On this day 1859, “On the Origin of Species” by British naturalist Charles Darwin was first published, selling out by the end of the day. It introduced the scientific theory that populations evolve over generations through a process of natural selection. It presented a body of evidence — gathered from Darwin’s nearly five-year journey around the world on the HMS Beagle — that the diversity of life arose by common descent through a branching pattern of evolution.
While various evolutionary ideas had been proposed since ancient times, there was renewed interest into the 19th century as scientific knowledge increased. (In fact, fellow British naturalist Alfred Russell Wallace had independently conceived the theory of evolution through natural selection, publishing his paper on the subject jointly with Darwin in 1858.)
However, Darwin’s book was brilliant in that it was written for lay readers who were not specialists in the subject. Moreover, because he was already a famed scientist, his findings were taken seriously, with the evidence he presented generating intense scientific, philosophical, and religious discussion. Within two decades, there was widespread scientific belief in evolution with a branching pattern of common descent, although people were slow to give much credit to Darwin’s specific finding of natural selection as the primary mechanism.
Indeed, from the 1880s to the 1930s there occurred an “eclipse of Darwinism” , wherein various other mechanisms of evolution were given more credit and Darwin’s fell to the wayside. Only in the 1940s, with the development of the modern evolutionary synthesis — a set of theoretical concepts that tried to harmonize and integrate different factors in evolution — did Darwin’s concept of evolutionary adaptation through natural selection become central to modern evolutionary theory, now becoming the unifying concept of the life sciences (botany, zoology, biology, etc.)
A word about the term theory as used in science: Contrary to popular belief, a theory in science is not the same as how theory is used in everyday language.
A scientific theory is an explanation of some aspect of the natural world that can be repeatedly tested and verified in accordance with the scientific method (rigorous observation, measurement, evaluation of results, etc.) Theories are tested under controlled conditions and/or through abductive reasoning (logical inference from a set of observations). Scientific theories are established following repeated tests and scrutiny.
By contrast, outside the scientific context, a theory usually defines explanation that is unsubstantiated or speculative–hence why so many people wrongfully believe that evolutionary is “just a theory” in the vernacular sense, rather than the more rigorously proven scientific kind of theory.