Walter Dalrymple Severn, bacteriologist and inventor, was the son of Walter Severn, a water colour painter, and his wife Mary Dalrymple Fergusson. He was educated in London at the Kensington Grammar School, King's College School, and the Kensington Normal School of Science. From October 1889 he attended Gonville and Caius College, University of Cambridge, for five terms. In 1891 he was appointed director of the West End Pathological Laboratory [presumably in the town West End, near Southampton].
In May 1903 Severn was appointed as an assistant in the laboratory of the Public Health and Local Government Department of the Cape Colony. The next year he became assistant to Dr A. Edington* at the Bacteriological Institute in Grahamstown, and a few years later assistant to the government bacteriologist in the Local Government and Health Branch of the Colonial Secretary's Department. After the formation of the Union of South Africa in 1910 he was appointed in April 1912 as second grade bacteriological assistant in the Public Health Laboratory, Cape Town. He remained there to his death in 1918, when he held the post of assistant government bacteriologist.
Severn has been credited with inventing the apparatus for taking photomicrographs by combining a vertical microscope and a horizontal camera (Bensusan, 1966). Just before his death he revised the chapter on photomicrography in the third edition of Photography in colour (1919) by George L. Johnson*. One Walter Dalrymple Severn, presumably him, with W.B. Thomson and G. Twycross, was granted a United States patent in 1918 for the manufacture of nitrated cellulose products.
By 1910 Severn was an associate of the Royal College of Science, a Fellow of the Chemical Society, and a member of the South African Association for the Advancement of Science. At the annual congress of the association held in Cape Town in 1910 he served as joint secretary of Section C (which included bacteriology).