The most striking behavioural characteristic of mammals is their learning ability. Compared with other animals, mammals can learn faster, learn more, reminder more and show more insight. Correlated with these abilities is the possession of well-developed sense organs, particularly the distance receptors concerned with olfaction, hearing and vision. These make it possible for behaviour to take into account and to be adapted to, an enlarged world, not limited to the objects in contact with or very close to the animal. Learning and memory enlarge this world still further, adding a time dimension and so taking in the past as well as the present. All this, however, does not mean that the innate basis of behaviour has a diminished importance. Different species of mammals have evolved to live in nearly all terrestrial and aquatic habitats on Earth. Mammals inhabit every terrestrial biome, from deserts to tropical rainforests to polar icecaps. Many species are arboreal, spending most or all of their time in the forest canopy. One group (bats) has even evolved powered flight, only the third time that this ability has evolved in vertebrates (the other two groups being birds and extinct Pterosaurs).