Rocket propellant is used as a reaction mass ejected from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines.
Overview
Rockets create thrust by expelling mass rear-ward, at high velocity. The thrust produced can be calculated by multiplying the mass flow rate of the propellants by their exhaust velocity relative to the rocket (specific impulse). A rocket can be thought of as being accelerated by the pressure of the combusting gases against the combustion chamber and nozzle, not by "pushing" against the air behind or below it. Rocket engines perform best in outer space
because of the lack of air pressure on the outside of the engine. In
space it is also possible to fit a longer nozzle without suffering from flow separation.
Most chemical propellants release energy through redox chemistry, more specifically combustion. As such, both an oxidizing agent and a reducing agent (fuel) must be present in the mixture. Decomposition, such as that of highly unstable peroxide bonds in monopropellant rockets, can also be the source of energy.
In the case of bipropellant liquid rockets, a mixture of reducing fuel and oxidizing oxidizer is introduced into a combustion chamber, typically using a turbopump to overcome the pressure. As combustion takes place, the liquid propellant mass
is converted into a huge volume of gas at high temperature and
pressure. This exhaust stream is ejected from the engine nozzle at high
velocity, creating an opposing force that propels the rocket forward in
accordance with Newton's laws of motion.
Chemical rockets can be grouped by phase. Solid rockets use propellant in the solid phase, liquid fuel rockets use propellant in the liquid phase, gas fuel rockets use propellant in the gas phase, and hybrid rockets use a combination of solid and liquid or gaseous propellants.
In the case of solid rocket motors, the fuel and oxidizer are
combined when the motor is cast. Propellant combustion occurs inside the
motor casing, which must contain the pressures developed. Solid rockets
typically have higher thrust, less specific impulse, shorter burn times, and a higher mass than liquid rockets, and additionally cannot be stopped once lit.
Rocket stages
In space, the maximum change in velocity that a rocket stage can impart on its payload is primarily a function of its mass ratio and its exhaust velocity. This relationship is described by the rocket equation. Exhaust velocity is dependent on the propellant and engine used and closely related to specific impulse,
the total energy delivered to the rocket vehicle per unit of propellant
mass consumed. The mass ratio can also be affected by the choice of a
given propellant.
Rocket stages that fly through the atmosphere usually use
lower-performing, high-molecular-mass, high-density propellants due to
the smaller and lighter tankage required. Upper stages, which mostly or
only operate in the vacuum of space, tend to use the high-energy,
high-performance, low-density liquid hydrogen
fuel. For future planetary missions the use of local resources and
solar energy for in situ propellant production is considered.
Solid chemical propellants
Solid propellants come in two main types. "Composites" are mostly a mixture of granules of solid oxidizer, such as ammonium nitrate, ammonium dinitramide, ammonium perchlorate, or potassium nitrate in a polymer binding agent, with flakes or powders of energetic fuel compounds such as RDX, HMX, aluminium, beryllium. Plasticizers, stabilizers, and burn rate modifiers (iron oxide, copper oxide) can be added.
Single-, double-, or triple-bases are homogeneous mixtures of one
to three primary ingredients, which must include fuel and oxidizer, and
often include binders and plasticizers. All components are
macroscopically indistinguishable and often blended as liquids and cured
in a single batch. Ingredients can have multiple roles: RDX is both
fuel and oxidizer, while nitrocellulose is fuel, oxidizer, and
structural polymer.
Many propellants contain elements of double-base and composite
propellants, and often contain energetic additives homogeneously mixed
into the binder. In the case of gunpowder (a pressed composite without a
polymeric binder), the fuel is charcoal, the oxidizer is potassium
nitrate, and sulphur serves as a reaction catalyst while also being
consumed to form reaction products such as potassium sulfide.
The newest nitramine solid propellants based on CL-20 (HNIW) can match the performance of NTO/UDMH storable liquid propellants, but cannot be throttled or restarted.
Extraterrestrial on-site production is being explored by combining aluminum and ice (ALICE).
Advantages
Solid-propellant
rockets are much easier to store and handle than liquid-propellant
rockets. High propellant density makes for compact size as well. These
features plus simplicity and low cost make solid-propellant rockets
ideal for military applications.
Their simplicity also makes solid rockets a good choice whenever large amounts of thrust are needed and cost is an issue. The Space Shuttle and many other orbital launch vehicles use solid-fueled rockets in their boost stages (solid rocket boosters) for this reason.
Disadvantages
Solid-fuel rockets have lower specific impulse,
a measure of propellant efficiency, than liquid-fuel rockets. As a
result, the overall performance of solid upper stages is less than
liquid stages even though the solid mass ratios are usually in the .91
to .93 range, as good as or better than most liquid-propellant upper
stages. The high mass ratios possible with these unsegmented solid upper
stages is a result of high propellant density and very high
strength-to-weight ratio filament-wound motor casings.
A drawback to solid rockets is that they cannot be throttled in
real time, although a programmed thrust schedule can be created by
adjusting the interior propellant geometry. Solid rockets can be vented
to extinguish combustion or reverse thrust as a means of controlling
range or accommodating stage separation. Casting large amounts of
propellant requires consistency and repeatability to avoid cracks and
voids in the completed motor. The blending and casting take place under
computer control in a vacuum, and the propellant blend is spread thin
and scanned to ensure that no large gas bubbles are introduced into the
motor.
Solid-fuel rockets are intolerant to cracks and voids and require
post-processing such as X-ray scans to identify faults. The combustion
process is dependent on the surface area of the fuel. Voids and cracks
represent local increases in burning surface area, increasing the local
temperature, which increases the local rate of combustion. This positive
feedback loop can easily lead to catastrophic failure of the case or
nozzle.
During the 1950s and 60s, researchers in the United States developed ammonium perchlorate composite propellant (APCP). This mixture is typically 69-70% finely ground ammonium perchlorate (an oxidizer), combined with 16-20% fine aluminium powder (a fuel), held together in a base of 11-14% polybutadiene acrylonitrile (PBAN) or hydroxyl-terminated polybutadiene
(polybutadiene rubber fuel). The mixture is formed as a thickened
liquid and then cast into the correct shape and cured into a firm but
flexible load-bearing solid. Historically, the tally
of APCP solid propellants is relatively small. The military, however,
uses a wide variety of different types of solid propellants, some of
which exceed the performance of APCP. A comparison of the highest
specific impulses achieved with the various solid and liquid propellant
combinations used in current launch vehicles is given in the article on solid-fuel rockets.
In the 1970s and 1980s, the U.S. switched entirely to solid-fueled ICBMs: the LGM-30 Minuteman and LG-118A Peacekeeper (MX). In the 1980s and 1990s, the USSR/Russia also deployed solid-fueled ICBMs (RT-23, RT-2PM, and RT-2UTTH), but retains two liquid-fueled ICBMs (R-36 and UR-100N). All solid-fueled ICBMs on both sides had three initial solid stages, and those with multiple independently targeted warheads had a precision maneuverable bus used to fine-tune the trajectory of the re-entry vehicles.
Liquid-fueled rockets have higher specific impulse
than solid rockets and are capable of being throttled, shut down, and
restarted. Only the combustion chamber of a liquid-fueled rocket needs
to withstand high combustion pressures and temperatures. Cooling can be
done regeneratively with the liquid propellant. On vehicles employing turbopumps,
the propellant tanks are at a lower pressure than the combustion
chamber, decreasing tank mass. For these reasons, most orbital launch
vehicles use liquid propellants.
The primary specific impulse advantage of liquid propellants is
due to the availability of high-performance oxidizers. Several
practical liquid oxidizers (liquid oxygen, dinitrogen tetroxide, and hydrogen peroxide) are available which have better specific impulse than the ammonium perchlorate used in most solid rockets when paired with suitable fuels.
The main difficulties with liquid propellants are also with the oxidizers. Storable oxidizers, such as nitric acid and nitrogen tetroxide,
tend to be extremely toxic and highly reactive, while cryogenic
propellants by definition must be stored at low temperature and can also
have reactivity/toxicity issues. Liquid oxygen (LOX) is the only flown cryogenic oxidizer. Others such as FLOX, a fluorine/LOX mix, have never been flown due to instability, toxicity, and explosivity. Several other unstable, energetic, and toxic oxidizers have been proposed: liquid ozone (O3), ClF3, and ClF5.
Liquid-fueled rockets require potentially troublesome valves,
seals, and turbopumps, which increase the cost of the launch vehicle.
Turbopumps are particularly troublesome due to high performance
requirements.
Current cryogenic types
Liquid oxygen (LOX) and highly refined kerosene (RP-1). Used for the first stages of the Atlas V, Falcon 9, Falcon Heavy, Soyuz, Zenit, Angara, and Long March 6,
among others. This combination is widely regarded as the most practical
for boosters that lift off at ground level and therefore must operate
at full atmospheric pressure.
Dinitrogen tetroxide (N2O4) and hydrazine (N2H4), MMH, or UDMH.
