point of view. These animals have been transported into every
climate and placed in every variety of circumstances; they have been
made, as a modern naturalist observes, the servant, the companion, the
guardian, and the intimate friend of man, and the power of a superior
genius has had a wonderful influence not only on their forms, but on
their manners and intelligence.[803] Different races have undergone
remarkable changes in the quantity and color of their clothing; the dogs
of Guinea are almost naked, while those of the arctic circle are covered
with a warm coat both of hair and wool, which enables them to bear the
most intense cold without inconvenience. There are differences also of
another kind no less remarkable, as in size, the length of their
muzzles, and the convexity of their foreheads.
But, if we look for some of those essential changes which would be
required to lend even the semblance of a foundation for the theory of
Lamarck, respecting the growth of new organs and the gradual
obliteration of others, we find nothing of the kind. For, in all these
varieties of the dog, says Cuvier, the relation of the bones with each
other remains essentially the same; the form of the teeth never changes
in any perceptible degree, except that, in some individuals, one
additional false grinder occasionally appears, sometimes on the one
side, and sometimes on the other.[804] The greatest departure from a
common type--and it constitutes the maximum of variation as yet known in
the animal kingdom--is exemplified in those races of dogs which have a
supernumerary toe on the hind foot with the corresponding tarsal bones;
a variety analogous to one presented by six-fingered families of the
human race.[805]
Lamarck has thrown out as a conjecture, that the wolf may have been the
original of the dog; and eminent naturalists are still divided in
opinion on this subject. It seems now admitted that both species agree
in the period of gestation, and Mr. Owen has been unable to confirm the
alleged difference in the structure of a part of the intestinal
canal.[806] Mr. Bell inclines to the opinion that all the various races
of dogs have descended from one common stock, of which the wolf is the
original source.
It is well known that the horse, the ox, the boar, and other domestic
animals which have been introduced into South America, and have run wild
in many parts, have entirely lost all marks of domesticity, and have
reverted to the original characters of their species. But dogs have also
become wild in Cuba, Hayti, and in all the Caribbean islands. In the
course of the seventeenth century, they hunted in packs from twelve to
fifty, or more, in number, and fearlessly attacked herds of wild boars
and other animals. It is natural, therefore, to inquire to what form
they reverted? Now, they are said by many travellers to have resembled
very nearly the shepherd's dog; but it is certain that they were never
turned into wolves. They were extremely savage, and their ravages appear
to have been as much dreaded as those of wolves; but when any of their
whelps were caught, and brought from the woods to the towns, they grew
up in the most perfect submission to man.[807]
Many examples might be adduced to prove that the extent to which the
alteration of species can be pushed in the domestic state depends on the
original capacity of the species to admit of variation. The horse has
been as long domesticated as the dog, yet its different races depart
much less widely from a common type; the ass has been still less
changed, the camel scarcely at all; yet these species have probably been
subjected to the influence of domestication as long as the horse.
_Mummies of animals in Egyptian tombs identical with species still
living._--As the advocates of the theory of transmutation trust much to
the slow and insensible changes which time may work, they are accustomed
to lament the absence of accurate descriptions, and figures of
particular animals and plants, handed down from the earliest periods of
history, such as might have afforded data for comparing the condition of
species, at two periods considerably remote. But, fortunately, we are in
some measure independent of such evidence: for, by a singular accident,
the priests of Egypt have bequeathed to us, in their cemeteries, that
information which the museums and works of the Greek philosophers have
failed to transmit.
For the careful investigation of these documents, we are greatly
indebted to the skill and diligence of those naturalists who accompanied
the French armies during their brief occupation of Egypt: that conquest
of four years, from which we may date the improvement of the modern
Egyptians in the arts and sciences, and the rapid progress which has
been made of late in our knowledge of the arts and sciences of their
remote predecessors. Instead of wasting their whole time, as so many
preceding travellers had done, in exclusively collecting human mummies,
M. Geoffrey and his associates examined diligently, and sent home great
numbers of embalmed bodies of consecrated animals, such as the bull, the
dog, the cat, the ape, the ichneumon, the crocodile, and the ibis.
