Previous chapters have dealt with problems of learning of a general nature and we have considered the fundamental mechanisms underlying the modifications in behaviour which enable organisms to adapt most effectively to changes in their environment. While we have stressed that the basic mechanisms of adaptation, conditional reflexes, are fundamentally the same in all organisms, there are great differences in the complexity of the adaptive behaviour. As we have seen, the behaviour of the most elementary organisms is largely governed by the inherited reflexes and is therefore rigid and stereotyped, whereas the behaviour of the mammals is highly adaptive and complex (perhaps most complex when working for man in such conditions as looking after sheep or performing in a circus).