Used in military, orbital, and deep-space rockets because both liquids
are storable for long periods at reasonable temperatures and pressures.
N2O4/UDMH is the main fuel for the Proton rocket, older Long March rockets (LM 1-4), PSLV, Fregat, and Briz-M upper stages. This combination is hypergolic,
making for attractively simple ignition sequences. The major
inconvenience is that these propellants are highly toxic and require
careful handling.
Monopropellants such as hydrogen peroxide, hydrazine, and nitrous oxide are primarily used for attitude control and spacecraft station-keeping
where their long-term storability, simplicity of use, and ability to
provide the tiny impulses needed outweighs their lower specific impulse
as compared to bipropellants. Hydrogen peroxide is also used to drive
the turbopumps on the first stage of the Soyuz launch vehicle.
Mixture ratio
The
theoretical exhaust velocity of a given propellant chemistry is
proportional to the energy released per unit of propellant mass
(specific energy). In chemical rockets, unburned fuel or oxidizer
represents the loss of chemical potential energy, which reduces the specific energy. However, most rockets run fuel-rich mixtures, which result in lower theoretical exhaust velocities.
However, fuel-rich mixtures also have lower molecular weight exhaust species. The nozzle of the rocket converts the thermal energy of the propellants into directed kinetic energy.
This conversion happens in the time it takes for the propellants to
flow from the combustion chamber through the engine throat and out the
nozzle, usually on the order of one millisecond. Molecules store thermal
energy in rotation, vibration, and translation, of which only the
latter can easily be used to add energy to the rocket stage. Molecules
with fewer atoms (like CO and H2) have fewer available vibrational and rotational modes than molecules with more atoms (like CO2 and H2O). Consequently, smaller molecules store less vibrational and rotational energy
for a given amount of heat input, resulting in more translation energy
being available to be converted to kinetic energy. The resulting
improvement in nozzle efficiency is large enough that real rocket
engines improve their actual exhaust velocity by running rich mixtures
with somewhat lower theoretical exhaust velocities.
The effect of exhaust molecular weight on nozzle efficiency is
most important for nozzles operating near sea level. High-expansion
rockets operating in a vacuum see a much smaller effect, and so are run
less rich.
LOX/hydrocarbon rockets are run slightly rich (O/F mass ratio of 3 rather than stoichiometric
of 3.4 to 4) because the energy release per unit mass drops off quickly
as the mixture ratio deviates from stoichiometric. LOX/LH2
rockets are run very rich (O/F mass ratio of 4 rather than
stoichiometric 8) because hydrogen is so light that the energy release
per unit mass of propellant drops very slowly with extra hydrogen. In
fact, LOX/LH2 rockets are generally limited in how rich they
run by the performance penalty of the mass of the extra hydrogen tankage
instead of the underlying chemistry.[10]
Another reason for running rich is that off-stoichiometric
mixtures burn cooler than stoichiometric mixtures, which makes engine
cooling easier. Because fuel-rich combustion products are less
chemically reactive (corrosive)
than oxidizer-rich combustion products, a vast majority of rocket
engines are designed to run fuel-rich. At least one exception exists:
the Russian RD-180 preburner, which burns LOX and RP-1 at a ratio of 2.72.
Additionally, mixture ratios can be dynamic during launch. This
can be exploited with designs that adjust the oxidizer-to-fuel ratio
(along with overall thrust) throughout a flight to maximize overall
system performance. For instance, during lift-off, thrust is more
valuable than specific impulse, and careful adjustment of the O/F ratio
may allow higher thrust levels. Once the rocket is away from the
launchpad, the engine O/F ratio can be tuned for higher efficiency.
Propellant density
Although liquid hydrogen gives a high Isp,
its low density is a disadvantage: hydrogen occupies about 7 times more
volume per kilogram than dense fuels such as kerosene. The fuel
tankage, plumbing, and pump must be correspondingly larger. This
increases the vehicle's dry mass, reducing performance. Liquid hydrogen
is also relatively expensive to produce and store, and causes
difficulties with design, manufacture, and operation of the vehicle.
However, liquid hydrogen is extremely well-suited to upper stage use
where Isp is at a premium and thrust-to-weight ratios are less relevant.
Dense-propellant launch vehicles have a higher takeoff mass due to lower Isp,
but can more easily develop high takeoff thrusts due to the reduced
volume of engine components. This means that vehicles with dense-fueled
booster stages reach orbit earlier, minimizing losses due to gravity drag and reducing the effective delta-v requirement.
The proposed tripropellant rocket uses mainly dense fuel while at low altitude and switches across to hydrogen at higher altitude. Studies in the 1960s proposed single-stage-to-orbit vehicles using this technique. The Space Shuttle
approximated this by using dense solid rocket boosters for the majority
of the thrust during the first 120 seconds. The main engines burned a
fuel-rich hydrogen and oxygen mixture, operating continuously throughout
the launch but providing the majority of thrust at higher altitudes
after SRB burnout.
Hybrid propellants consist of a storable liquid oxidizer used with a
solid fuel, which retains most virtues of both liquids (high ISP) and solids (simplicity).
A hybrid-propellant rocket usually has a solid fuel and a liquid or NEMA oxidizer.The fluid oxidizer can make it possible to throttle and restart the
motor just like a liquid-fueled rocket. Hybrid rockets can also be
environmentally safer than solid rockets since some high-performance
solid-phase oxidizers contain chlorine (specifically composites with ammonium perchlorate),
versus the more benign liquid oxygen or nitrous oxide often used in
hybrids. This is only true for specific hybrid systems. There have been
hybrids which have used chlorine or fluorine compounds as oxidizers and hazardous materials such as beryllium
compounds mixed into the solid fuel grain. Because just one constituent
is a fluid, hybrids can be simpler than liquid rockets that depend on
the rocket's acceleration to transport the fluid into the combustion
chamber. Fewer fluids typically mean fewer and smaller piping systems,
valves, and pumps.
Hybrid motors suffer two major drawbacks. The first, shared with
solid rocket motors, is that the casing around the fuel grain must be
built to withstand the full combustion pressure and often extreme
temperatures as well. However, modern composite structures handle this
problem well, and when used with nitrous oxide
and a solid rubber propellant (HTPB), a relatively small percentage of
fuel is needed anyway, so the combustion chamber is not especially
large.
The primary remaining difficulty with hybrids is with mixing the
propellants during the combustion process. In solid propellants, the
oxidizer and fuel are mixed in a factory in carefully controlled
conditions. Liquid propellants are generally mixed by the injector at
the top of the combustion chamber, which directs many small swift-moving
streams of fuel and oxidizer into one another. Liquid-fueled rocket
injector design has been studied at great length and still resists
reliable performance prediction. In a hybrid motor, the mixing happens
at the melting or evaporating surface of the fuel. The mixing is not a
well-controlled process and generally, quite a lot of propellant is left
unburned, which limits the efficiency of the motor. The combustion rate of the
fuel is largely determined by the oxidizer flux and exposed fuel surface
area. This combustion rate is not usually sufficient for high-power
operations such as boost stages unless the surface area or oxidizer flux
is high. Too-high oxidizer flux can lead to flooding and loss of
flame-holding, which locally extinguishes the combustion. Surface area
can be increased, typically by longer grains or multiple ports, but this
can increase combustion chamber size, reduce grain strength, and reduce
volumetric loading. Additionally, as the burn continues, the hole down
the center of the grain (the "port") widens, and the mixture ratio tends
to become more oxidizer-rich.
There has been much less development of hybrid motors than of
solid and liquid motors. For military use, ease of handling and
maintenance have driven the use of solid rockets. For orbital work,
liquid fuels are more efficient than hybrids, and most development has
concentrated there. There has recently been an increase in hybrid motor development for nonmilitary suborbital work:
Several universities have recently experimented with hybrid rockets. Brigham Young University, the University of Utah, and Utah State University launched a student-designed rocket called Unity IV in 1995 which burned the solid fuel hydroxy-terminated polybutadiene (HTPB) with an oxidizer of gaseous oxygen, and in 2003 launched a larger version which burned HTPB with nitrous oxide. Stanford University researches nitrous-oxide/paraffin wax hybrid motors. UCLA has launched hybrid rockets through an undergraduate student group since 2009 using HTPB.
The Rochester Institute of Technology
was building an HTPB hybrid rocket to launch small payloads into space
and to several near-Earth objects. Its first launch was in the Summer
of 2007.
Scaled Composites SpaceShipOne, the first private crewed spacecraft, was powered by a hybrid rocket burning HTPB with nitrous oxide: RocketMotorOne. The hybrid rocket engine was manufactured by SpaceDev.
SpaceDev partially based its motors on experimental data collected from
the testing of AMROC's (American Rocket Company) motors at NASA's Stennis Space Center's E1 test stand.
Some rocket designs impart energy to their propellants with external energy sources. For example, water rockets use a compressed gas, typically air, to force the water reaction mass out of the rocket.