To those who have never been accustomed to connect the facts of Natural
History with philosophical speculations, who have never raised their
conceptions of the end and import of such studies beyond the mere
admiration of isolated and beautiful objects, or the exertion of skill
in detecting specific differences, it will seem incredible that amidst
the din of arms, and the stirring excitement of political movements, so
much enthusiasm could have been felt in regard to these precious
remains.
In the official report drawn up by the Professors of the Museum at
Paris, on the value of these objects, there are some eloquent passages,
which may appear extravagant, unless we reflect how fully these
naturalists could appreciate the bearing of the facts thus brought to
light on the past history of the globe.
"It seems," say they, "as if the superstition of the ancient Egyptians
had been inspired by Nature, with a view of transmitting to after ages a
monument of her history. That extraordinary and eccentric people, by
embalming with so much care the brutes which were the objects of their
stupid adoration, have left us in their secret grottoes, cabinets of
zoology almost complete. The climate has conspired with the art of
embalming to preserve the bodies from corruption, and we can now assure
ourselves by our own eyes what was the state of a great number of
species three thousand years ago. We can scarcely restrain the
transports of our imagination, on beholding thus preserved, with their
minutest bones, with the smallest portions of their skin, and in every
particular most perfectly recognizable, many an animal, which at Thebes
or Memphis, two or three thousand years ago, had its own priests and
altars."[808]
Among the Egyptian mummies thus procured were not only those of numerous
wild quadrupeds, birds, and reptiles; but what was perhaps of still
higher importance in deciding the great question under discussion, there
were the mummies of domestic animals, among which those above mentioned,
the bull, the dog, and the cat, were frequent. Now, such was the
conformity of the whole of these species to those now living, that there
was no more difference, says Cuvier, between them than between the human
mummies and the embalmed bodies of men of the present day. Yet some of
these animals have since that period been transported by man to almost
every climate, and forced to accommodate their habits to the greatest
variety of circumstances. The cat, for example, has been carried over
the whole earth, and within the last three centuries, has been
naturalized in every part of the new world,--from the cold regions of
Canada to the tropical plains of Guiana; yet it has scarcely undergone
any perceptible mutation, and is still the same animal which was held
sacred by the Egyptians.
Of the ox, undoubtedly, there are many very distinct races; but the bull
Apis, which was led in solemn processions by the Egyptian priests, did
not differ from some of those now living. The black cattle that have run
wild in America, where there were many peculiarities in the climate not
to be found, perhaps, in any part of the old world, and where scarcely a
single plant on which they fed was of precisely the same species,
instead of altering their form and habits, have actually reverted to the
exact likeness of the aboriginal wild cattle of Europe.
In answer to the arguments drawn from the Egyptian mummies, Lamarck said
they were identical with their living descendants in the same country,
because the climate and physical geography of the banks of the Nile have
remained unaltered for the last thirty centuries. But why, it may be
asked, have other individuals of these species retained the same
characters in many different quarters of the globe, where the climate
and many other conditions are so varied?
_Seeds and plants from the Egyptian tombs._--The evidence derived from
the Egyptian monuments was not confined to the animal kingdom; the
fruits, seeds, and other portions of twenty different plants, were
faithfully preserved in the same manner; and among these the common
wheat was procured by Delille, from closed vessels in the sepulchres of
the kings, the grain of which retained not only their form but even
their color; so effectual has proved the process of embalming with
bitumen in a dry and equable climate. No difference could be detected
between this wheat and that which now grows in the East and elsewhere;
and in regard to the barley, I am informed by Mr. Brown, the celebrated
botanist, that its identity with the grain of our own times can be
tested by the closest comparison. On examining, for example, one of the
seeds from Mr. Sam's Egyptian collection in the British Museum, it is
found that "the structure of the husks or that part of the flower which
is persistent, agrees precisely with the barley of the present day, in
having one perfect flower and the filiform rudiments of a second." Some
naturalists believe that the perfect identification of the ancient
Egyptian cerealia with the varieties now cultivated has been carried
still further, by sowing the seeds taken out of the catacombs, and
raising plants from them; but we want more evidence of this fact.
