Tonight sees the unveiling of a belated 200th-birthday present to Charles Darwin in London's Natural History Museum - a sculpture consisting of a 5mm section of a 200-year-old oak tree, including trunk, roots and branches, installed on the ceiling. Sculptor Tania Kovats dreamed up her work in a camper-van in Patagonia (on a trip round South America, during which she used Darwin's The Voyage of the Beagle "as a travel guide - and a surprisingly fresh one").