Alfred Russel Wallace OM, FRS (8 January 1823 – 7 November 1913) was a British naturalist, explorer, biologist and social activist. He is best known for proposing a theory of natural selection. This was published in 1858 together with Charles Darwin's idea.

Wallace did extensive natural history exploring. He went first to the Amazon River basin with Henry Walter Bates, and later to Malaya and Indonesia. He wrote books on both these adventures. While in Indonesia he drew the Wallace Line which divides Indonesia into two parts. On one side are animals of Australasia. On the other side are species mostly of Asian origin. He wrote a wonderful book on biogeography (the distribution of animals).

The great adventure 
After a few years of working as a railway surveyor with his brother, Wallace's life was changed by meeting Henry Walter Bates in Leicester in 1847.

The Amazon 
Wallace and Bates discussed the idea of an expedition to the Amazon. The plan was to defray expenses by sending specimens back to London, where an agent would sell them for a commission. Also, for the travellers to "gather facts towards solving the problem of the origin of species", as Wallace put it in a letter to Bates. The two friends, who were both by now experienced amateur entomologists, met in London to prepare themselves by viewing South American plants and animals in the main collections.

Bates and Wallace sailed from Liverpool in April 1848, arriving in Pará (now Belém) at the end of May. For the first year they settled in a villa near the city, collecting birds and insects. After that they agreed to collect independently.

Wallace continued charting the Amazon for four years, collecting specimens and making notes on the peoples, the languages, the geography, flora, and fauna. On 12 July 1852, Wallace embarked for England on the brig Helen. After twenty-eight days at sea, balsam in the ship's cargo caught fire and the crew was forced to abandon ship. All of the specimens Wallace had on the ship, most of his collection, were lost. He could only save part of his diary and a few sketches. Wallace and the crew spent ten days in an open boat before being picked up by the brig Jordeson.

The East Indies 

From 1854 to 1862, age 31 to 39, Wallace travelled through the Dutch East Indies (now Malaysia and Indonesia), to collect specimens for sale and to study nature.  Wallace collected more than 125,000 specimens in the Dutch East Indies (more than 80,000 beetles alone).  More than a thousand of them represented species new to science. 

His observations of the clear zoological differences across a narrow strait in the archipelago led to his proposing the zoogeographical boundary now known as the Wallace Line. Bali and Lombok were two islands in the archipelago only 17 miles apart at the widest (28 km), roughly the same size and with the same climate, soil, elevation and aspect. Yet their flora and fauna were so different. 

"In this archipelago there are two distinct faunas rigidly circumscribed, which differ as much as do those of Africa and South America... yet there is nothing on the map or on the face of the islands to mark their limits. The boundary line passes between islands closer together than others belonging to the same group. I believe the western part to be a separated portion of continental Asia, while the eastern is a fragmentary prolongation of a former west Pacific continent".

Why was it, he wondered, that the animals and plants on the Bali side of the channel were of Asian types, while those on the Lombok side were Australasian in type? This had to mean that the western group had evolved from common western stock, while the eastern group had evolved from a common eastern stock. 

While he was exploring the archipelago, he 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 own theory, in the same year.

Accounts of his studies and adventures there were eventually published in 1869 as The Malay Archipelago. It became one of the most popular natural history travel journals of the 19th century. It was praised by Charles Darwin and Charles Lyell and by others, such as the novelist Joseph Conrad, who called it his "favorite bedside companion".

Natural selection 
Unlike Darwin, Wallace began his career as a travelling naturalist already believing in evolution. Both he and Bates had read Vestiges, a controversial work of popular science, published anonymously in 1844. This advocated an evolutionary origin for the solar system, the earth, and living things. He also read Malthus's Principle of population, which he says "twenty years later gave me the long-sought clue to the effective agent in the evolution of organic species". He meant by that the struggle for existence, which led Wallace to natural selection.

Wallace wrote his ideas on the small isle of Ternate, in what was then the Dutch East Indies. He was suffering from malaria. While the natives looked after him, his thoughts turned to Malthus's book. 
"I thought of [Malthus's] clear exposition of the 'positive checks to increase' – disease, accidents, wars, famine – which keep down the population... It then occurred to me that the same causes, or their equivalents, are
continually acting in the case of animals also... Why do some die and some live? And the answer came clearly, that on the whole the best fitted live... Then it suddenly flashed upon me, that this self-acting process would necessarily improve the race..." 
This was the origin of his letter to Darwin, which set out the idea of natural selection.