Thermal rockets use inert propellants of low molecular weight that are chemically compatible with the heating mechanism at high temperatures. Solar thermal rockets and nuclear thermal rockets typically propose to use liquid hydrogen for a specific impulse
of around 600–900 seconds, or in some cases water that is exhausted as
steam for a specific impulse of about 190 seconds. Nuclear thermal
rockets use the heat of nuclear fission
to add energy to the propellant. Some designs separate the nuclear fuel
and working fluid, minimizing the potential for radioactive
contamination, but nuclear fuel loss was a persistent problem during
real-world testing programs. Solar thermal rockets use concentrated
sunlight to heat a propellant, rather than using a nuclear reactor.
For low-performance applications, such as attitude control jets, compressed gases such as nitrogen have been employed. Energy is stored in the pressure of the inert gas. However, due to the
low density of all practical gases and high mass of the pressure vessel
required to contain it, compressed gases see little current use.
Wallace did extensive fieldwork, starting in the Amazon River basin. He then did fieldwork in the Malay Archipelago, where he identified the faunal divide now termed the Wallace Line,
which separates the Indonesian archipelago into two distinct parts: a
western portion in which the animals are largely of Asian origin, and an
eastern portion where the fauna reflect Australasia.
He was considered the 19th century's leading expert on the geographical
distribution of animal species, and is sometimes called the "father of biogeography", or more specifically of zoogeography.
Wallace was one of the leading evolutionary thinkers of the 19th century, working on warning coloration in animals and reinforcement (sometimes known as the Wallace effect), a way that natural selection could contribute to speciation by encouraging the development of barriers against hybridisation. Wallace's 1904 book Man's Place in the Universe was the first serious attempt by a biologist to evaluate the likelihood of life on other planets. He was one of the first scientists to write a serious exploration of whether there was life on Mars.
Aside from scientific work, he was a social activist, critical of
what he considered to be an unjust social and economic system in
19th-century Britain. His advocacy of spiritualism and his belief in a non-material origin
for the higher mental faculties of humans strained his relationship
with other scientists. He was one of the first prominent scientists to
raise concerns over the environmental impact of human activity.
He wrote prolifically on both scientific and social issues; his account
of his adventures and observations during his explorations in Southeast
Asia, The Malay Archipelago, was first published in 1869. It continues to be both popular and highly regarded.
Biography
Early life
Alfred Russel Wallace was born on 8 January 1823 in Llanbadoc, Monmouthshire. He was the eighth of nine children born to Mary Anne Wallace (née Greenell)
and Thomas Vere Wallace. His mother was English, while his father was
of Scottish ancestry. His family claimed a connection to William Wallace, a leader of Scottish forces during the Wars of Scottish Independence in the 13th century.
Wallace's father graduated in law but never practised it. He
owned some income-generating property, but bad investments and failed
business ventures resulted in a steady deterioration of the family's
financial position. Wallace's mother was from a middle-class family of Hertford, to which place his family moved when Wallace was five years old. He attended Hertford Grammar School until 1837, when he reached the age of 14, the normal leaving age for a pupil not going on to university.
A photograph from Wallace's autobiography shows the building Wallace and his brother John designed and built for the Neath Mechanics' Institute.
Wallace then moved to London to board with his older brother John, a
19-year-old apprentice builder. This was a stopgap measure until
William, his oldest brother, was ready to take him on as an apprentice surveyor. While in London, Alfred attended lectures and read books at the London Mechanics Institute. Here he was exposed to the radical political ideas of the Welsh social reformer Robert Owen and of the English-born political theorist Thomas Paine.
He left London in 1837 to live with William and work as his apprentice
for six years. They moved repeatedly to different places in Mid-Wales.
Then at the end of 1839, they moved to Kington, Herefordshire, near the Welsh border, before eventually settling at Neath in Wales. Between 1840 and 1843, Wallace worked as a land surveyor in the countryside of the west of England and Wales. The natural history of his surroundings aroused his interest; from 1841 he collected flowers and plants as an amateur botanist.
One result of Wallace's early travels is a modern controversy about his nationality. Since he was born in Monmouthshire, some sources have considered him to be Welsh. Other historians have questioned this because neither of his parents
were Welsh, his family only briefly lived in Monmouthshire, the Welsh
people Wallace knew in his childhood considered him to be English, and
because he consistently referred to himself as English rather than
Welsh. One Wallace scholar has stated that the most reasonable
interpretation is therefore that he was an Englishman born in Wales.
In 1843 Wallace's father died, and a decline in demand for surveying meant William's business no longer had work available. For a short time Wallace was unemployed, then early in 1844 he was engaged by the Collegiate School in Leicester to teach drawing, mapmaking, and surveying. He had already read George Combe's The Constitution of Man, and after Spencer Hall lectured on mesmerism, Wallace as well as some of the older pupils tried it out. Wallace spent many hours at the town library in Leicester; he read An Essay on the Principle of Population by Thomas Robert Malthus, Alexander von Humboldt's Personal Narrative, Darwin's Journal (The Voyage of the Beagle), and Charles Lyell's Principles of Geology. One evening Wallace met the entomologist Henry Bates, who was 19 years old, and had published an 1843 paper on beetles in the journal Zoologist. He befriended Wallace and started him collecting insects.
When Wallace's brother William died in March 1845, Wallace left
his teaching position to assume control of his brother's firm in Neath,
but his brother John and he were unable to make the business work. After
a few months, he found work as a civil engineer for a nearby firm that
was working on a survey for a proposed railway in the Vale of Neath.
Wallace's work on the survey was largely outdoors in the countryside,
allowing him to indulge his new passion for collecting insects. Wallace
persuaded his brother John to join him in starting another architecture
and civil engineering firm. It carried out projects including the design
of a building for the Neath Mechanics' Institute, founded in 1843. During this period, he exchanged letters with Bates about books. By the end of 1845, Wallace was convinced by Robert Chambers's anonymously published treatise on progressive development, Vestiges of the Natural History of Creation, but he found Bates was more critical. Wallace re-read Darwin's Journal,
and on 11 April 1846 wrote "As the Journal of a scientific traveller,
it is second only to Humboldt's 'Personal Narrative'—as a work of
general interest, perhaps superior to it."
William Jevons, the founder of the Neath institute, was impressed
by Wallace and persuaded him to give lectures there on science and
engineering. In the autumn of 1846, Wallace and his brother John
purchased a cottage near Neath, where they lived with their mother and
sister Fanny.
Exploration and study of the natural world
Inspired by the chronicles of earlier and contemporary travelling naturalists, Wallace decided to travel abroad. He later wrote that Darwin's Journal and Humboldt's Personal Narrative were "the two works to whose inspiration I owe my determination to visit the tropics as a collector." After reading A Voyage up the River Amazon by William Henry Edwards,
Wallace and Bates estimated that by collecting and selling natural
history specimens such as birds and insects they could meet their costs,
with the prospect of good profits. They therefore engaged as their agent Samuel Stevens
who would advertise and arrange sales to institutions and private
collectors, for a commission of 20% on sales plus 5% on despatching
freight and remittances of money.
In 1848, Wallace and Bates left for Brazil aboard the Mischief. They intended to collect insects and other animal specimens in the Amazon Rainforest
for their private collections, selling the duplicates to museums and
collectors back in Britain to fund the trip. Wallace hoped to gather
evidence of the transmutation of species. Bates and he spent most of their first year collecting near Belém,
then explored inland separately, occasionally meeting to discuss their
findings. In 1849, they were briefly joined by another young explorer,
the botanist Richard Spruce, along with Wallace's younger brother Herbert. Herbert soon left (dying two years later from yellow fever), but Spruce, like Bates, would spend over ten years collecting in South America. Wallace spent four years charting the Rio Negro, collecting specimens and making notes on the peoples and languages he encountered as well as the geography, flora, and fauna.
On 12 July 1852, Wallace embarked for the UK on the brig Helen.
After 25 days at sea, the ship's cargo caught fire, and the crew was
forced to abandon ship. All the specimens Wallace had on the ship,
mostly collected during the last, and most interesting, two years of his
trip, were lost. He managed to save a few notes and pencil sketches,
but little else. Wallace and the crew spent ten days in an open boat
before being picked up by the brig Jordeson, which was sailing from Cuba to London. The Jordeson's
provisions were strained by the unexpected passengers, but after a
difficult passage on short rations, the ship reached its destination on 1
October 1852.
The lost collection had been insured for £200 by Stevens. After his return to Britain, Wallace spent 18 months in London living
on the insurance payment, and selling a few specimens that had been
shipped home. During this period, despite having lost almost all the
notes from his South American expedition, he wrote six academic papers
(including "On the Monkeys of the Amazon") and two books, Palm Trees of the Amazon and Their Uses and Travels on the Amazon. At the same time, he made connections with several other British naturalists.
