Book excerpt: Charles Darwin's On the Origin of Species is unquestionably one of the chief landmarks in biology. The Origin (as it is widely known) was literally only an abstract of the manuscript Darwin had originally intended to complete and publish as the formal presentation of his views on evolution. Compared with the Origin, his original long manuscript work on Natural Selection, which is presented here and made available for the first time in printed form, has more abundant examples and illustrations of Darwin's argument, plus an extensive citation of sources.