ABSTRACT
A stimulus is a small change of energy outside or inside an animal which leads to a large change of energy output by the animal's effectors. Sense organs may be looked on as instruments which measure either the intensity or the amount of some form of energy falling on them. To make sense, any statement concerning stimuli must specify what kind of energy is involved. The change in energy output, that is, in behaviour, is mediated by the central nervous system. The existence of a number of tame strains of a species which is easily available in the wild form makes Rattus norvegicus a particularly convenient animal in which to study behaviour. Some behaviour consists of species-characteristic fixed action patterns: in them the effects of learning are often small or barely observable. An important feature of the changes of behaviour is that they are adaptive: that is, they tend to make more probable the survival of the individual.