Certain it is, that when the experiment was recently made in the botanic
garden at Kew, with 100 seeds of wheat, barley, and lentils, from the
Egyptian collection before mentioned of the British Museum, not one of
them would germinate.[809]
_Native country of the common wheat._--And here I may observe that there
is an obvious answer to Lamarck's objection, that the botanist cannot
point out a country where the common wheat grows wild, unless in places
where it may have been derived from neighboring cultivation.[810] All
naturalists are well aware that the geographical distribution of a great
number of species is extremely limited; that it was to be expected that
every useful plant should first be cultivated successfully in the
country where it was indigenous; and that, probably, every station which
it partially occupied, when growing wild, would be selected by the
agriculturist as best suited to it when artificially increased.
Palestine has been conjectured, by a late writer on the cerealia, to
have been the original habitation of wheat and barley; a supposition
which is rendered the more plausible by Hebrew and Egyptian traditions,
and by tracing the migrations of the worship of Ceres, as indicative of
the migrations of the plant.[811]
If we are to infer that some one of the wild grasses has been
transformed into the common wheat, and that some animal of the genus
_Canis_, still unreclaimed, has been metamorphosed into the dog, merely
because we cannot find the domestic dog, or the cultivated wheat, in a
state of nature, we may be next called upon to make similar admissions
in regard to the camel; for it seems very doubtful whether any race of
this species of quadruped is now wild.
_Changes in plants produced by cultivation._--But if agriculture, it
will be said, does not supply examples of extraordinary changes of form
and organization, the horticulturist can, at least, appeal to facts
which may confound the preceding train of reasoning. The crab has been
transformed into the apple; the sloe into the plum; flowers have changed
their color, and become double; and these new characters can be
perpetuated by seed; a bitter plant, with wavy sea-green leaves, has
been taken from the sea-side, where it grew like wild charlock; has been
transplanted into the garden, lost its saltness, and has been
metamorphosed into two distinct vegetables, as unlike each other as is
each to the parent plant--the red cabbage and the cauliflower. These,
and a multitude of analogous facts, are undoubtedly among the wonders of
nature, and attest more strongly, perhaps, the extent to which species
may be modified, than any examples derived from the animal kingdom. But
in these cases we find that we soon reach certain limits, beyond which
we are unable to cause the individuals descending from the same stock to
vary; while, on the other hand, it is easy to show that these
extraordinary varieties could seldom arise, and could never be
perpetuated in a wild state for many generations, under any imaginable
combination of accidents. They may be regarded as extreme cases,
brought about by human interference, and not as phenomena which indicate
a capability of indefinite modification in the natural world.
The propagation of a plant by buds or grafts, and by cuttings, is
obviously a mode which nature does not employ; and this multiplication,
as well as that produced by roots and layers, seems merely to operate as
an extension of the life of an individual, and not as a reproduction of
the species such as happens by seed. All plants increased by grafts or
layers retain precisely the peculiar qualities of the individual to
which they owe their origin, and, like an individual, they have only a
determinate existence; in some cases longer, and in others shorter.[812]
It seems now admitted by horticulturists, that none of our garden
varieties of fruit are entitled to be considered strictly permanent, but
that they wear out after a time;[813] and we are thus compelled to
resort again to seeds; in which case there is so decided a tendency in
the seedlings to revert to the original type, that our utmost skill is
sometimes baffled in attempting to recover the desired variety.
_Varieties of the cabbage._--The different races of cabbages afford, as
was admitted, an astonishing example of deviation from a common type;
but we can scarcely conceive them to have originated, much less to have
lasted for several generations, without the intervention of man. It is
only by strong manures that these varieties have been obtained, and in
poorer soils they instantly degenerate. If, therefore, we suppose in a
state of nature the seed of the wild _Brassica oleracea_ to have been
wafted from the sea-side to some spot enriched by the dung of animals,
and to have there become a cauliflower, it would soon diffuse its seed
to some comparatively sterile soils around, and the offspring would
relapse to the likeness of the parent stock.
