It was a challenge to the economic doctrines of the time, especially Mercantilism. The Mercantilists thought that the only way to increase wealth was to exploit neighbours and colonists, but they failed to see that commerce and agriculture afforded equally satisfactory methods. Nor must we forget the Physiocrats’ influence upon practical politics. Sully, the French minister, betrays evidence of their influence when he remarks that the only two sources of national wealth are land and labour. Let us also remember that, despite some glaring mistakes, agriculture has never lost the pre-eminence which they gave it, and that the recent revival of agricultural Protection is directly traceable to their influence. They were always staunch Free Traders themselves, but we can hardly blame them for not being sufficiently sanguine to expect such whole-hearted acceptance of their views as to anticipate some of the more curious developments of their doctrines. It is almost certain that if they were living to-day they would not be found supporting the Protectionist movement. At least this is the opinion of M. Oncken, the economist, who has made the most thorough study of their ideas.[38] Although the Physiocratic distinction between agriculture and industry was largely imaginary, it is nevertheless true that agriculture does possess certain special features, such as the power of engendering the forces of life, whether vegetable or animal. This mysterious force, which under the term “nature” was only very dimly understood by the Physiocrats, and still is too often confused with the physico-chemical forces, does really possess some characteristics which help us to differentiate between agriculture and industry. At some moments agriculture seems inferior because its returns are limited by the exigencies of time and place; but more often superior because agriculture alone can produce the necessaries of life. This is no insignificant fact; but we are trenching on the difficult problems connected with the name of Malthus. III: THE CIRCULATION OF WEALTH The Physiocrats were the first to attempt a synthesis of distribution. They were anxious to know—and it was surely a praiseworthy ambition—how wealth passed from one class in society to another, why it always followed the same routes, whose meanderings they were successful in unravelling, and how this continual circulation, as Turgot said, “constituted the very life of the body politic, just as the circulation of the blood did of the physical.”