As the sheer volume of his correspondence indicates, 1862 was a very productive year for Darwin. This was not only the case in his published output (two botanical papers and a book on the pollination mechanisms of orchids), but more particularly in the extent and breadth of the botanical experiments he carried out. The promotion of his theory of natural selection also continued: Darwin's own work on it expanded, Thomas Henry Huxley gave lectures about it, and Henry Walter Bates invoked it to explain mimicry in butterflies. As well as monitoring the progress of his scientific work, the correspondence also records the continuing effects of Darwin's ill-health. Serious illness in two of his children also disrupts his work.