On November 24th 1859, Charles Darwin published On the Origin of Species. It would prove to be the foundational document of the entire science of evolutionary biology. Darwin proposed that biological species evolve over time due to natural selection. In the ensuing years this theory has been confirmed in multiple experiments, to the point where biological evolution is accepted as established fact within the scientific community today.
The book presented an exhaustive argument for biological evolution based on Darwin's own voyage aboard HMS Beagle and a large body of correspondence with other contemporary natural philosophers. Written for non-specialists, Darwin intended to reach a broad audience of educated people with an interest in natural philosophy. The success of the book, and the widespread acceptance of Darwin's theory, led to the secularization of science in British universities. Prior to this, the study of science had been tied very closely to the study of natural theology.
You can read more about The Origin of Species
here.