Bats show a remarkable variety of adaptations to their environment and consequently vary considerably in size, appearance, and morphology. For example, the world's smallest bat, the 2 g Craseonycteris thonglongyai (Figure 1) from Thailand, is 600 times smaller than Pteropus vampyrus, which weighs up to 1.2 kg and has a wingspan of 1.8 m (Jones 1996). Bats occur worldwide, except in extreme polar and desert habitats. The species diversity of bats is highest in equatorial regions, notably in tropical forests, with progressively fewer species encountered with increasing latitudes in temperate climates (Schoeman et al. 2013, Herkt et al. 2016).

The oldest known bat fossil, Onychonycteris finneyi, was discovered recently in fossil beds in Wyoming, USA, and dates to 52 million years ago. Its diagnostic characters are intermediate between bats and non-flying mammals (Simmons et al. 2008). The fossil bat species had claws on each finger and relatively long hind legs in relation to its forelimbs, similar in ratio to sloths and lemurs. Its short, broad wings indicate that it probably alternated between flapping flight and gliding, and that it was also capable of clambering in trees. The small cochlea (ear) bones show that it could not echolocate, suggesting that flight evolved before echolocation in bats. By 50 million years ago, during the Eocene, there were already at least three families and 13 species of bats. Eocene fossil bats are known from North America, Europe and Australia. All these ancestors were already fully developed as bats, although they did not closely resemble extant species. Recent discoveries in Egypt of extinct bats from the late Eocene and early Oligocene (37–27 million years ago), which appear more closely related to modern species, suggest that bats may have diversified in Africa following migration of primitive ancestors from Europe (Gunnell et al. 2008).

Taxonomic classifications are essential to the universal communication of verifiable scientific knowledge (Cotterill 1995a, Ghiselin 2005), in which descriptions of species, and any other taxon, should adhere to scientific conventions of nomenclature (Winston 1999, Gardner and Hayssen 2004). In this respect, the persisting confusion over the real species diversity of African house bats (genus Scotophilus) testifies to why precise and accurate taxonomy is so critical to all of biology and conservation. This example especially highlights the relevance of type material, preserved in museums, if we are to apply scientific evidence to clarify the distinctiveness of a population in a taxonomic revision.