But if we go so far as to imagine the soil, in the spot first occupied,
to be constantly manured by herds of wild animals, so as to continue as
rich as that of a garden, still the variety could not be maintained;
because we know that each of these races is prone to fecundate others,
and gardeners are compelled to exert the utmost diligence to prevent
cross-breeds. The intermixture of the pollen of varieties growing in the
poorer soil around would soon destroy the peculiar characters of the
race which occupied the highly manured tract; for, if these accidents so
continually happen, in spite of our care, among the culinary varieties,
it is easy to see how soon this cause might obliterate every marked
singularity in a wild state.
Besides, it is well known that, although the pampered races which we
rear in our gardens for use or ornament may often be perpetuated by
seed, yet they rarely produce seed in such abundance, or so prolific in
quality, as wild individuals; so that if the care of man were withdrawn,
the most fertile variety would always, in the end, prevail over the more
sterile.
Similar remarks may be applied to the double flowers, which present
such strange anomalies to the botanist. The ovarium, in such cases, is
frequently abortive; and the seeds, when prolific, are generally much
fewer than where the flowers are single.
_Changes caused by soil._--Some curious experiments, recently made on
the production of blue instead of red flowers in the _Hydrangea
hortensis_, illustrate the immediate effect of certain soils on the
colors of the calyx and petals. In garden-mould or compost, the flowers
are invariably red; in some kinds of bog-earth they are blue; and the
same change is always produced by a particular sort of yellow loam.
_Varieties of the primrose._--Linnaeus was of opinion that the primrose,
oxlip, cowslip, and polyanthus, were only varieties of the same species.
The majority of the modern botanists, on the contrary, consider them to
be distinct, although some conceived that the oxlip might be a cross
between the cowslip and the primrose. Mr. Herbert has lately recorded
the following experiment:--"I raised from the natural seed of one umbel
of a highly manured red cowslip a primrose, a cowslip, oxlips of the
usual and other colors, a black polyanthus, a hose-in-hose cowslip, and
a natural primrose bearing its flower on a polyanthus stalk. From the
seed of that very hose-in-hose cowslip I have since raised a
hose-in-hose primrose. I therefore consider all these to be only local
varieties, depending upon soil and situation."[814] Professor Henslow,
of Cambridge, has since confirmed this experiment of Mr. Herbert; so
that we have an example, not only of the remarkable varieties which the
florist can obtain from a common stock, but of the distinctness of
analogous races found in a wild state.[815]
On what particular ingredient, or quality in the earth, these changes
depend, has not yet been ascertained.[816] But gardeners are well aware
that particular plants, when placed under the influence of certain
circumstances, are changed in various ways, according to the species;
and as often as the experiments are repeated, similar results are
obtained. The nature of these results, however, depends upon the
species, and they are, therefore, part of the specific character; they
exhibit the same phenomena, again and again, and indicate certain fixed
and invariable relations between the physiological peculiarities of the
plant, and the influence of certain external agents. They afford no
ground for questioning the instability of species, but rather the
contrary; they present us with a class of phenomena, which, when they
are more thoroughly understood, may afford some of the best tests for
identifying species, and proving that the attributes originally
conferred endure so long as any issue of the original stock remains upon
the earth.
CHAPTER XXXV.
WHETHER SPECIES HAVE A REAL EXISTENCE IN NATURE--_continued_.
Limits of the variability of species--Species susceptible of
modification may be altered greatly in a short time, and in a few
generations; after which they remain stationary--The animals now
subject to man had originally an aptitude to domesticity--Acquired
peculiarities which become hereditary have a close connexion with
the habits or instincts of the species in a wild state--Some
qualities in certain animals have been conferred with a view of
their relation to man--Wild elephant domesticated in a few years,
but its faculties incapable of further development.
