Charles Darwin is credited with developing the theory of evolution, but Alfred Russel Wallace contributed to Darwin's ideas. Wallace proposed a theory of natural selection as a key part of evolution before Darwin published his own work, and many of Darwin's concepts duplicated Wallace's earlier writings.

While Darwin documented his findings extensively and produced far more published material, Wallace first came up with some of the innovative ideas. The two men shared notes and drafts of papers, and Darwin became aware that Wallace had independently developed concepts on evolution and natural selection that were similar to those of Darwin's own theories.

Wallace reached his ground-breaking realizations simultaneously with Darwin, but Darwin's methodical approach, detailed records, and numerous papers and books allowed the latter to become predominant in the field of evolution and natural selection.

Despite this, the historical record is clear that Wallace was one of the first to identify the role of natural selection in evolution.

A. R. Wallace was born in 1823 to a British middle class family. He tried his hand at a number of different areas of work but gravitated toward field studies of flora and fauna due to his preference for scientific studies outdoors.

The major events of his early adult biography are:

• Apprenticeship. As a young man, Wallace apprenticed in a number of trades, including surveying and map-making. He discovered he enjoyed the outdoor surveying work and became interested in botany, animal life and the biology of his surroundings.
  
• Education. While teaching surveying in Leicester, Wallace frequented local libraries and read several major works on natural history and biology. Largely self-taught, he made friends with a young British naturalist, Henry Walter Bates, who introduced Wallace to entomology.
• Amazon voyage. Wallace and Bates decided to pursue their entomological activities in the Amazon basin of South America. They set sail for the mouth of the Amazon in 1848, and Wallace spent the next four years collecting specimens and studying evolutionary change.
• Return to England. In 1852 Wallace decided to return to England due to ill health. On the way back his ship caught fire and sank. He survived and was picked up from a lifeboat, but his collections were lost.
• First publications. Back in England he published two works based on his Amazon trip, Palm Trees of the Amazon and Their Uses and A Narrative of Travels on the Amazon and Rio Negro.

While Wallace's observations in the Amazon laid the basis for his future work on evolution and natural selection, he was not able to connect the variation in characteristics within species to survival of individuals best adapted to their environment. He would only come to this realization with further reading and travel.

In 1854 Wallace resumed his specimen collection activities and traveled to the Malay archipelago, now called Indonesia, Malaysia and Singapore.

Based on his observations of the variation of characteristics in species on different islands, he published On the Law Which Has Regulated the Introduction of New Species in 1855. Two further studies on geographical influences on biology and organic change followed in 1856 and 1857.

Wallace was on the brink of a breakthrough but wasn't quite there yet. The theory of evolution has two parts. One part describes how the characteristics of species change over time. This part of evolution is often called descent with modification.

The other part of the theory of evolution details the mechanism through which species change. This mechanism is natural selection or survival of the fittest.

Wallace's 1855 paper dealt with the first part of evolution. He described his observations that species had varying characteristics or traits and that the traits seemed to be influenced by being passed on from parents to offspring.

Wallace published his paper but did not get an enthusiastic response from the scientific community. He sent the paper to Darwin, who took little notice of it.

Wallace remained in Indonesia studying Indonesian butterflies and the displacement of Asiatic people by Melanesian people in the islands. At one point he caught malaria. While sick, he thought about the work of Robert Thomas Malthus, a British scholar and economist he had previously studied.

Malthus wrote that human population growth will always out pace the food supply. Unless war, disease or natural disasters intervene, those worst off will die of starvation.

Wallace realized that this thinking could also be applied to animal species. Many animals produce more young than their surroundings can support. As a result, those least adapted to their environment will die off while the rest, with favorable traits, survive.

As soon as he recovered from his malaria, Wallace put his ideas down on paper and wrote On the Tendency of Varieties to Depart Indefinitely From the Original Type. He was the first to write a paper detailing the evolutionary mechanism of natural selection.

Because he remembered the lack of enthusiasm for his previous paper, Wallace wondered whether Charles Darwin could help him get more attention. He sent the paper to Darwin asking for comments and possibly help in getting it published. He had been in occasional contact with Darwin for several years and knew Darwin was interested in "the species question."

Darwin was aghast. He had been working on the subject of evolution and an evolutionary mechanism for over 20 years, and his conclusions were almost identical to those in Wallace's paper. He did not want to be scooped by Wallace but also did not want to unfairly deprive Wallace of his due.

