In 1846 he completed his third geological book, and turned from supervising the publication of expert reports on the findings from the voyage to examining barnacle specimens himself. This grew into an eight-year study, making use of his theory to find hitherto unknown relationships between the many species of barnacle, and establishing his expertise as a biologist. His faith in Christianity dwindled and he stopped going to church. In 1851 his treasured daughter suffered a long illness and died. In 1854 he resumed his work on the species question which led on to the publication of Darwin's theory.