The most striking behavioural characteristic of mammals is their learning ability. Compared with other animals, mammals can learn faster, learn more, remernher 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. The dependence of learning on the adapted innate basis has already been discussed in the last chapter. lt does, however, imply certain characteristics in that innate basis. As we have already seen, complex behaviour which is yet sufficiently flexible to cope with a variety of circumstances has in many cases been achieved on the basis of built-in units which are small, often individual movements. These can be adaptively tailored by learning to fit a multiplicity of situations.