Five years after his death, sons Robert and Samuel Wilberforce published a five-volume biography about their father, and subsequently a collection of his letters in 1840. The biography was controversial in that the authors emphasised Wilberforce's role in the abolition movement and played down the important work of Thomas Clarkson. Incensed, Clarkson came out of retirement to write a book refuting their version of events, and the sons eventually made a half-hearted private apology to him and removed the offending passages in a revision of their biography. However, for more than a century, Wilberforce's role in the campaign dominated the history books. Later historians have noted the warm and highly productive relationship between Clarkson and Wilberforce, and have termed it one of history's great partnerships: without both the parliamentary leadership supplied by Wilberforce and the research and public mobilisation organised by Clarkson, abolition could not have been achieved.