_Variability of a species compared to that of an individual._--I
endeavored, in the last chapter, to show, that a belief in the reality
of species is not inconsistent with the idea of a considerable degree of
variability in the specific character. This opinion, indeed, is little
more than an extension of the idea which we must entertain of the
identity of an individual, throughout the changes which it is capable of
undergoing.
If a quadruped, inhabiting a cold northern latitude, and covered with a
warm coat of hair or wool, be transported to a southern climate, it will
often, in the course of a few years, shed a considerable portion of its
coat, which it gradually recovers on being again restored to its native
country. Even there the same changes are, perhaps, superinduced to a
certain extent by the return of winter and summer. We know that the
Alpine hare (_Lepus variabilis_, Pal.) and the ermine, or stoat,
(_Mustela erminea_, Linn.) become white during winter, and again obtain
their full color during the warmer season; that the plumage of the
ptarmigan undergoes a like metamorphosis in color and quantity, and that
the change is equally temporary. We are aware that, if we reclaim some
wild animal, and modify its habits and instincts by domestication, it
may, if it escapes, become in a few years nearly as wild and untractable
as ever; and if the same individual be again retaken, it may be reduced
to its former tame state. A plant is sown in a prepared soil, in order
that the petals of its flowers may multiply, and their color be
heightened or changed: if we then withhold our care, the flowers of this
same species become again single. In these, and innumerable other
instances, we must suppose that the species was produced with a certain
number of qualities; and, in the case of animals, with a variety of
instincts, some of which may or may not be developed according to
circumstances, or which, after having been called forth, may again
become latent when the exciting causes are removed.
Now, the formation of races seems the necessary consequence of such a
capability in species to vary, if it be a general law that the offspring
should very closely resemble the parent. But, before we can infer that
there are no limits to the deviation from an original type which may be
brought about in the course of an indefinite number of generations, we
ought to have some proof that, in each successive generation,
individuals may go on acquiring an equal amount of new peculiarities,
under the influence of equal changes of circumstances. The balance of
evidence, however, inclines most decidedly on the opposite side; for in
all cases we find that the quantity of divergence diminishes after a few
generations in a very rapid ratio.
_Species susceptible of modification may be greatly altered in a few
generations._--It cannot be objected, that it is out of our power to go
on varying the circumstances in the same manner as might happen in the
natural course of events during some great geological cycle. For in the
first place, where a capacity is given to individuals to adapt
themselves to new circumstances, it does not generally require a very
long period for its development: if, indeed, such were the case, it is
not easy to see how the modification would answer the ends proposed, for
all the individuals would die before new qualities, habits, or instincts
were conferred.
When we have succeeded in naturalizing some tropical plant in a
temperate climate, nothing prevents us from attempting gradually to
extend its distribution to higher latitudes, or to greater elevations
above the level of the sea, allowing equal quantities of time, or an
equal number of generations, for habituating the species to successive
increments of cold. But every husbandman and gardener is aware that such
experiments will fail; and we are more likely to succeed in making some
plants, in the course of the first two generations, support a
considerable degree of difference of temperature, than a very small
difference afterwards, though we persevere for many centuries.
It is the same if we take any other cause instead of temperature; such
as the quality of the food, or the kind of dangers to which an animal is
exposed, or the soil in which a plant lives. The alteration in habits,
form, or organization, is often rapid during a short period; but when
the circumstances are made to vary farther, though in ever so slight a
degree, all modification ceases, and the individual perishes. Thus some
herbivorous quadrupeds may be made to feed partially on fish or flesh;
but even these can never be taught to live on some herbs which they
reject, and which would even poison them, although the same may be very
nutritious to other species of the same natural order. So when man uses
force or stratagem against wild animals, the persecuted race soon
becomes more cautious, watchful, and cunning; new instincts seem often
to be developed, and to become hereditary in the first two or three
generations: but let the skill and address of man increase, however
gradually, no farther variation can take place, no new qualities are
elicited by the increasing dangers. The alteration of the habits of the
species has reached a point beyond which no ulterior modification is
possible, however indefinite the lapse of ages during which the new
circumstances operate. Extirpation then follows, rather than such a
transformation as could alone enable the species to perpetuate itself
under the new state of things.