He showed the Wallace paper to several associates including geologist Charles Lyell and botanist Joseph Hooker with whom he had previously discussed his work. The group decided the best way forward would be to present the as yet unpublished works of Wallace and Darwin together.

On July 1, 1858, Wallace's paper was read at a meeting of the Linnean Society, a British science group, along with some of Darwin's unpublished writings on natural selection. The two papers were published together later that year and received a lot of attention.

The Wallace and Darwin papers were revolutionary in that they explained how species changed over time to adapt to their surroundings. The state of knowledge at the time recognized that species changed, but religious advocates believed it was according to God's plan while many scientists thought the environment directly caused certain traits.

The Darwin-Wallace theory of evolution and the associated theory of natural selection were based on the following new premises:

• Many traits were inherited.
  
• Some inherited traits were favorable while others were unfavorable.
• Favorable traits made individuals more likely to survive and reproduce.
• Favorable traits were passed on to offspring while individuals without favorable traits died off and couldn't pass on their unfavorable traits.
• Over generations, individuals with favorable traits would come to dominate the population.

The papers attracted both positive reviews and criticism. This is where Darwin came into his own because he had spent 20 years assembling his evidence, first for the theory of evolution and then for the theory of natural selection.

Darwin had spent the last 20 years cataloging his specimens and assembling what he hoped would be the definitive work on evolutionary theory. He had not finished his work when Wallace's paper landed on his desk.

When he chose to publish a brief paper together with Wallace's work, he knew he would have to quickly publish more material to support his theories.

He was unable to bring all of his material forward for rapid publication but assembled his work with the finches of the Galapagos Islands and his work on the mechanism of natural selection into a book.

Darwin's On the Origin of Species was published in 1859, and it presented in greater detail how evolution functioned. Mainly due to this publication, the theory of evolution it describes is now known as Darwinian evolution.

As a result of the attention his paper received, Wallace continued with his studies of species in the Indonesian islands. Based on this work he wrote a paper on the geographical limits he observed when looking at the animal populations of different islands. He presented On the Zoological Geography of the Malay Archipelago to the Linnean Society in 1859.

The paper details a geographic boundary between species originating in Asia and Australian species. The boundary winds between the islands of Indonesia and is known as the Wallace Line.

In 1862 Wallace returned to England with a substantial nest egg from selling his specimens and from his writings. He subsequently wrote The Origin of Human Races Deduced From the Theory of Natural Selection and presented it to the Anthropological Society of London. He settled down and married but kept writing and became a respected member of the British scientific community.

Alfred Russel Wallace wrote on many different subjects. His body of work includes books on spiritual subjects such as, The Scientific Aspect of the Supernatural, published in 1866, and A Defence of Modern Spiritualism, published in 1874. Additional works include The Wonderful Century, published in 1898, and Man's Place in the Universe, published in 1903. However, it is his scientific writings for which he is best-known.

He went back to writing about his Malay Archipelago expedition and natural selection several times. Notable books include:

• The Malay Archipelago, 1869.
  
• Contributions to the Theory of Natural Selection, 1870.
• The Geographical Distribution of Animals, 1876.
• Island Life, 1880.
• Darwinism, 1889.

In addition to writing, he received several honors as a senior British scientist. These included:

• President of the Entomological Society of London, 1872 to 1874.
• Darwin Medal of the Royal Society, 1890.
• Elected Fellow of the Royal Society, 1893.
• Darwin-Wallace Medal of the Linnean Society of London, 1908.

While Wallace is best known for his scientific contributions, starting in 1880 he became more and more involved in social issues. He began advocating for government intervention to provide basic necessities so anyone could enjoy an acceptable standard of living. He was an early and consistent supporter of women's suffrage and supported the labor movement as well as the organization of unions.

In many respects, he was far ahead of his time. His ideas on labor included the concept that unions should eventually accumulate funds to buy out the employers. He wrote on dealing with inherited wealth and trusts and reforming the House of Lords to make it more democratic.

One of his main preoccupations was with public lands. He thought the state should buy up large tracts of land for public use and benefit. He helped organize the Land Nationalization Society and became its first president, promoting local use, green belts, parks and rural re-population.

Overall, Wallace's legacy is multifaceted and complex, reflecting his own complicated character. His contributions to the field of evolution are better known, but some of his other works reveal even more unique ideas and radical thought.