Ida, Lady Darwin (née Farrer; 7 November 1854 – 5 July 1946) was the wife of Sir Horace Darwin, member of the Ladies Dining Society, and a co-founder in 1913 of the Central Association for the Care of the Mentally Defective (in 1921 renamed the Central Association for Mental Welfare).
Darwin was born Emma Cecilia Farrer and took the name Ida from Hans Christian Andersen’s fairy tale of Little Ida's Flowers. Her father was Permanent Secretary to the Board of Trade Thomas Farrer and her mother was Frances Erskine, daughter of the historian and orientalist William Erskine and granddaughter of Sir James Mackintosh. Thomas Farrer was a friend of Charles Darwin and, following the death of Frances Farrer, married Katherine Wedgwood, niece of Emma Darwin. On 3 January 1880, she married her stepmother's cousin, Sir Horace Darwin, youngest son of Charles and Emma Darwin, at St Mary's, Bryanston Square. The couple had a son and two daughters:
Erasmus Darwin IV (7 December 1881 – 24 April 1915) who was killed in the Second Battle of Ypres during the First World War.
Ruth Frances Darwin (1883–1972) who was an advocate of eugenics and married Dr William Rees-Thomas.
Emma Nora Darwin (1885–1989) who edited the 1959 edition of The Autobiography of Charles Darwin and married the civil servant Sir Alan Barlow.
Ida and Horace Darwin settled in Cambridge, where they lived at "The Orchards", a 24-room mansion on Huntingdon Road. A full complement of servants gave Darwin the leisure to pursue activities outside the home. In 1883 she was a founding member of the Cambridge Association for the Care of Girls, an organisation that identified working-class girls who were deemed to be wayward or out of control and placed them in domestic service or sent them to training institutions for service, and that also ran recreational clubs for girls. She later joined the Ladies Dining Society.
Darwin was also on the committee of the Cambridge Charity Organisation Society and in 1908, in response to the recommendations of Royal Commission for the Care and Control of the Feeble-Minded, together with Florence Ada Keynes formed a sub-committee to enquire into the number of "defective" children in Borough schools. This sub-committee became the Cambridge Association for the Care of the Feeble-Minded and members included the Mayor of Cambridge, the Regius Professor of Medicine and representatives from the Borough Education Committee, the Eastern Counties Asylum in Colchester and the Cambridge Association for the Care of Girls. The group campaigned for the passage of legislation that would put the recommendations of the Royal Commission into force and organised meetings and conferences. In 1912 the Association, jointly with Cambridge University Eugenics Society, held a meeting in the Guildhall, where Ellen Pinsent read a paper on Mental Defect and its Social Dangers.