Whilst learnt rules, which the individual came to obey habitually and almost as unconsciously as inherited instincts, increasingly replaced the latter, we cannot precisely distinguish between these two determinants of conduct because they interact in complicated ways. Practices learnt as infants have become as much part of our personalities as what governed us already when we began to learn. Even some structural changes in the human body have occurred because they helped man to take fuller advantage of opportunities provided by cultural developments. Neither is it important for our present
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