Not every need can be obtained, not every foe avoided. But a living species must have acquired a behaviour that strikes a compromise in avoiding the deadliest foes and satisfying the most urgent needs from the sources of easiest access, so that it does survive. A favourable mutation makes certain sources more easily accessible, or reduces the danger from certain foes, or both. It thereby increases the chance of survival of the individuals endowed with it, but in addition it shifts the most favourable compromise, because it changes the relative weights of those needs or foes on which it bears.
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