The evolution of ontologies has been identified by W3C as a significant issue. Automatic ontology evolution is increasingly important to allow runtime communication between vast collections of online agents. For this application, manual, offline evolution is both too slow and too expensive. Very many ontologies are classifications of a set of objects. In order to gain insight into how the evolution of classifications might be automated, we discuss some historic examples of such evolution. From this discussion, we extract some general principles that might be employed to help automate the ontology evolution process.