Having laboured for nearly five years on human evolution, sexual selection, and the expression of emotions, Darwin was able to devote 1873 almost exclusively to his beloved plants. He resumed work on the digestive powers of sundews and Venus fly traps, and the comparative fertility and vigour of self- and cross-pollinated species, work that would culminate in two books, Insectivorous plants (1875) and Cross and self fertilisation (1876). Darwin's son Francis became increasingly involved in this botanical research, eventually renouncing plans for a medical career to become his father's scientific secretary.