Zola was determined to write the history of a certain family at the
time of the Empire, on the ground of conditions produced by it, in
consideration of the law of heredity.

There was a question even about something more than this
consideration, because this heredity had to become the physiological
foundation of the work. There is a certain contradiction in the
premises. Speaking historically Rougon-Macquart had to be a picture
of French society during its last times. According to their moral
manifestations of life, therefore, they ought to be of themselves more
or less a normal family. But in such a case what shall one do with
heredity? To be sure, moral families are such on the strength of
the law of heredity--but it is impossible to show it in such
conditions--one can do it only in exceptional cases of the normal
type. Therefore the Rougon are in fact a sick family. They are
children of nervousness. It was contracted by the first mother of the
family, and since that time the coming generations, one after another,
followed with the same stigma on their foreheads. This is the way the
author wishes to have it, and one must agree with him. In what way,
however, can a history of one family exceptionally attainted with a
mental disorder be at the same time a picture of French society, the
author does not explain to us. Had he said that during the Empire
all society was sick, it would be a trick. A society can walk in the
perilous road of politics or customs and be sick as a community, and
at the same time have healthy individuals and families. These are two
different things. Therefore one of the two: either the Rougon are
sick, and in that case the cycle of novels about them is not a picture
of French society during the Empire--it is only a psychological
study--or the whole physiological foundations, all this heredity
on which the cycle is based, in a word Zola's whole doctrine, is