Sir Horace Darwin, KBE, FRS (13 May 1851 – 29 September 1928), a son of the English naturalist Charles Darwin, was a civil engineer.
Darwin was born in Down House in 1851, the fifth son and ninth child of the British naturalist Charles Darwin and his wife Emma, the youngest of their seven children that survived to adulthood.

He was educated at a private school in Woodbridge, Suffolk and at Trinity College, Cambridge, where he graduated BA in 1874.[1] He founded the Cambridge Scientific Instrument Company in 1885 and was Mayor of Cambridge between 1896 and 1897. He was elected a Fellow of the Royal Society in 1903. He was knighted in 1918.

Darwin married Emma Cecilia "Ida" Farrer (1854–1946), daughter of Thomas Farrer, 1st Baron Farrer in January 1880, and they had one son and two daughters:
Erasmus Darwin (December 7, 1881 – April 24, 1915) who was killed in the Second Battle of Ypres during the First World War.[2]

Ruth Frances Darwin (1883–1972), married Dr William Rees-Thomas, was a notable advocate of eugenics.

Emma Nora Darwin (1885–1989) edited the 1959 edition of The Autobiography of Charles Darwin and married the civil servant Sir Alan Barlow.

His family home, "the Orchard", in Huntingdon Road, Cambridge, is now the site of Murray Edwards College.

He is buried at the Parish of the Ascension Burial Ground in Cambridge.

From Venn’s

Adm. pens. at TRINITY, Oct. 30, 1868. [5th] s. of Charles Robert (1827), of Down, Bromley, Kent.
B. there May 13, 1851.
School, Woodbridge, Suffolk (private). Matric. Michs. 1870; B.A. 1874; M.A. 1877.
Took up an apprenticeship at the works of Messrs Easton and Anderson, engineering firm; designed and made a klinostat for recording the rate of growth of small plants.
Returned to Cambridge and co-operated with A. G. Dew Smith in the making of scientific instruments; [this business became the Cambridge Scientific Instrument Co. Ltd., in 1895]. Designed a number of anthropometric instruments.
Took a keen interest in Local Government.
Mayor of Cambridge, 1896-7. For several years a director of the Cambridge Gas Co. Chairman of the Cambridge Instrument Co. Ltd., where the most delicate and intricate of scientific instruments were made.
F.R.S., 1903.
A member of the Advisory Committee on Aeronautics.
Chairman of the Air Invention Committee, 1917.
K.B.E., 1918.
Married Hon. Emma (née Farrer) and had issue.
Interested in the training of mentally deficient children; assisted in establishing a home for them at Girton.
Contributed towards the endowment of a studentship in the University for the study of mental diseases.
A distinguished inventor whose genius and stimulating influence served to raise the standard of instrument-making in Great Britain to a high level. 'By his passing Cambridge has lost yet another distinguished son, a man whose inventitive and scientific genius has increased the debt which the world of science owes to this famous family....' Died at the Orchard, Huntingdon Road, Cambridge, Sept. 22, 1928.
Buried in Huntingdon Road cemetery, Cambridge.
Brother of George H. (1863), etc.
(The Times, Sept. 22, 1928.)