A map from The Malay Archipelago
shows the physical geography of the archipelago and Wallace's travels
around the area. The thin black lines indicate where Wallace travelled;
the red lines indicate chains of volcanoes.
Bates and others were collecting in the Amazon area, Wallace was more interested in new opportunities in the Malay Archipelago as demonstrated by the travel writings of Ida Laura Pfeiffer, and valuable insect specimens she collected which Stevens sold as her agent. In March 1853 Wallace wrote to Sir James Brooke, Rajah of Sarawak, who was then in London, and who arranged assistance in Sarawak for Wallace. In June Wallace wrote to Murchison at the Royal Geographical Society (RGS) for support, proposing to again fund his exploring entirely from sale of duplicate collections. He later recalled that, while researching in the insect-room of the British Museum,
he was introduced to Darwin and they "had a few minutes' conversation."
After presenting a paper and a large map of the Rio Negro to the RGS,
Wallace was elected a Fellow of the society on 27 February 1854. Free passage arranged on Royal Navy ships was stalled by the Crimean War, but eventually the RGS funded first class travel by P&O
steamships. Wallace and a young assistant, Charles Allen, embarked at
Southampton on 4 March 1854. After the overland journey to Suez and
another change of ship at Ceylon they disembarked at Singapore on 19
April 1854.
From 1854 to 1862, Wallace travelled around the islands of the Malay Archipelago or East Indies (now Singapore, Malaysia and Indonesia). His main objective "was to obtain specimens of natural history, both
for my private collection and to supply duplicates to museums and
amateurs". In addition to Allen, he "generally employed one or two, and
sometimes three Malay servants" as assistants, and paid large numbers of
local people at various places to bring specimens. His total was
125,660 specimens, most of which were insects including more than 83,000
beetles, Several thousand of the specimens represented species new to science, Overall, more than thirty men worked for him at some stage as full-time
paid collectors. He also hired guides, porters, cooks and boat crews,
so well over 100 individuals worked for him.
After collecting expeditions to Bukit Timah Hill in Singapore, and to Malacca, Wallace and Allen reached Sarawak in October 1854, and were welcomed at Kuching by Sir James Brooke's (then) heir Captain John Brooke. Wallace hired a Malay named Ali as a general servant and cook, and spent the early 1855 wet season in a small Dyak house at the foot of Mount Santubong, overlooking a branch outlet of the Sarawak River. He read about species distribution, notes on Pictets's Palaeontology, and wrote his "Sarawak Paper". In March he moved to the Simunjon coal-works, operated by the Borneo Company under Ludvig Verner Helms, and supplemented collecting by paying workers a cent for each insect. A specimen of the previously unknown gliding tree frog Rhacophorus nigropalmatus (now called Wallace's flying frog) came from a Chinese workman who told Wallace that it glided down. Local people also assisted with shooting orangutans. They spent time with Sir James, then in February 1856 Allen chose to stay on with the missionaries at Kuching.
On reaching Singapore in May 1856, Wallace hired a bird-skinner. With Ali as cook, they collected for two days on Bali, then from 17 June to 30 August on Lombok. In December 1856, Darwin had written to contacts worldwide to get specimens for his continuing research into variation under domestication. At Lombok's port city, Ampanam, Wallace wrote telling his agent, Stevens, about specimens shipped, including a domestic duck variety "for Mr. Darwin & he would perhaps also like the jungle cock, which is often domesticated here & is doubtless one of the originals of the domestic breed of poultry." In the same letter, Wallace said birds from Bali and Lombok, divided by a
narrow strait, "belong to two quite distinct zoological provinces, of
which they form the extreme limits", Java, Borneo, Sumatra and Malacca, and Australia and the Moluccas. Stevens arranged publication of relevant paragraphs in the January 1857 issue of The Zoologist. After further investigation, the zoogeographical boundary eventually became known as the Wallace Line.
Ali became Wallace's most trusted assistant, a skilled collector
and researcher. Wallace collected and preserved the delicate insect
specimens, while most of the birds were collected and prepared by his
assistants; of those, Ali collected and prepared around 5000. While exploring the archipelago, Wallace refined his thoughts about evolution, and had his famous insight on natural selection.
In 1858 he sent an article outlining his theory to Darwin; it was
published, along with a description of Darwin's theory, that same year.
Accounts of Wallace's studies and adventures were eventually published in 1869 as The Malay Archipelago.
This became one of the most popular books of scientific exploration of
the 19th century, and has never been out of print. It was praised by
scientists such as Darwin (to whom the book was dedicated), by Lyell,
and by non-scientists such as the novelist Joseph Conrad. Conrad called the book his "favorite bedside companion" and used information from it for several of his novels, especially Lord Jim. A set of 80 bird skeletons Wallace collected in Indonesia are held in the Cambridge University Museum of Zoology, and described as of exceptional historical significance.
In 1862, Wallace returned to Britain, where he moved in with his
sister Fanny Sims and her husband Thomas. While recovering from his
travels, Wallace organised his collections and gave numerous lectures
about his adventures and discoveries to scientific societies such as the
Zoological Society of London. Later that year, he visited Darwin at Down House, and became friendly with both Lyell and the philosopher Herbert Spencer. During the 1860s, Wallace wrote papers and gave lectures defending
natural selection. He corresponded with Darwin about topics including sexual selection, warning coloration, and the possible effect of natural selection on hybridisation and the divergence of species. In 1865, he began investigating spiritualism.
After a year of courtship, Wallace became engaged in 1864 to a
young woman whom, in his autobiography, he would only identify as Miss
L. Miss L. was the daughter of Lewis Leslie who played chess with
Wallace, but to Wallace's great dismay, she broke off the engagement. In 1866, Wallace married Annie Mitten. Wallace had been introduced to
Mitten through the botanist Richard Spruce, who had befriended Wallace
in Brazil and who was a friend of Annie Mitten's father, William Mitten, an expert on mosses. In 1872, Wallace built the Dell, a house of concrete, on land he leased in Grays
in Essex, where he lived until 1876. The Wallaces had three children:
Herbert (1867–1874), Violet (1869–1945), and William (1871–1951).
Financial struggles
In
the late 1860s and 1870s, Wallace was very concerned about the
financial security of his family. While he was in the Malay Archipelago,
the sale of specimens had brought in a considerable amount of money,
which had been carefully invested by the agent who sold the specimens
for Wallace. On his return to the UK, Wallace made a series of bad
investments in railways and mines that squandered most of the money, and
he found himself badly in need of the proceeds from the publication of The Malay Archipelago.
Despite assistance from his friends, he was never able to secure a
permanent salaried position such as a curatorship in a museum. To
remain financially solvent, Wallace worked grading government
examinations, wrote 25 papers for publication between 1872 and 1876 for
various modest sums, and was paid by Lyell and Darwin to help edit some
of their works.
In 1876, Wallace needed a £500 advance from the publisher of The Geographical Distribution of Animals to avoid having to sell some of his personal property. Darwin was very aware of Wallace's financial difficulties and lobbied
long and hard to get Wallace awarded a government pension for his
lifetime contributions to science. When the £200 annual pension was
awarded in 1881, it helped to stabilise Wallace's financial position by
supplementing the income from his writings.
Social activism
Article written by Professor Wallace, published in the report of the proceedings of the International Worker's Congress
In 1881, Wallace was elected as the first president of the newly
formed Land Nationalisation Society. In the next year, he published a
book, Land Nationalisation; Its Necessity and Its Aims, on the subject. He criticised the UK's free trade policies for the negative impact they had on working-class people. In 1889, Wallace read Looking Backward by Edward Bellamy and declared himself a socialist, despite his earlier foray as a speculative investor. After reading Progress and Poverty, the bestselling book by the progressive land reformist Henry George, Wallace described it as "Undoubtedly the most remarkable and important book of the present century."
Wallace opposed eugenics,
an idea supported by other prominent 19th-century evolutionary
thinkers, on the grounds that contemporary society was too corrupt and
unjust to allow any reasonable determination of who was fit or unfit. In his 1890 article "Human Selection" he wrote, "Those who succeed in
the race for wealth are by no means the best or the most
intelligent ..." He said, "The world does not want the eugenicist to set it straight,"
"Give the people good conditions, improve their environment, and all
will tend towards the highest type. Eugenics is simply the meddlesome
interference of an arrogant, scientific priestcraft."
Wallace wrote on other social and political topics, including in support of women's suffrage and repeatedly on the dangers and wastefulness of militarism. In an 1899 essay, he called for popular opinion to be rallied against
warfare by showing people "that all modern wars are dynastic; that they
are caused by the ambition, the interests, the jealousies, and the
insatiable greed of power of their rulers, or of the great mercantile
and financial classes which have power and influence over their rulers;
and that the results of war are never good for the people, who yet bear
all its burthens (burdens)". In a letter published by the Daily Mail
in 1909, with aviation in its infancy, he advocated an international
treaty to ban the military use of aircraft, arguing against the idea
"that this new horror is 'inevitable', and that all we can do is to be
sure and be in the front rank of the aerial assassins—for surely no
other term can so fitly describe the dropping of, say, ten thousand
bombs at midnight into an enemy's capital from an invisible flight of
airships."