_Animals now subject to man had originally an aptitude to
domesticity._--It has been well observed by M. F. Cuvier and M. Dureau
de la Malle, that unless some animals had manifested in a wild state an
aptitude to second the efforts of man, their domestication would never
have been attempted. If they had all resembled the wolf, the fox, and
the hyaena, the patience of the experimentalist would have been exhausted
by innumerable failures before he at last succeeded in obtaining some
imperfect results; so if the first advantages derived from the
cultivation of plants had been elicited by as tedious and costly a
process as that by which we now make some slight additional improvements
in certain races, we should have remained to this day in ignorance of
the greater number of their useful qualities.
_Acquired instincts of some animals become hereditary._--It is
undoubtedly true, that many new habits and qualities have not only been
acquired in recent times by certain races of dogs, but have been
transmitted to their offspring. But in these cases it will be observed,
that the new peculiarities have an intimate relation to the habits of
the animal in a wild state, and therefore do not attest any tendency to
a departure to an indefinite extent from the original type of the
species. A race of dogs employed for hunting deer in the platform of
Sante Fa, in Mexico, affords a beautiful illustration of a new
hereditary instinct. The mode of attack, observes M. Roulin, which they
employ consists in seizing the animal by the belly and overturning it by
a sudden effort, taking advantage of the moment when the body of the
deer rests only upon the fore-legs. The weight of the animal thus thrown
over is often six times that of its antagonist. The dog of pure breed
inherits a disposition to this kind of chase, and never attacks a deer
from before while running. Even should the deer, not perceiving him,
come directly upon him, the dog steps aside and makes his assault on the
flank; whereas other hunting dogs, though of superior strength, and
general sagacity, which are brought from Europe, are destitute of this
instinct. For want of similar precautions, they are often killed by the
deer on the spot, the vertebrae of their neck being dislocated by the
violence of the shock.[817]
A new instinct has also become hereditary in a mongrel race of dogs
employed by the inhabitants of the banks of the Magdalena almost
exclusively in hunting the white-lipped pecari. The address of these
dogs consists in restraining their ardor, and attaching themselves to no
animal in particular, but keeping the whole herd in check. Now, among
these dogs some are found, which the very first time they are taken to
the woods, are acquainted with this mode of attack; whereas, a dog of
another breed starts forward at once, is surrounded by the pecari, and,
whatever may be his strength, is destroyed in a moment.
Some of our countrymen, engaged of late in conducting one of the
principal mining associations in Mexico, that of Real del Monte, carried
out with them some English greyhounds of the best breed, to hunt the
hares which abound in that country. The great platform which is the
scene of sport is at an elevation of about nine thousand feet above the
level of the sea, and the mercury in the barometer stands habitually at
the height of about nineteen inches. It was found that the greyhounds
could not support the fatigues of a long chase in this attenuated
atmosphere, and before they could come up with their prey, they lay down
gasping for breath; but these same animals have produced whelps which
have grown up, and are not in the least degree incommoded by the want of
density in the air, but run down the hares with as much ease as the
fleetest of their race in this country.
The fixed and deliberate stand of the pointer has with propriety been
regarded as a mere modification of a habit, which may have been useful
to a wild race accustomed to wind game, and steal upon it by surprise,
first pausing for an instant, in order to spring with unerring aim. The
faculty of the retriever, however, may justly be regarded as more
inexplicable and less easily referable to the instinctive passions of
the species. M. Majendie, says a French writer in a recently published
memoir, having learnt that there was a race of dogs in England which
stopped and brought back game of their own accord, procured a pair, and
having obtained a whelp from them, kept it constantly under his eyes,
until he had an opportunity of assuring himself that, without having
received any instruction, and on the very first day that it was carried
to the chase, it brought back game with as much steadiness as dogs which
had been schooled into the same manoeuvre by means of the whip and
collar.
