CrossRef, a reference-linking service for scholarly and professional literature, has announced that The Royal Society has registered all of its historical back-file content with CrossRef. The Oldenburg Epistle Dedicatory, dating from 1665 and published in the first volume of Philosophical Transactions of the Royal Society, is the earliest publication linkable via CrossRef DOI (Digital Object Identifier). In registering DOIs for its complete journal archive, the Royal Society joins several CrossRef member publishers who have recently completed, or are in the midst of, vast retro-digitization initiatives, including Elsevier, Springer, Sage, Kluwer, Wiley, Blackwell, and the American Association for the Advancement of Science, among others. The complete archive is available via http://www.pubs.royalsoc.ac.uk/archive. It contains material from events such as the early experiments to demonstrate Newton's Law of Gravity and Harvey's theories of circulation, as well as the drawings that Charles Darwin produced while he was developing the theory of evolution.