After Cornwallis was removed from command of the Channel Fleet in early 1806, Whitby and his family were invited to live on his Newlands estate at Milford on Sea.
John died shortly afterwards, on 6 April 1806, but Mary remained as a companion for Cornwallis during his retirement – he was prone to depression, and as he had spent much of his life at sea, he had few close friends in society and had never married.
In 1835, Whitby was travelling in Italy when she encountered stories of an English businessman who had made substantial profits from silkworms on a mulberry plantation near Milan.
[10] Here she met Charles Darwin, who later asked her to perform a series of experiments on heritability in silkworms, looking at whether characteristics such as silklessness were passed down between generations of worms – "for in a work which I intend some few years hence to publish on variation, there will be hardly any facts in the insect world".
[11] She provided him with a set of specimens of moths, noting their sexual dimorphism, and promised to carry out some more detailed investigations;[12] her selective breeding experiments over the next two years persuaded Darwin that characteristics in the larval stage were inherited, a result which he later published in The Variation of Animals and Plants Under Domestication (1868).