Seventy-five species of vertebrate are endemic to the Eastern Arc, as well as thousands of species of invertebrates. Sixteen plant genera are endemic to these mountains, one of them being Saintpaulia, the African violet, now used as a houseplant around the world. Many of these endemics have evolved recently, but some are remnants of populations that were at one time more widespread. The Uluguru Mountains and the eastern Usambara Mountains have the greatest degree of biodiversity. Many endemic species are found on a single mountain range. Four of the endemic birds are similar to Asian species, and may have evolved at a time when the Arabian Peninsula had a coastal fringe of vegetation to act as a passageway; the Udzungwa forest partridge (Xenoperdix udzungwensis) is a relict and example of this, it is found only on the Rubeho Mountains and Udzungwa Mountains, and its closest relatives appear to be the hill partridges of Asia.