In 1898, Wallace published The Wonderful Century: Its Successes and Its Failures,
about developments in the 19th century. The first part of the book
covered the major scientific and technical advances of the century; the
second part covered what Wallace considered to be its social failures
including the destruction and waste of wars and arms races, the rise of
the urban poor and the dangerous conditions in which they lived and
worked, a harsh criminal justice system that failed to reform criminals,
abuses in a mental health system based on privately owned sanatoriums,
the environmental damage caused by capitalism, and the evils of European
colonialism. Wallace continued his social activism for the rest of his life, publishing the book The Revolt of Democracy just weeks before his death.
Further scientific work
In 1880, he published Island Life as a sequel to The Geographic Distribution of Animals.
In November 1886, Wallace began a ten-month trip to the United States
to give a series of popular lectures. Most of the lectures were on
Darwinism (evolution through natural selection), but he also gave
speeches on biogeography,
spiritualism, and socio-economic reform. During the trip, he was
reunited with his brother John who had emigrated to California years
before. He spent a week in Colorado, with the American botanist Alice Eastwood as his guide, exploring the flora of the Rocky Mountains and gathering evidence that would lead him to a theory on how glaciation
might explain certain commonalities between the mountain flora of
Europe, Asia and North America, which he published in 1891 in the paper
"English and American Flowers". He met many other prominent American
naturalists and viewed their collections. His 1889 book Darwinism used information he collected on his American trip and information he had compiled for the lectures.
Death
Wallace's grave in Broadstone Cemetery, Dorset, restored by the A. R. Wallace Memorial Fund in 2000. It features a fossil tree trunk 7 feet (2.1 m) tall from Portland, mounted on a block of Purbeck limestone.
On 7 November 1913, Wallace died at home, aged 90, in the country
house he called Old Orchard, which he had built a decade earlier. His death was widely reported in the press. The New York Times
called him "the last of the giants [belonging] to that wonderful group
of intellectuals composed of Darwin, Huxley, Spencer, Lyell, Owen, and
other scientists, whose daring investigations revolutionized and
evolutionized the thought of the century". Another commentator in the same edition said: "No apology need be made
for the few literary or scientific follies of the author of that great
book on the 'Malay Archipelago'." (Vol.1, Vol.2)
Some of Wallace's friends suggested that he be buried in Westminster Abbey, but his wife followed his wishes and had him buried in the small cemetery at Broadstone, Dorset. Several prominent British scientists formed a committee to have a
medallion of Wallace placed in Westminster Abbey near where Darwin had
been buried. The medallion was unveiled on 1 November 1915.
Theory of evolution
Early evolutionary thinking
Wallace
began his career as a travelling naturalist who already believed in the
transmutation of species. The concept had been advocated by Jean-Baptiste Lamarck, Geoffroy Saint-Hilaire, Erasmus Darwin, and Robert Grant, among others. It was widely discussed, but not generally accepted by leading naturalists, and was considered to have radical, even revolutionary connotations.Prominent anatomists and geologists such as Georges Cuvier, Richard Owen, Adam Sedgwick, and Lyell attacked transmutation vigorously. It has been suggested that Wallace accepted the idea of the
transmutation of species in part because he was always inclined to
favour radical ideas in politics, religion and science, and because he was unusually open to marginal, even fringe, ideas in science.
Wallace was profoundly influenced by Robert Chambers's Vestiges of the Natural History of Creation, a controversial work of popular science published anonymously in 1844. It advocated an evolutionary origin for the Solar System, the Earth, and living things. Wallace wrote to Henry Bates in 1845 describing it as "an ingenious
hypothesis strongly supported by some striking facts and analogies, but
which remains to be proven by ... more research". In 1847, he wrote to Bates that he would "like to take some one family
[of beetles] to study thoroughly ... with a view to the theory of the
origin of species."
Wallace planned fieldwork to test the evolutionary hypothesis
that closely related species should inhabit neighbouring territories. During his work in the Amazon basin,
he came to realise that geographical barriers—such as the Amazon and
its major tributaries—often separated the ranges of closely allied
species. He included these observations in his 1853 paper "On the
Monkeys of the Amazon". Near the end of the paper he asked the question,
"Are very closely allied species ever separated by a wide interval of
country?"
In February 1855, while working in Sarawak on the island of Borneo, Wallace wrote "On the Law which has Regulated the Introduction of New Species". The paper was published in the Annals and Magazine of Natural History in September 1855. In this paper, he discussed observations of the geographic and geologic
distribution of both living and fossil species, a field that became
biogeography. His conclusion that "Every species has come into existence
coincident both in space and time with a closely allied species" has
come to be known as the "Sarawak Law", answering his own question in his
paper on the monkeys of the Amazon basin. Although it does not mention
possible mechanisms for evolution, this paper foreshadowed the momentous
paper he would write three years later.
The paper challenged Lyell's belief that species were immutable.
Although Darwin had written to him in 1842 expressing support for
transmutation, Lyell had continued to be strongly opposed to the idea.
Around the start of 1856, he told Darwin about Wallace's paper, as did Edward Blyth
who thought it "Good! Upon the whole! ... Wallace has, I think put the
matter well; and according to his theory the various domestic races of
animals have been fairly developed into species." Despite this hint, Darwin mistook Wallace's conclusion for the progressive creationism
of the time, writing that it was "nothing very new ... Uses my simile
of tree [but] it seems all creation with him." Lyell was more impressed,
and opened a notebook on species in which he grappled with the
consequences, particularly for human ancestry. Darwin had already shown
his theory to their mutual friend Joseph Hooker
and now, for the first time spelt out the full details of natural
selection to Lyell. Although Lyell could not agree, he urged Darwin to
publish to establish priority. Darwin demurred at first, but began
writing up a species sketch of his continuing work in May 1856.
By February 1858, Wallace had been convinced by his biogeographical
research in the Malay Archipelago that evolution was real. He later
wrote in his autobiography that the problem was of how species change
from one well-marked form to another. He stated that it was while he was in bed with a fever that he thought
about Malthus's idea of positive checks on human population, and had the
idea of natural selection. His autobiography says that he was on the
island of Ternate at the time; but the evidence of his journal suggests that he was in fact on the island of Gilolo. From 1858 to 1861, he rented a house on Ternate from the Dutchman Maarten Dirk van Renesse van Duivenbode, which he used as a base for expeditions to other islands such as Gilolo.
Wallace describes how he discovered natural selection as follows:
It then occurred to me that these
causes or their equivalents are continually acting in the case of
animals also; and as animals usually breed much more quickly than does
mankind, the destruction every year from these causes must be enormous
to keep down the numbers of each species, since evidently they do not
increase regularly from year to year, as otherwise the world would long
ago have been crowded with those that breed most quickly. Vaguely
thinking over the enormous and constant destruction which this implied,
it occurred to me to ask the question, why do some die and some live?
And the answer was clearly, on the whole the best fitted live ... and
considering the amount of individual variation that my experience as a
collector had shown me to exist, then it followed that all the changes
necessary for the adaptation of the species to the changing conditions
would be brought about ... In this way every part of an animals
organization could be modified exactly as required, and in the very
process of this modification the unmodified would die out, and thus the
definite characters and the clear isolation of each new species would be
explained.
Wallace had once briefly met Darwin, and was one of the
correspondents whose observations Darwin used to support his own
theories. Although Wallace's first letter to Darwin has been lost,
Wallace carefully kept the letters he received. In the first letter, dated 1 May 1857, Darwin commented that Wallace's
letter of 10 October which he had recently received, as well as
Wallace's paper "On the Law which has regulated the Introduction of New
Species" of 1855, showed that they thought alike, with similar
conclusions, and said that he was preparing his own work for publication
in about two years time. The second letter, dated 22 December 1857, said how glad he was that
Wallace was theorising about distribution, adding that "without
speculation there is no good and original observation" but commented
that "I believe I go much further than you". Wallace believed this and sent Darwin his February 1858 essay, "On the Tendency of Varieties to Depart Indefinitely From the Original Type", asking Darwin to review it and pass it to Charles Lyell if he thought it worthwhile. Although Wallace had sent several articles for journal publication
during his travels through the Malay archipelago, the Ternate essay was
in a private letter. Darwin received the essay on 18 June 1858. Although
the essay did not use Darwin's term "natural selection", it did outline
the mechanics of an evolutionary divergence of species from similar
ones due to environmental pressures. In this sense, it was very similar
to the theory that Darwin had worked on for 20 years, but had yet to
publish. Darwin sent the manuscript to Charles Lyell with a letter
saying "he could not have made a better short abstract! Even his terms
now stand as heads of my chapters ... he does not say he wishes me to
publish, but I shall, of course, at once write and offer to send to any
journal." Distraught about the illness of his baby son, Darwin put the problem to Charles Lyell and Joseph Hooker,
who decided to publish the essay in a joint presentation together with
unpublished writings which highlighted Darwin's priority. Wallace's
essay was presented to the Linnean Society of London
on 1 July 1858, along with excerpts from an essay which Darwin had
disclosed privately to Hooker in 1847 and a letter Darwin had written to
Asa Gray in 1857.