His views on human evolution 

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 human beings. Huxley had already published his view that evolution applied to mankind as it did to other living things.

Wallace thought that natural selection could not account for mathematical, artistic, or musical genius, as well as metaphysical musings, wit and humour. He eventually said that something in "the unseen universe of Spirit" had interceded at least three times in history. The first was the creation of life from inorganic matter. The second was the introduction of consciousness in the higher animals. And the third was the generation of the higher mental faculties in mankind. He also believed that the purpose of the universe was the development of the human spirit. 

Many, including Huxley, Hooker, and Darwin himself, were highly critical of these ideas. As one historian of science has pointed out, Wallace's views in this area were at odds with two major tenets of Darwinism. These are: evolution does not have a goal, and it is not aimed at or centered around mankind. It is thought by most biographers that his thinking on the evolution of man was influenced by his adoption of spiritualism, which happened at the same time.

Spiritualism 
Wallace was not a believer in revealed religion of any kind, but he did believe in spiritualism. This has puzzled biographers, who struggle to think why such a man would believe in spirits. Early in his life, he experimented with hypnosis, which was then doubted and criticised. He used some of his students in Leicester as subjects, with considerable success. Apparently, this persuaded him not to reject ideas which were doubted. Even when Huxley told him that one of his favourite mediums was a proven fraud, he refused to believe it. He preferred the evidence of his own experience.

Wallace's public support of spiritualism, and his defence of spiritualist mediums against allegations of fraud, damaged his scientific reputation. It strained his relationships with friends such as Bates, Huxley and Darwin, who felt he was overly credulous. Others became openly hostile to Wallace over the issue. Wallace and other scientists who defended spiritualism were subject to much criticism from the press, with The Lancet, the leading English medical journal, being particularly harsh. The controversy affected the public's idea of Wallace for the rest of his career, though he was always respected in other ways.

 1878. Miracles and modern spiritualism: three essays. Spiritualist Press, London.

Other interests 
Wallace had a wide range of interests, and wrote books about all of them. He wrote against vaccination, for phrenology, for spiritualism, for land nationalisation, against the moon having canals, and for social change and the progress and improvement of mankind. He did not believe in religion, but he did believe in spiritualism. He was a radical in politics, economics and social reform. He was a kind and honourable person, but he could be a tough opponent if he thought something unfair was going on. Even the titles of some of his books were sensational, as for example:

 Bad Times: an essay on the depression of trade, tracing it to its sources in enormous foreign loans, excessive war expenditure, the increase of speculation and of millionaires, and their depopulation of the rural districts; with suggested remedies. With appendix on the nationalisation of house property. Macmillan, London. 1885.
 Land nationalisation: its necessity and its aims. Being a comparison of the system of landlord and tenant with that of occupying ownership in their influence on the wellbeing of the people. Swan Sonnenschein, London 1892.
 Vaccination a delusion: its penal enforcement a crime. This essay was written for the purpose of influencing Parliament, and securing the speedy abolition of the unjust, cruel, and pernicious Vaccination laws. For this purpose, it has been necessary to speak plainly of the ignorance and incompetence displayed by the Royal Commission, proofs of which I give from their Final Report and the evidence they have collected and printed. (Preface). Reprinted as a separate from chapter XVIII of The wonderful century. Swan Sonnenschein, London 1898.

Books 
Wallace wrote about 22 books, depending on how one counts them.
His greatest works include:
 1853 Travels on the Amazon and Rio Negro
 1858 On the tendency of varieties to depart indefinitely from the original type (this is the famous paper on natural selection)
 1869 The Malay Archipelago.
 1870 Contributions to the theory of natural selection.
 1876 The geographical distribution of animals
 1878 Tropical Nature, and other essays
 1880 Island Life
 1889 Darwinism

References 

1823 births
1913 deaths
British naturalists
British evolutionary biologists
British explorers
Spirituality
Order of Merit
Fellows of the Royal Society
Science writers