Charles Darwin first published the Origin of Species in 1859. The book was immediately revised and a second edition was released in 1860. These two editions were followed by four others, in 1861, 1866, 1869 and 1872. Each of them was revised by Darwin, some more heavily than others. These revisions make the textual history of the Origin complex and intriguing. This article describes the new Online Variorum, presents the types of changes that Darwin introduced in his text and offers hypotheses about the reasons for the changes.