5.1 The nervous system
Development has so far mostly been assessed in terms of the gross performance, the overt behaviour, of the organism. Relationships have been established between certain environmental events and certain behaviours. In the next three sections, Sections 5 to 7, there is a major shift in emphasis, from considering behaviour to considering the structure of the nervous system. Essentially, this is doing no more than stepping down a level to look at what is going on inside the organism, in particular with its nervous system, during development. By understanding something of how the nervous system develops, it is possible to explain the mechanism(s) by which environmental stimuli affect behavioural development. The final two sections, Sections 8 and 9, return again to consider behaviour, except that in these two sections the issue is not the extent to which environmental events can affect the development of behaviour, but the extent to which genes can affect behaviour.