Maynard Smith™s analysis of units of evolution is compared to traditional approaches generalizing Darwin™s princi- ples. Maynard Smith™s key principle of multiplication is elaborated into a general account of the process of reproduc- tion that integrates concepts of heredity and development and is applicable to all levels of the biological hierarchy. The amended analysis suggests a new unit of evolution, the ihreproducer,l. which generalizes the concept of a replicator. The theory of evolutionary transition, the evolutionary origin of new levels of biological organization, is revised to reflect these amendments to the analysis of units. A three-stage scenario for evolutionary transition is sug- gested.