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The behavioural development of an individual needs to be considered in two quite distinct ways:
the way in which the parts of the behavioural equipment in active use change from one phase of the life-cycle to the next;
the way in which each part of that equipment comes to take the particular form it does.
Both themes are of the greatest interest to psychoanalysts. The first is dealt with only briefly in this chapter but is referred to again at the end of Chapter 12. The second, which concerns the ontogeny of behavioural systems, is of much intricacy and importance and is discussed in the next chapter.
To ensure survival of individual and species, it is necessary for an animal to be equipped with an appropriately balanced repertoire of instinctive behavioural systems at each stage of its life-cycle. Not only must an adult be so equipped but the young animal must also have a balanced and efficient equipment of its own. This is likely to differ in many respects from that of the adult. Furthermore, because the unit of biological adaptation is a population and the survival of populations of higher species is dependent on the co-operation of individuals, much of the equipment of one individual is complementary to that of another of different age or sex in the same population. Behaviour patterns mediating attachment of young to adults are complementary to those mediating care of young by adults; in the same way, systems mediating adult masculine behaviour in one individual are complementary to those mediating adult feminine behaviour in another. This emphasises afresh that instinctive behaviour is never intelligible in terms of a single individual but in terms only of a population of individuals collaborating together.
In all species of bird and mammal certain parts of the behavioural equipment that are operative during the immature phase of the life-cycle are different from certain parts that are operative during the adult phase. Such differences are of two main kinds:
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