The narrative then switches over to the Macquart side of the family, whose grim working-class struggles to survive are juxtaposed keenly with the Rougons' seemingly trivial quest for greater wealth and influence in genteel drawing-room society. Descended from a drunken ne'er-do-well and a madwoman, Zola effectively predestines the Macquarts to lives of toil and misery. Zola's theories of heredity, laid out in the original preface to this novel, were a cornerstone of his entire philosophy and a major reason for his embarking on the mammoth Rougon-Macquart project in the first place in order to illustrate them. Largely discredited nowadays, the theories are largely "present but unseen" in most of the novels in the Rougon-Macquart cycle, allowing those books to be enjoyed without the overshadowing effect of Zola's somewhat suspect scientific ideas. Due to the original story nature of La Fortune des Rougon, the theories are placed much more to the fore, and can appear somewhat