The Origin of SpeciesCharles Darwin

Darwin published On the Origin of Species in 1859 after two decades of accumulating evidence. The book presents a single argument sustained across fourteen chapters: species are not fixed creations but products of descent with modification through natural selection.

The argument begins with variation under domestication. Breeders select for desired traits; nature, Darwin proposes, does the same, but unconsciously and over immense spans of time. All organisms produce more offspring than can survive. Those individuals whose variations give them an advantage in the struggle for existence are more likely to survive and reproduce. Over generations, favorable variations accumulate, populations diverge, and new species arise.

Darwin marshals evidence from biogeography, embryology, the fossil record, and comparative anatomy. Island species resemble those of the nearest mainland. Embryos of different species share structures that diverge only later in development. The fossil record, though incomplete, shows sequences of forms that change over time. Vestigial organs (the human appendix, the wing-stumps of flightless birds) make no sense as fresh designs but perfect sense as inheritances from differently adapted ancestors.

The book says almost nothing about human evolution (a single sentence: "Light will be thrown on the origin of man and his history"). Darwin saved that argument for The Descent of Man, twelve years later. But the implication was clear from the start, and no one missed it.