How Did Wallace Contribute To The Theory Of Evolution?

Alfred Russell Wallace, a naturalist -an expert in or student of natural history- Born in Usk, United Kingdom on January 08, 1823, made two expeditions to South America and Indonesia and formulated the modern theory of evolution! Wallace met Charles Darwin at scientific meetings and eventually shared ideas and became friends. Wallace wrote letters to Darwin about this as Darwin also stumbled upon the same results and therefore listed Wallace as an author to the theory of modern evolution on July 1, 1858 to the Linnaean Society in London. Wallace later "wrote a volume entitled Contributions to the Theory of Natural Selection in 1870." (Explorers & Discoverers of the World, 1993. Web)
Both Darwin and Wallace concluded that "if an animal has