This first day cover is for the Royal Mail "Darwin" stamp issue. The cover design shows Bletchley Park Lake with the Mansion in the background.
Darwin died in 1882 and Bletchley Park was built in 1883 so he would never have visited it. But Sir Herbert and Lady Leon made Bletchley Park a naturalist's delight with rare plants and trees to encourage wildlife and a well stocked lake. Sir Herbert was also a keen orchid grower and even sent gardeners on overseas trips to collect rare specimens. Both enjoyed farming and breeding cattle, sheep and other animals, particularly rare breeds. No doubt the library at Bletchley Park possessed a fair number of Darwin's works.