Charles Darwin, born on the same day as Abraham Lincoln, February 12, 1809, had been appointed Naturalist for the world wide voyage of HMS Beagle which departed Devonport on December 27, 1831 and returned to England on October 2, 1836. It was on that voyage that Darwin conducted many of his experiments and observations that allowed him to later formulate his theory of evolution by natural selection. What we now call Darwinism are the ideas and theories that Darwin published in his two books, Origin of Species in 1859 and Descent of Man in 1871.
Darwinism has persisted with updates, enlargment and addition but without refutation for 156 years. One might wonder why it has endured while so many other scientific theories have either been found to be erroneous or severely curtailed and changed by new understanding.
Perhaps the question is not really why Darwinism has been so successful, but how? One answer, to my mind the best answer, is stated by historian Paul Johnson in his excellent biography of Darwin: Portrait of a Genius (2012), at page 146:
There was, it was true, a moment early in the twentieth century when it seemed possible Darwinism might be toppled. As Mendel and his theory of genetics were rediscovered, spread rapidly, and were accepted and enlarged upon, some thought genetics was incompatible with Darwin, But after the First World War, Ronald Fisher showed that it was possible to reconcile Darwin and Mendel. Indeed, said Fisher, “Mendelism supplied the missing parts of the structure erected by Darwin.” Darwin showed the what of evolution and the why, natural selection. Now Mendel had produced the how, genetics. This was reinforced when Herbert Joseph Muller showed that genes are artificially mutable. Thanks to Fisher and Muller, and others such as J.B.S. Haldane, by the 1930s, Darwin-Mendelism was triumphant. The way was then open for James Watson and Francis Crick to discover the double helix structure of DNA. So on to the genome and the present infinite possibilities of the science.
Paul Johnson’s book on Darwin is amazing for its in-depth coverage of Darwin and Darwinism in a short 160-page, easy-to-read book. His chapters on The Evils of Social Darwinism [definitely not a part of Darwinism as originally constructed nor as we know it today] and the Triumph and Reversal of Natural Selection take the reader beyond the life of Charles Darwin and into what a few others attempted to make of his theory and why they did so, and then looks at what natural selection has wrought, which is what it was already doing before Charles Darwin was born and has continued without any notice of Darwin or his discovery of it. Finally, Johnson looks at what natural selection has accomplished in the making of homo sapiens, and into the future.