Charles Darwin's On the Origin of Species is one of the central publication nodes in the history of evolutionary thought. It matters not only as a famous book, but as a durable entry point into descent with modification, natural selection, variation, and the long public controversy over what evolutionary theory does and does not claim.
The book belongs between broad chronology and later concept work because it is both a historical event and a recurring point of reference in explanation, controversy, and public misunderstanding. Readers return to it both for what it argued and for what later debates repeatedly say about it.
It remains useful not simply as a canonical title, but as a way to keep together the scientific, historical, and rhetorical strands that later discussion often separates too sharply.