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Nor will it be out of place here to utter a warning against any over-hasty
generalization of what we have brought to light concerning the distribution of
the various mental functions between the two systems. We are describing the
state of affairs as it appears in the adult human being, in whom the system Ucs. operates, strictly speaking, only as a preliminary stage of the higher
organization. The question of what the content and connections of that system are
during the development of the individual, and of what significance it possesses in
animals - these are points on which no conclusion can be deduced from our
description: they must be investigated independently. Moreover, in human beings we
must be prepared to find possible pathological conditions under which the two
systems alter, or even exchange, both their content and their characteristics.