Communication with Wallace in the far-off Malay Archipelago
involved months of delay, so he was not part of this rapid publication.
Wallace accepted the arrangement after the fact, happy that he had been
included at all, and never expressed bitterness in public or in private.
Darwin's social and scientific status was far greater than Wallace's,
and it was unlikely that, without Darwin, Wallace's views on evolution
would have been taken seriously. Lyell and Hooker's arrangement
relegated Wallace to the position of co-discoverer, and he was not the
social equal of Darwin or the other prominent British natural
scientists. All the same, the joint reading of their papers on natural
selection associated Wallace with the more famous Darwin. This, combined
with Darwin's (as well as Hooker's and Lyell's) advocacy on his behalf,
would give Wallace greater access to the highest levels of the
scientific community. The reaction to the reading was muted, with the president of the
Linnean Society remarking in May 1859 that the year had not been marked
by any striking discoveries; but, with Darwin's publication of On the Origin of Species
later in 1859, its significance became apparent. When Wallace returned
to the UK, he met Darwin. Although some of Wallace's opinions in the
ensuing years would test Darwin's patience, they remained on friendly
terms for the rest of Darwin's life.
Over the years, a few people have questioned this version of events. In the early 1980s, two books, one by Arnold Brackman and another by John Langdon Brooks,
suggested not only that there had been a conspiracy to rob Wallace of
his proper credit, but that Darwin had actually stolen a key idea from
Wallace to finish his own theory. These claims have been examined and
found unconvincing by a number of scholars. Shipping schedules show that, contrary to these accusations, Wallace's
letter could not have been delivered earlier than the date shown in
Darwin's letter to Lyell.
Defence of Darwin and his ideas
After Wallace returned to England in 1862, he became one of the staunchest defenders of Darwin's On the Origin of Species.
In an incident in 1863 that particularly pleased Darwin, Wallace
published the short paper "Remarks on the Rev. S. Haughton's Paper on
the Bee's Cell, And on the Origin of Species". This rebutted a paper by a
professor of geology at the University of Dublin that had sharply
criticised Darwin's comments in the Origin on how hexagonal honey bee cells could have evolved through natural selection. An even longer defence was a 1867 article in the Quarterly Journal of Science called "Creation by Law". It reviewed George Campbell, the 8th Duke of Argyll's book, The Reign of Law, which aimed to refute natural selection. After an 1870 meeting of the British Science Association,
Wallace wrote to Darwin complaining that there were "no opponents left
who know anything of natural history, so that there are none of the good
discussions we used to have".
Differences between Darwin and Wallace
Historians
of science have noted that, while Darwin considered the ideas in
Wallace's paper to be essentially the same as his own, there were
differences. Darwin emphasised competition between individuals of the same species
to survive and reproduce, whereas Wallace emphasised environmental
pressures on varieties and species forcing them to become adapted to
their local conditions, leading populations in different locations to
diverge.The historian of science Peter J. Bowler has suggested that in the paper he mailed to Darwin, Wallace might have been discussing group selection. Against this, Malcolm Kottler showed that Wallace was indeed discussing individual variation and selection.
Others have noted that Wallace appeared to have envisioned
natural selection as a kind of feedback mechanism that kept species and
varieties adapted to their environment (now called 'stabilizing", as
opposed to 'directional' selection). They point to a largely overlooked passage of Wallace's famous 1858 paper, in which he likened "this principle ... [to] the centrifugal governor of the steam engine, which checks and corrects any irregularities". The cybernetician and anthropologist Gregory Bateson
observed in the 1970s that, although writing it only as an example,
Wallace had "probably said the most powerful thing that'd been said in
the 19th Century". Bateson revisited the topic in his 1979 book Mind and Nature: A Necessary Unity, and other scholars have continued to explore the connection between natural selection and systems theory.
Illustration of Batesian mimicry: a wasp (top) mimicked by a beetle in Wallace's 1889 book Darwinism
Warning coloration was one of Wallace's contributions to the evolutionary biology of animal coloration. In 1867, Darwin wrote to Wallace about a problem in explaining how some
caterpillars could have evolved conspicuous colour schemes. Darwin had
come to believe that many conspicuous animal colour schemes were due to
sexual selection, but he saw that this could not apply to caterpillars.
Wallace responded that he and Bates had observed that many of the most
spectacular butterflies had a peculiar odour and taste, and that he had
been told by John Jenner Weir
that birds would not eat a certain kind of common white moth because
they found it unpalatable. Since the moth was as conspicuous at dusk as a
coloured caterpillar in daylight, it seemed likely that the conspicuous
colours served as a warning to predators and thus could have evolved
through natural selection. Darwin was impressed by the idea. At a later
meeting of the Entomological Society, Wallace asked for any evidence
anyone might have on the topic. In 1869, Weir published data from experiments and observations
involving brightly coloured caterpillars that supported Wallace's idea. Wallace attributed less importance than Darwin to sexual selection. In his 1878 book Tropical Nature and Other Essays, he wrote extensively about the coloration of animals and plants, and proposed alternative explanations for a number of cases Darwin had attributed to sexual selection. He revisited the topic at length in his 1889 book Darwinism. In 1890, he wrote a critical review in Nature of his friend Edward Bagnall Poulton's The Colours of Animals
which supported Darwin on sexual selection, attacking especially
Poulton's claims on the "aesthetic preferences of the insect world".
In 1889, Wallace wrote the book Darwinism, which explained and
defended natural selection. In it, he proposed the hypothesis that
natural selection could drive the reproductive isolation of two
varieties by encouraging the development of barriers against
hybridisation. Thus it might contribute to the development of new
species. He suggested the following scenario: When two populations of a
species had diverged beyond a certain point, each adapted to particular
conditions, hybrid offspring would be less adapted than either parent
form and so natural selection would tend to eliminate the hybrids.
Furthermore, under such conditions, natural selection would favour the
development of barriers to hybridisation, as individuals that avoided
hybrid matings would tend to have more fit offspring, and thus
contribute to the reproductive isolation of the two incipient species.
This idea came to be known as the Wallace effect, later called reinforcement. Wallace had suggested to Darwin that natural selection could play a
role in preventing hybridisation in private correspondence as early as
1868, but had not worked it out to this level of detail. It continues to be a topic of research in evolutionary biology today,
with both computer simulation and empirical results supporting its
validity.
Application of theory to humans, and role of teleology in evolution
An illustration from the chapter on the application of natural selection to humans in Wallace's 1889 book Darwinism shows a chimpanzee.
In 1864, Wallace published a paper, "The Origin of Human Races and
the Antiquity of Man Deduced from the Theory of 'Natural Selection'", applying the theory to humankind. Darwin had not yet publicly addressed the subject, although Thomas Huxley had in Evidence as to Man's Place in Nature.
Wallace explained the apparent stability of the human stock by pointing
to the vast gap in cranial capacities between humans and the great apes.
Unlike some other Darwinists, including Darwin himself, he did not
"regard modern primitives as almost filling the gap between man and
ape". He saw the evolution of humans in two stages: achieving a bipedal
posture that freed the hands to carry out the dictates of the brain, and
the "recognition of the human brain as a totally new factor in the
history of life". Wallace seems to have been the first evolutionist to see that the human
brain effectively made further specialisation of the body unnecessary. Wallace wrote the paper for the Anthropological Society of London to address the debate between the supporters of monogenism, the belief that all human races shared a common ancestor and were one species, and the supporters of polygenism,
who held that different races had separate origins and were different
species. Wallace's anthropological observations of Native Americans in
the Amazon, and especially his time living among the Dayak people
of Borneo, had convinced him that human beings were a single species
with a common ancestor. He still felt that natural selection might have
continued to act on mental faculties after the development of the
different races; and he did not dispute the nearly universal view among
European anthropologists of the time that Europeans were intellectually
superior to other races. According to political scientist Adam Jones, "Wallace found little difficulty in reconciling the extermination of native peoples with his progressive political views". In 1864, in the aforementioned paper, he stated "It is the same great
law of the preservation of favored races in the struggle for life, which
leads to the inevitable extinction of all those low and mentally
undeveloped populations with which Europeans come in contact." He argued that the natives die out due to an unequal struggle.
Shortly afterwards, Wallace became a spiritualist.
At about the same time, he began to maintain that natural selection
could not account for mathematical, artistic, or musical genius,
metaphysical musings, or wit and humour. He stated that something in
"the unseen universe of Spirit" had interceded at least three times in
history: the creation of life from inorganic matter; the introduction of
consciousness in the higher animals; and the generation of the higher
mental faculties in humankind. He believed that the raison d'être of the universe was the development of the human spirit.
