[T]he human brain is an imperfect instrument
built up through long geological periods. Some
of its levels of operation are more primitive and
archaic than others. Our heads, modern man has
learned, may contain weird and irrational
shadows out of the subhuman past—shadows
that under stress can sometimes elongate
and fall darkly across the threshold of our
rational lives. Man has lost the faith of the
eighteenth century in the enlightening power of
pure reason, for he has come to know that he is
not a consistently reasoning animal. We have
frightened ourselves with our own black nature
and instead of thinking “We are men now, not
beasts, and must live like men,” we have eyed
each other with wary suspicion and whispered
in our hearts, “We will trust no one. Man is
evil. Man is an animal. He has come from the
dark wood and the caves.”

LOREN EISELEY
Darwin’s Century¹