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The only scientific way to deal with adaptation is to get the facts for each case. Only after the facts are known is it possible to tell just how much of the adaptedness of a given phenomenon is due to inherited evolutionary prearrangements, and how much to direct adjustive interactions. The ratio varies greatly and unpredictably from species to species, from function to function, from unit to unit.
Whereas, in some lower orders of animal, behavioural systems on first appearance in the life of an individual are, like Venus, already of almost perfect form, in higher orders they more usually take at first a primitive form and proceed thence to undergo an elaborate process of development. Although newborn birds and mammals are equipped with a few behavioural systems capable at once of fulfilling a vital function, e.g. food-intake, initially they are apt to fulfil it only very inefficiently. A number of other systems, moreover, on first manifesting themselves are so ill organised that not only is the behaviour for which they are responsible incomplete but the functional consequences that will later commonly follow it are conspicuous by their absence. In general, therefore, it is only as an individual bird or mammal grows older that its behavioural systems become complete and that the functional consequences of their activation come, as a rule, to follow with regularity and efficiency.
The behavioural equipment of newborn birds and mammals not only is limited in scope but is simple in form; and of no mammal is this more true than the human baby. Yet by the age of two years a human child is already talking, and soon afterwards he can use language as a means of ordering and controlling behaviour. Thus during this brief proportion of his life-span the sophistication of the behavioural systems operative