While some historians have concluded that Wallace's belief that
natural selection was insufficient to explain the development of
consciousness and the higher functions of the human mind was directly
caused by his adoption of spiritualism, other scholars have disagreed,
and some maintain that Wallace never believed natural selection applied
to those areas. Reaction to Wallace's ideas on this topic among leading naturalists at
the time varied. Lyell endorsed Wallace's views on human evolution
rather than Darwin's. Wallace's belief that human consciousness could not be entirely a
product of purely material causes was shared by a number of prominent
intellectuals in the late 19th and early 20th centuries. All the same, many, including Huxley, Hooker, and Darwin himself, were critical of Wallace's views.
As the historian of science and sceptic Michael Shermer
has stated, Wallace's views in this area were at odds with two major
tenets of the emerging Darwinian philosophy. These were that evolution
was not teleological (purpose-driven), and that it was not anthropocentric (human-centred). Much later in his life Wallace returned to these themes, that evolution
suggested that the universe might have a purpose, and that certain
aspects of living organisms might not be explainable in terms of purely
materialistic processes. He set out his ideas in a 1909 magazine article
entitled The World of Life, later expanded into a book of the same name. Shermer commented that this anticipated ideas about design in nature
and directed evolution that would arise from religious traditions
throughout the 20th century.
Assessment of Wallace's role in history of evolutionary theory
In many accounts of the development of evolutionary theory, Wallace
is mentioned only in passing as simply being the stimulus to the
publication of Darwin's own theory. In reality, Wallace developed his own distinct evolutionary views which
diverged from Darwin's, and was considered by many (especially Darwin)
to be a leading thinker on evolution in his day, whose ideas could not
be ignored. One historian of science has pointed out that, through both
private correspondence and published works, Darwin and Wallace exchanged
knowledge and stimulated each other's ideas and theories over an
extended period. Wallace is the most-cited naturalist in Darwin's Descent of Man, occasionally in strong disagreement. Darwin and Wallace agreed on the importance of natural selection, and
some of the factors responsible for it: competition between species and
geographical isolation. But Wallace believed that evolution had a
purpose ("teleology") in maintaining species' fitness to their
environment, whereas Darwin hesitated to attribute any purpose to a
random natural process. Scientific discoveries since the 19th century
support Darwin's viewpoint, by identifying additional mechanisms and
triggers such as mutations triggered by environmental radiation or
mutagenic chemicals. Wallace remained an ardent defender of natural selection for the rest
of his life. By the 1880s, evolution was widely accepted in scientific
circles, but natural selection less so. Wallace's 1889 Darwinism was a response to the scientific critics of natural selection. Of all Wallace's books, it is the most cited by scholarly publications.
Other scientific contributions
Biogeography and ecology
A map of the world from The Geographical Distribution of Animals shows Wallace's six biogeographical regions.
In 1872, at the urging of many of his friends, including Darwin, Philip Sclater, and Alfred Newton,
Wallace began research for a general review of the geographic
distribution of animals. Initial progress was slow, in part because
classification systems for many types of animals were in flux. He resumed the work in earnest in 1874 after the publication of a number of new works on classification. Extending the system developed by Sclater for birds—which divided the
earth into six separate geographic regions for describing species
distribution—to cover mammals, reptiles and insects as well, Wallace
created the basis for the zoogeographic regions
in use today. He discussed the factors then known to influence the
current and past geographic distribution of animals within each
geographic region.
These factors included the effects of the appearance and disappearance of land bridges (such as the one currently connecting North America and South America)
and the effects of periods of increased glaciation. He provided maps
showing factors, such as elevation of mountains, depths of oceans, and
the character of regional vegetation, that affected the distribution of
animals. He summarised all the known families and genera of the higher
animals and listed their known geographic distributions. The text was
organised so that it would be easy for a traveller to learn what animals
could be found in a particular location. The resulting two-volume work,
The Geographical Distribution of Animals, was published in 1876 and served as the definitive text on zoogeography for the next 80 years.
The book included evidence from the fossil record to discuss the
processes of evolution and migration that had led to the geographical
distribution of modern species. For example, he discussed how fossil
evidence showed that tapirs had originated in the Northern Hemisphere,
migrating between North America and Eurasia and then, much more
recently, to South America after which the northern species became
extinct, leaving the modern distribution of two isolated groups of tapir
species in South America and Southeast Asia. Wallace was very aware of, and interested in, the mass extinction of megafauna in the late Pleistocene. In The Geographical Distribution of Animals
(1876) he wrote, "We live in a zoologically impoverished world, from
which all the hugest, and fiercest, and strangest forms have recently
disappeared". He added that he believed the most likely cause for the rapid extinctions was glaciation, but by the time he wrote World of Life (1911) he had come to believe those extinctions were "due to man's agency".
The line separating the Indo-Malayan and the Austro-Malayan region in Wallace's On the Physical Geography of the Malay Archipelago (1863)
In 1880, Wallace published the book Island Life as a sequel to The Geographical Distribution of Animals.
It surveyed the distribution of both animal and plant species on
islands. Wallace classified islands into oceanic and two types of
continental islands. Oceanic islands, in his view, such as the Galapagos and Hawaiian Islands
(then called Sandwich Islands) formed in mid-ocean and never part of
any large continent. Such islands were characterised by a complete lack
of terrestrial mammals and amphibians, and their inhabitants (except
migratory birds and species introduced by humans) were typically the
result of accidental colonisation and subsequent evolution. Continental
islands, in his scheme, were divided into those that were recently
separated from a continent (like Britain) and those much less recently
(like Madagascar).
Wallace discussed how that difference affected flora and fauna. He
discussed how isolation affected evolution and how that could result in
the preservation of classes of animals, such as the lemurs
of Madagascar that were remnants of once widespread continental faunas.
He extensively discussed how changes of climate, particularly periods
of increased glaciation, may have affected the distribution of flora and
fauna on some islands, and the first portion of the book discusses
possible causes of these great ice ages. Island Life
was considered a very important work at the time of its publication. It
was discussed extensively in scientific circles both in published
reviews and in private correspondence.
Environmentalism
Wallace's extensive work in biogeography made him aware of the impact of human activities on the natural world. In Tropical Nature and Other Essays
(1878), he warned about the dangers of deforestation and soil erosion,
especially in tropical climates prone to heavy rainfall. Noting the
complex interactions between vegetation and climate, he warned that the
extensive clearing of rainforest for coffee cultivation in Ceylon (now called Sri Lanka) and India would adversely impact the climate in those countries and lead to their impoverishment due to soil erosion. In Island Life, Wallace again mentioned deforestation and invasive species. On the impact of European colonisation on the island of Saint Helena,
he wrote that the island was "now so barren and forbidding that some
persons find it difficult to believe that it was once all green and
fertile". He explained that the soil was protected by the island's vegetation;
once that was destroyed, the soil was washed off the steep slopes by
heavy tropical rain, leaving "bare rock or sterile clay". He attributed the "irreparable destruction" to feral goats, introduced in 1513. The island's forests were further damaged by the "reckless waste" of the East India Company from 1651, which used the bark of valuable
redwood and ebony trees for tanning, leaving the wood to rot unused. Wallace's comments on environment grew more urgent later in his career. In The World of Life
(1911) he wrote that people should view nature "as invested with a
certain sanctity, to be used by us but not abused, and never to be
recklessly destroyed or defaced."
Title page to Man's Place in the Universe (1903)
Astrobiology
Wallace's 1904 book Man's Place in the Universe was the first serious attempt by a biologist to evaluate the likelihood of life on other planets. He concluded that the Earth was the only planet in the Solar System that could possibly support life, mainly because it was the only one in which water could exist in the liquid phase. His treatment of Mars in this book was brief, and in 1907, Wallace returned to the subject with the book Is Mars Habitable? to criticise the claims made by the American astronomer Percival Lowell that there were Martian canals
built by intelligent beings. Wallace did months of research, consulted
various experts, and produced his own scientific analysis of the Martian
climate and atmospheric conditions. He pointed out that spectroscopic analysis had shown no signs of water vapour in the Martian atmosphere,
that Lowell's analysis of Mars's climate badly overestimated the
surface temperature, and that low atmospheric pressure would make liquid
water, let alone a planet-girding irrigation system, impossible. Richard Milner comments that Wallace "effectively debunked Lowell's illusionary network of Martian canals." Wallace became interested in the topic because his anthropocentric
philosophy inclined him to believe that man would be unique in the
universe.
Other activities
Spiritualism
Wallace was an enthusiast of phrenology. Early in his career, he experimented with hypnosis, then known as mesmerism, managing to hypnotise some of his students in Leicester. When he began these experiments, the topic was very controversial: early experimenters, such as John Elliotson, had been harshly criticised by the medical and scientific establishment. Wallace drew a connection between his experiences with mesmerism and
spiritualism, arguing that one should not deny observations on "a priori
grounds of absurdity or impossibility".