_Attributes of animals in their relation to man._--Such attainments, as
well as the habits and dispositions which the shepherd's dog and many
others inherit, seem to be of a nature and extent which we can hardly
explain by supposing them to be modifications of instincts necessary for
the preservation of the species in a wild state. When such remarkable
habits appear in races of this species we may reasonably conjecture that
they were given with no other view than for the use of man and the
preservation of the dog, which thus obtains protection.
As a general rule, I fully agree with M. F. Cuvier, that, in studying
the habits of animals, we must attempt, as far as possible, to refer
their domestic qualities to modifications of instincts which are
implanted in them in a state of nature; and that writer has successfully
pointed out, in an admirable essay on the domestication of the
mammalia[818], the true origin of many dispositions which are vulgarly
attributed to the influence of education alone. But we should go too far
if we did not admit that some of the qualities of particular animals and
plants may have been given solely with a view to the connection which it
was foreseen would exist between them and man--especially when we see
that connexion to be in many cases so intimate, that the greater number,
and sometimes, as in the case of the camel, all the individuals of the
species which exist on the earth are in subjection to the human race.
We can perceive in a multitude of animals, especially in some of the
parasitic tribes, that certain instincts and organs are conferred for
the purpose of defence or attack against some other species. Now if we
are reluctant to suppose the existence of similar relations between man
and the instincts of many of the inferior animals, we adopt an
hypothesis no less violent, though in the opposite extreme to that which
has led some to imagine the whole animate and inanimate creation to have
been made solely for the support, gratification, and instruction of
mankind.
Many species, most hostile to our persons or property, multiply, in
spite of our efforts to repress them; others, on the contrary, are
intentionally augmented many hundred fold in number by our exertions. In
such instances, we must imagine the relative resources of man, and of
species friendly or inimical to him, to have been prospectively
calculated and adjusted. To withhold assent to this supposition, would
be to refuse what we must grant in respect to the economy of nature in
every other part of the organic creation; for the various species of
contemporary plants and animals have obviously their relative forces,
nicely balanced, and their respective tastes, passions, and instincts so
contrived, that they are all in perfect harmony with each other. In no
other manner could it happen that each species, surrounded, as it is, by
countless dangers, should be enabled to maintain its ground for periods
of considerable duration.
The docility of the individuals of some of our domestic species,
extending, as it does, to attainments foreign to their natural habits
and faculties, may, perhaps, have been conferred with a view to their
association with man. But, lest species should be thereby made to vary
indefinitely, we find that such habits are never transmissible by
generation.
A pig has been trained to hunt and point game with great activity and
steadiness[819]; and other learned individuals, of the same species,
have been taught to spell; but such fortuitous acquirements never become
hereditary, for they have no relation whatever to the exigencies of the
animal in a wild state, and cannot, therefore, be developments of any
instinctive propensities.
_Influence of domestication._--An animal in domesticity, says M. F.
Cuvier, is not essentially in a different situation, in regard to the
feeling of restraint, from one left to itself. It lives in society
without constraint, because, without doubt, it was a social animal; and
it conforms itself to the will of man, because it had a chief, to which,
in a wild state, it would have yielded obedience. There is nothing in
its new situation that is not conformable to its propensities; it is
satisfying its wants by submission to a master, and makes no sacrifice
of its natural inclinations. All the social animals, when left to
themselves, form herds more or less numerous; and all the individuals of
the same herd know each other, are mutually attached, and will not allow
a strange individual to join them. In a wild state, moreover, they obey
some individual, which, by its superiority, has become the chief of the
herd. Our domestic species had, originally, this sociability of
disposition; and no solitary species, however easy it may be _to tame
it_, has yet afforded true domestic races. We merely, therefore,
develope, to our own advantage, propensities which propel the
individuals of certain species to draw near to their fellows.