No slave system has ever been able to continue to function on the slaves provided by its own biological reproduction because the rate of human reproduction is too slow and the expense from infant mortality and years of unproductive upkeep of the young make this prohibitively expensive. This relationship is one of the basic causes of the American Civil War, and was even more significant in destroying ancient Rome.

Chapter 9, Classical Civilization, p. 318 - The Evolution of Civilizations (1961) (Second Edition 1979)