It is evident, therefore, that the doctrine does not imply economic fatalism. It does not deny that ideals may influence historical developments and individual conduct. While, as we shall see in a later chapter, it is part of the doctrine that classes are formed upon a basis of unity of material interests, it does not deny that men may, and often do, act in accordance with the promptings of noble impulses and humanitarian ideals, when their material interests would lead them to do otherwise. We have a conspicuous example of this in the life of Marx himself; in his splendid devotion to the cause of the workers through years of terrible poverty and hardship when he might have chosen wealth and fame. It is known, for example, that Bismarck made the most extravagant offers to enlist the services of Marx, who declined them at the very time when he was suffering awful privations. Marx himself has noted more than one instance of individual idealism triumphing over material interests and class environment, and, by a perversity that is astonishing, and not wholly disingenuous, some of his critics, notably Ludwig Slonimski,[81] have used these instances as arguments against his theory, claiming that they disprove it! We are to understand the materialistic theory, then, as teaching, not that history is determined by economic forces only, but that in human evolution the chief factors are social factors, and that these factors in turn are _mainly_ molded by economic circumstances.[82] This, then, is the basis of the Socialist philosophy, which Engels regarded as "destined to do for history what Darwin's theory has done for biology." Marx himself made a similar comparison.[83] Marx was, so Liebknecht tells us, one of the first to recognize the importance of Darwin's investigations to sociology. His first important treatment of the materialistic theory, in "A Contribution to the Critique of Political Economy," appeared in 1859, the year in which "The Origin of Species" appeared. "We spoke for months of nothing else but Darwin, and the revolutionizing power of his scientific conquests,"[84] says Liebknecht. Darwin, however, had little knowledge of political economy, as he acknowledged in a letter to Marx, thanking the latter for a copy of "Das Kapital." "I heartily wish that I possessed a greater knowledge of the deep and important subject of economic questions, which would make me a more worthy recipient of your gift," he wrote.[85] IV