His two 1914 papers on the dynamical theory of diffraction of X-rays from perfect crystals became often cited classics, minting the Darwin Curve of reflectivity.

A year later William Lawrence Bragg had him transferred to the Royal Engineers to participate in the work on the localisation of enemy artillery by sound ranging.

He then resigned his post in Edinburgh to become Master of Christ's College, beginning his career as an active and able administrator, becoming director of the National Physical Laboratory on the approach of war in 1938.

He served in the role into the post-war period, unafraid to seek improved laboratory performance through re-organisation, but spending much of the war years working on the Manhattan Project co-ordinating the American, British, and Canadian efforts.

His conclusions were pessimistic and entailed a resigned belief in an inevitable Malthusian catastrophe, as described in his 1952 book The Next Million Years.

He first argued in this book that voluntary birth control (family planning) establishes a selective system that ensures its own failure.

He and his late wife are commemorated with a memorial at St Botolph's Church, Cambridge; she was cremated, the funeral was in Wimbledon, where she had been living.