One feature of biological organisms is that they normally occur in discrete species, with distinct differences between different species. It seems likely that the existence of such discreteness is related to the discreteness of underlying genetic programs. Currently there are a few million species known. Most are distinguished just by their habitats, visual appearance or various simple numerical characteristics. Sometimes, however, it is known that members of different species have the traditional defining characteristic that they cannot normally mate, though this may well be more a matter of the mechanics of mating and development than a fundamental feature.