After Darwin's complete online collection, Alfred Russel Wallace's complete work is now also online. Surveying this online collection, it's impressive to see the diversity of Wallace's work, and his capacity to start entire new fields of enquiry. I particularly admire Wallace, because, unlike Darwin, Wallace did not come from a privileged social environment. His family was relatively poor, and Wallace could not enjoy a university education, being largely self-taught on books by Malthus, Lyell and Chambers (the then anonymously published Vestiges).
Unlike Darwin, who married very richly, Wallace struggled financially all his life. This led Wallace to engage in endeavors that his richer gentlemen-scholar colleagues frowned upon, such as a £500 wager against a flat-Earth theorist (John Hampden) that he could prove the Flat Earth theory incorrect. Wallace was also a controversial figure, who was amongst other things, into spiritism, and a proponent of intelligent design to explain the human intellect. Below the fold, I'd like highlight just two of Wallace's lesser-known intellectual achievements, which still have a large impact on scientific practice today:
Astrobiology: Wallace pioneered the field of astrobiology, presenting what can be counted among the first careful scientific analyses of life conditions on other planets. He introduced astrobiology in his book Man's place in the Universe (1903). Wallace argued that the physical conditions for organic life are very unlikely to occur. He argued that earth was the only planet in the solar system that had conditions congenial to life. Also interesting is his monograph Is Mars habitable? (1907) which attacked the then popular idea that Mars was inhabited by intelligent aliens. The alleged discovery of canals on Mars had fueled this idea. Wallace attacked this idea in this book, using careful calculations to argue that the surface temperature of Mars was too cold for liquid water. He concluded that plant-based complex life on Mars was not possible. I recently attended a talk by the astrobiologist Milan M. Cirkovic at Oxford, who said "Without Wallace, astrobiology as we know it would not exist". Indeed, the dispassionate scientific analysis of conditions for life, using a variety of inductions (mass of the planet, distance to the sun, atmosphere) is a method initiated by Wallace that is still used in the quest of habitable planets today.
Sexual selection theory: As is well known, Darwin came up with the theory of sexual selection to account for ornaments, such as peacock tails, in animals. He thought that such traits could evolved, exclusively because the (mainly female) sexual partners preferred such traits. Darwin's work was heavily influenced by 18th century aesthetics as he thought these traits were purely non-functional: they were only preferred because they were attractive, and for no other reason. Wallace doubted this. He did not deny that sexual selection played a role, but argued that beauty was correlated with fitness-related traits, like vigor and health: "If, however, heightened color is correlated with health and vigour; and if these healthy and vigorous birds provide best for their young, and leave offspring which, being equally healthy and vigorous, can best provide for themselves---which cannot be denied---then natural selection becomes a preserver and intensifier of colour" (Wallace 1877).
As Prum and Cronin have argued, current sexual selection theory is much more in line with Wallace than with Darwin. The idea is that ornaments are costly signals, i.e., signals of an animal's quality. While Wallace's critique of Darwinian aesthetic sexual selection theory was not well received, Prum remarks "… to modern ears, Wallace’s opinions do not sound so half-baked! Wallace’s view is entirely consistent with orthodox opinion today. [It] would be entirely at home in any modern biology textbook, or the introduction to practically any con- temporary paper on intersexual signalling. Indeed, it is likely to capture the opinions of the majority of the readers of this paper." (Prum, 2012, 2256). Current research in sexual selection theory is much more congenial to Wallace's hypothesis than to Darwin's arbitrary aesthetic model.