Alfred Russel Wallace was born in Wales in 1823. He has been described variously as a naturalist, a geographer, and a social critic. He even weighed in on the debate as to whether or not life could exist on Mars. However, what he is best known for is his work on the theory of natural selection.

Like fellow naturalist and colleague Charles Darwin, Wallace traveled the world, observing and collecting samples of species. He traveled to Brazil and various islands of the Malay Archipelago that make up modern-day Indonesia and the Philippines, where he collected thousands of specimens of insects, birds, and other animals. After four years in Brazil, Wallace fell ill and decided to return home to England. But 26 days into their voyage home, his ship caught fire and sank in the Atlantic. Wallace’s team and the ship’s crew spent 10 days adrift before being picked up by a passing ship, and all of Wallace’s notes and samples were lost at sea.

Despite this setback, Wallace set off on another voyage in 1854 to Southeast Asia to collect more samples. By 1855, his observations led him to the conclusion that living things change over long periods of time—they evolve. However, he could not explain how or why they evolve. Then, in 1858, while still in Southeast Asia, he became ill again. Wracked with a fever, he suffered hallucinations, but when the fever broke, the answer came to him—species evolve by adapting to their environment!

Wallace knew Darwin was working on similar research. In 1858, he sent Darwin a letter outlining his ideas about evolution. The two collaborated on a scientific paper, discussing their evidence for natural selection and evolution.

In 1859, Darwin published his book On the Origin of Species, which presented his theory of natural selection to a broader audience. The theory of evolution by natural selection became known as Darwin’s theory. Though Wallace’s contributions to the study of evolution were considerable, they are often forgotten.

Wallace spent eight years studying and collecting biological specimens in Southeast Asia. During that time, he gathered over 125,000 specimens. His research on the geographic distribution of animals provided critical support for his evolutionary theories and led him to draw a boundary line through Southeast Asia that divides Asian and Australian animal groups. Wallace’s Line, as it was later named, runs from the Indian Ocean to the Philippine Sea. It signifies the unexpected distribution of animals on either side of the line. Several mammal, bird, and fish species are found in abundance on one side of the line and only in small numbers, or not at all, on the other side.

Wallace wrote over 20 books and published more than 700 articles and letters on a wide variety of topics. He died in 1913 at the age of 90.