This has been facilitated by many special studies. Gibbon covers much of the ground in chapters 6, 14, 17, 18, 29, 35, 36 and 41; and Professor Guglielmo Ferrero sheds new light at some points in his great work, _The Greatness and Decline of Rome_ (Eng. trans. 5 vols. 1907-1911), though his economics at times calls for revision. Cp. Alison on "The Fall of Rome," in _Essays_, 1850, vol. iii (a useful conspectus, though flawed by some economic errors); Spalding's _Italy and the Italian Islands_, 3rd ed. 1845, i, 371-400; Dureau de la Malle, _Économie politique des Romains_, 1840, t. ii; Robiou et Delaunay, _Les Institutions de l'ancienne Rome_, 1888, vol. iii, ch. 1; Fustel de Coulanges, _Le Colonat romain_, etc.; Finlay, _History of Greece from its Conquest by the Romans_, ed. 1877, ch. i, §§ 5-8; Long, _Decline of the Roman Republic_, vol. i, 1864, chs. xi, xii, xx (a work full of sound criticism of testimonies); W.T. Arnold, _Roman Provincial Administration_, 1879; Brooks Adams, _The Law of Civilisation and Decay_, 1897, ch. i; and Dr. Cunningham's _Western Civilisation_, vol. i, 1899. Among many learned and instructive German treatises may be noted the Preisschrift of R. Pöhlmann, _Die Uebervölkerung der antiken Grossstädte_, Leipzig, 1884. Special notice is due to the recent work of W.R. Patterson, _The Nemesis of Nations_, 1907--a valuable study of slavery. As we have first traced them, the Romans are a cluster of agricultural and pastoral tribes, chronically at war with their neighbours, and centring round certain refuge-fortresses on one or two of the "Seven Hills." Whether before or after conquest by monarchic Etruscans, these tribes tended normally to fall into social grades in which relative wealth and power tended to go together. The first source of subsistence for all was cattle-breeding and agriculture, and that of the richer was primarily slave labour, a secondary source being usury. Slaves there were in the earliest historic times. But from the earliest stages wealth was in some degree procured through war, which yielded plunder in the form of cattle,[161] the principal species of riches in the ages before the precious metals stood for the command of all forms of wealth. Thus the rich tended to grow richer even in that primitive community, their riches enabling them specially to qualify themselves for war, so getting more slaves and cattle, and to acquire fresh slave labour in time of peace, while in time of war the poor cultivator ran a special risk of being himself reduced to slavery at home, in that his farm was untilled, while that of the slave-owner went on as usual.[162] Long before the ages described as decadent, the lapse of the poor into slavery was a frequent event. "The law of debt, framed by creditors, and for the protection of creditors, was the most horrible that has ever been known