Two new derivations of Darwinʼs theorem on the equality of the added mass for translation of a body moving in an ideal fluid of infinite extent and the drift mass are given. The first is based on the idea of time lag, used by Rayleigh (1876), Ursell (1953), and Longuet-Higgins (1953) to study fluid drift. The second is truly elementary, relying only on the concept of continuity and Newtonʼs second law of motion. A geometrical interpretation of the result in the first derivation is given, and a few examples are provided.