Atavism: "the reappearance of a characteristic in an organism after several generations of absence, caused by a recessive gene or complementary genes" (American Heritage Dictionary)
 

"Our civilization is nothing but a thin film or crust lying over a volcanic pit."
    --W. H. White, Rutherford's Deliverance (1885)

"I have read of men who, when forced by their calling to live for long periods in utter solitude--save for a few black faces--have made it a rule to dress regularly for dinner in order to maintain their self-respect and prevent and relapse into barbarism."
    --first sentence of Erskine Childers, The Riddle of the Sands (1903)

"You think that a wall as solid as the earth separates civilization from barbarism. I tell you the division is a thread, a sheet of glass."
    --John Buchan, The Power-house (1913)