Before Darwin, most naturalists held that each species was separately and permanently created. Darwin proposed something simpler and stranger: that species change, and that today's forms descend, with modification, from earlier ones.

He rarely used the word “evolution.” His phrase was “descent with modification” — the slow accumulation of small inherited changes that, over vast time, turns one form into many.

Follow the branches back far enough and they converge. “Probably all the organic beings which have ever lived on this earth have descended from some one primordial form.”

The book is, in his own words, “one long argument”: every chapter a piece of evidence for this one idea, and an honest answer to its objections.