The year 2009 will mark the bicentennial of Charles Darwin's birth and the 150th anniversary of the publication of The Origin of Species. From 1840 to his death in 1882, Darwin was constantly plagued by chronic illnesses that allowed him to work only a few hours at a time and by an obsession with his physical health. Was this the psychosomatic product of stress resulting from the development and public reception to his theory of evolution or the result of a disease or parasite obtained during the world traveler's excursions?   In 1977 Ralph Colp Jr. argued persuasively for the former explanation in his book To Be an Invalid: The Illness of Charles Darwin, now out of print, but considered to be one of the century's most important works on Darwin's life. Expanding and reworking his earlier arguments to take into account new information (including Darwin's "Diary of Health," included as an appendix), Darwin's Illness paints a more intimate portrait of the nature and possible causes of Darwin's lifelong illness, of the ways he and Victorian physicians tried treating it, and how it influenced his scientific work and relations with his family and friends.