Merdeis an unusual (very unusual) and witty investigation into a subject you may always have wondered about--but didn't know quite what to ask.          History, biology, anthropology, culture, animal behavior--all of these are the real subjects of Merde. Why can some animals do it on the run, and others can't? Why does camel dung make good fires? What are the fascinating stories of the dung beetles?          Myths and legends, physical features, health and disease, uses for construction and as fertilizers--even nutritional values!--Ralph Lewin writes about them all in the most ingratiating and sophisticated and yet scientific way. Merde is also full of personal adventures and observations, as well as anecdotes and examples.          The scattered literature on this subject is voluminous, but until now no one has perused and compiled it all and given it a personal touch, so to speak. It will be hard not to talk about this treasure trove of a book after you've finished it--or perhaps even when you're in the middle of it.