Excerpt from Zola's preface:

I wish to explain how a family, a small group of human beings, conducts itself in a given social system after blossoming forth and giving birth to ten or twenty members, who, though they may appear, at the first glance, profoundly dissimilar one from the other, are, as analysis demonstrates, most closely linked together from the point of view of affinity. Heredity, like gravity, has its laws.

By resolving the duplex question of temperament and environment, I shall endeavour to discover and follow the thread of connection which leads mathematically from one man to another. And when I have possession of every thread, and hold a complete social group in my hands, I shall show this group at work, participating in an historical period; I shall depict it in action, with all its varied energies, and I shall analyse both the will power of each member, and the general tendency of the whole.

The great characteristic of the Rougon-Macquarts, the group or family which I propose to study, is their ravenous appetite, the great outburst of our age which rushes upon enjoyment. Physiologically the Rougon-Macquarts represent the slow succession of accidents pertaining to the nerves or the blood, which befall a race after the first organic lesion, and, according to environment, determine in each individual member of the race those feelings, desires and passions—briefly, all the natural and instinctive manifestations peculiar to humanity—whose outcome assumes the conventional name of virtue or vice. Historically the Rougon-Macquarts proceed from the masses, radiate throughout the whole of contemporary society, and ascend to all sorts of positions by the force of that impulsion of essentially modern origin, which sets the lower classes marching through the social system. And thus the dramas of their individual lives recount the story of the Second Empire, from the ambuscade of the Coup d’Etat to the treachery of Sedan.