section_path: Biography/Descent of Man, sexual selection, and botany
section_name: Descent of Man, sexual selection, and botany
paragraph_text: Descent of Man, sexual selection, and botany
  
  Despite repeated bouts of illness during the last twenty-two years of his life, Darwin's work continued. Having published On the Origin of Species as an abstract of his theory, he pressed on with experiments, research, and writing of his " big book ". He covered human descent from earlier animals, including the evolution of society and of mental abilities, as well as explaining decorative beauty in wildlife and diversifying into innovative plant studies.
  
  Enquiries about insect pollination led in 1861 to novel studies of wild orchids, showing adaptation of their flowers to attract specific moths to each species and ensure cross fertilisation. In 1862 Fertilisation of Orchids gave his first detailed demonstration of the power of natural selection to explain complex ecological relationships, making testable predictions. As his health declined, he lay on his sickbed in a room filled with inventive experiments to trace the movements of climbing plants. Admiring visitors included Ernst Haeckel, a zealous proponent of Darwinism incorporating Lamarckism and Goethe 's idealism. Wallace remained supportive, though he increasingly turned to Spiritualism.
  
  Darwin's book The Variation of Animals and Plants Under Domestication (1868) was the first part of his planned "big book", and included his unsuccessful hypothesis of pangenesis attempting to explain heredity. It sold briskly at first, despite its size, and was translated into many languages. He wrote most of a second part, on natural selection, but it remained unpublished in his lifetime.
  
  Lyell had already popularised human prehistory, and Huxley had shown that anatomically humans are apes. With The Descent of Man, and Selection in Relation to Sex published in 1871, Darwin set out evidence from numerous sources that humans are animals, showing continuity of physical and mental attributes, and presented sexual selection to explain impractical animal features such as the peacock 's plumage as well as human evolution of culture, differences between sexes, and physical and cultural racial classification, while emphasizing that humans are all one species. According to an editorial in Nature journal: "Although Charles Darwin opposed slavery and proposed that humans have a common ancestor, he also advocated a hierarchy of races, with white people higher than others."
  
  His research using images was expanded in his 1872 book The Expression of the Emotions in Man and Animals, one of the first books to feature printed photographs, which discussed the evolution of human psychology and its continuity with the behaviour of animals. Both books proved very popular, and Darwin was impressed by the general assent with which his views had been received, remarking that "everybody is talking about it without being shocked." His conclusion was "that man with all his noble qualities, with sympathy which feels for the most debased, with benevolence which extends not only to other men but to the humblest living creature, with his god-like intellect which has penetrated into the movements and constitution of the solar system—with all these exalted powers—Man still bears in his bodily frame the indelible stamp of his lowly origin."
  
  His evolution-related experiments and investigations led to books on Insectivorous Plants, The Effects of Cross and Self Fertilisation in the Vegetable Kingdom, different forms of flowers on plants of the same species, and The Power of Movement in Plants. He continued to collect information and exchange views from scientific correspondents all over the world, including Mary Treat, whom he encouraged to persevere in her scientific work. He was the first person to recognize the significance of carnivory in plants. His botanical work was interpreted and popularised by various writers including Grant Allen and H. G. Wells, and helped transform plant science in the late 19th century and early 20th century.