Abstract: What is the gradient? Amongst the earliest remarked, and best documented, of broad-scale spatial patterns of life on Earth is a trend for more species to be found towards lower latitudes. The latitudinal gradient in species richness is such that most extant eukaryote, perhaps also prokaryote, species are found in the tropics. Even when counted over quite small areas in which it is present, the species richness of a taxonomic group might commonly vary by orders of magnitude between high and low latitudes.