Wallace began investigating spiritualism in the summer of 1865, possibly at the urging of his older sister Fanny Sims. After reviewing the literature and attempting to test what he witnessed at séances,
he came to believe in it. For the rest of his life, he remained
convinced that at least some séance phenomena were genuine, despite
accusations of fraud and evidence of trickery. One biographer suggested
that the emotional shock when his first fiancée broke their engagement
contributed to his receptiveness to spiritualism. Other scholars have emphasised his desire to find scientific explanations for all phenomena. In 1874, Wallace visited the spirit photographer Frederick Hudson. He declared that a photograph of him with his deceased mother was genuine. Others reached a different conclusion: Hudson's photographs had previously been exposed as fraudulent in 1872.
Wallace's public advocacy of spiritualism and his repeated
defence of spiritualist mediums against allegations of fraud in the
1870s damaged his scientific reputation. In 1875 he published the
evidence he believed proved his position in On Miracles and Modern Spiritualism. His attitude permanently strained his relationships with previously friendly scientists such as Henry Bates, Thomas Huxley, and even Darwin. Others, such as the physiologist William Benjamin Carpenter and zoologist E. Ray Lankester became publicly hostile to Wallace over the issue. Wallace was heavily criticised by the press; The Lancet was particularly harsh. When, in 1879, Darwin first tried to rally support among naturalists to get a civil pension awarded to Wallace, Joseph Hooker
responded that "Wallace has lost caste considerably, not only by his
adhesion to Spiritualism, but by the fact of his having deliberately and
against the whole voice of the committee of his section of the British
Association, brought about a discussion on Spiritualism at one of its
sectional meetings ... This he is said to have done in an underhanded
manner, and I well remember the indignation it gave rise to in the B.A.
Council." Hooker eventually relented and agreed to support the pension request.
In 1870, a flat-Earth proponent named John Hampden offered a £500 wager (roughly equivalent to £60,000 in 2023)
in a magazine advertisement to anyone who could demonstrate a convex
curvature in a body of water such as a river, canal, or lake. Wallace,
intrigued by the challenge and short of money at the time, designed an
experiment in which he set up two objects along a six-mile (10 km)
stretch of canal. Both objects were at the same height above the water,
and he mounted a telescope on a bridge at the same height above the
water as well. When seen through the telescope, one object appeared
higher than the other, showing the curvature of the Earth. The judge for the wager, the editor of Field
magazine, declared Wallace the winner, but Hampden refused to accept
the result. He sued Wallace and launched a campaign, which persisted for
several years, of writing letters to various publications and to
organisations of which Wallace was a member denouncing him as a swindler
and a thief. Wallace won multiple libel suits against Hampden, but the
resulting litigation cost Wallace more than the amount of the wager, and
the controversy frustrated him for years.
In the early 1880s, Wallace joined the debate over mandatory smallpox vaccination. Wallace originally saw the issue as a matter of personal liberty; but,
after studying statistics provided by anti-vaccination activists, he
began to question the efficacy of vaccination. At the time, the germ theory of disease was new and far from universally accepted. Moreover, no one knew enough about the human immune system
to understand why vaccination worked. Wallace discovered instances
where supporters of vaccination had used questionable, in a few cases
completely false, statistics to support their arguments. Always
suspicious of authority, Wallace suspected that physicians had a vested
interest in promoting vaccination, and became convinced that reductions
in the incidence of smallpox that had been attributed to vaccination
were due to better hygiene and improvements in public sanitation.
Another factor in Wallace's thinking was his belief that, because
of the action of natural selection, organisms were in a state of
balance with their environment, and that everything in nature, served a
useful purpose. Wallace pointed out that vaccination, which at the time was often unsanitary, could be dangerous.
In 1890, Wallace gave evidence to a royal commission investigating the controversy. It found errors in his testimony, including some questionable statistics. The Lancet
averred that Wallace and other activists were being selective in their
choice of statistics. The commission found that smallpox vaccination was
effective and should remain compulsory, though they recommended some
changes in procedures to improve safety, and that the penalties for
people who refused to comply be made less severe. Years later, in 1898,
Wallace wrote a pamphlet, Vaccination a Delusion; Its Penal Enforcement a Crime, attacking the commission's findings. It, in turn, was attacked by The Lancet, which stated that it repeated many of the same errors as his evidence given to the commission.
Legacy and historical perception
Honours
Wallace and his signature on the frontispiece of Darwinism (1889)
As a result of his writing, Wallace became a well-known figure both
as a scientist and as a social activist, and was often sought out for
his views. He became president of the anthropology section of the British Association in 1866, and of the Entomological Society of London in 1870. He was elected to the American Philosophical Society in 1873. The British Association elected him as head of its biology section in 1876. He was elected to the Royal Society in 1893. He was asked to chair the International Congress of Spiritualists meeting in London in 1898. He received honorary doctorates and professional honours, such the Royal Society's Royal Medal in 1868 and its Darwin Medal in 1890, and the Order of Merit in 1908.
Obscurity and rehabilitation
Wallace's
fame faded quickly after his death. For a long time, he was treated as a
relatively obscure figure in the history of science. Reasons for this lack of attention may have included his modesty, his
willingness to champion unpopular causes without regard for his own
reputation, and the discomfort of much of the scientific community with
some of his unconventional ideas. The reason that the theory of evolution is popularly credited to Darwin is likely the impact of Darwin's On the Origin of Species.
Recently, Wallace has become better known, with the publication
of at least five book-length biographies and two anthologies of his
writings published since 2000. A web page dedicated to Wallace scholarship is maintained at Western Kentucky University. In a 2010 book, the environmentalist Tim Flannery
argued that Wallace was "the first modern scientist to comprehend how
essential cooperation is to our survival", and suggested that Wallace's
understanding of natural selection and his later work on the atmosphere
should be seen as a forerunner to modern ecological thinking. A collection of his medals, including the Order of Merit, were sold at auction for £273,000 in 2022.
The Natural History Museum, London, co-ordinated commemorative events for the Wallace centenary worldwide in the 'Wallace100' project in 2013. On 24 January, his portrait was unveiled in the Main Hall of the museum by Bill Bailey, a fervent admirer. Bailey further championed Wallace in his 2013 BBC Two series "Bill Bailey's Jungle Hero". On 7 November 2013, the 100th anniversary of Wallace's death, Sir David Attenborough unveiled a statue of Wallace at the museum. The statue, sculpted by Anthony Smith, was donated by the A. R. Wallace Memorial Fund. It depicts Wallace as a young man, collecting in the jungle. November 2013 marked the debut of The Animated Life of A. R. Wallace, a paper-puppet animation film dedicated to Wallace's centennial. In addition, Bailey unveiled a bust of Wallace, sculpted by Felicity Crawley, in Twyn Square in Usk, Monmouthshire in November 2021.
Bicentenary celebrations
Commemorations
of the 200th anniversary of Wallace's birth celebrated during the
course of 2023 range from naturalist walk events to scientific congresses and presentations. A Harvard Museum of Natural History event in April 2023 will also include a mixologist-designed special cocktail to honor Wallace's legacy.
Memorials
Mount Wallace in California's Sierra Nevada mountain range was named in his honour in 1895. In 1928, a house at Richard Hale School (then called Hertford Grammar School, where he had been a pupil) was named after Wallace. The Alfred Russel Wallace building is a prominent feature of the Glyntaff campus at the University of South Wales, by Pontypridd, with several teaching spaces and laboratories for science courses. The Natural Sciences Building at Swansea University and lecture theatre at Cardiff University are named after him, as are impact craters on Mars and the Moon. In 1986, the Royal Entomological Society mounted a year-long expedition to the Dumoga-Bone National Park in North Sulawesi named Project Wallace. A group of Indonesian islands is known as the Wallacea
biogeographical region in his honour, and Operation Wallacea, named
after the region, awards "Alfred Russel Wallace Grants" to undergraduate
ecology students. Several hundred species of plants and animals, both living and fossil, have been named after Wallace, such as the gecko Cyrtodactylus wallacei, and the freshwater stingray Potamotrygon wallacei. More recently, several new species have been named during the bicentenary year of Wallace's birth, including a large spider from Peru, Linothele wallacei Sherwood et al., 2023 and a South Africanweevil, Nama wallacei Meregalli & Borovec, 2023.
Writings
Wallace was a prolific author. In 2002, historian of science Michael Shermer
published a quantitative analysis of Wallace's publications. He found
that Wallace had published 22 full-length books and at least 747 shorter
pieces, 508 of which were scientific papers (191 of them published in Nature).
He further broke down the 747 short pieces by their primary subjects:
29% were on biogeography and natural history, 27% were on evolutionary
theory, 25% were social commentary, 12% were on anthropology, and 7%
were on spiritualism and phrenology. An online bibliography of Wallace's writings has more than 750 entries.
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