Tarde also insisted that whilst the human race had advanced intellectually and technologically, its moral development lagged far behind. Atavistic impulses had adapted themselves to changing conditions and to technological progress. Tarde had worked as a judge’s assistant and as an examining magistrate at Sarlat, in the Périgord, since 1867. His Comparative Criminology was written out of twenty years of practical experience of criminal investigation and it also had a practical objective. Tarde was concerned to ensure that the administration of justice did not consist merely in the application of summary sentences but that each case would be judged with reference to all the particularities of character and circumstance that attended it.

Echoes of this criminological debate are sensed in a number of ways in La Bête humaine. Most obvious is the suggestion, introduced early in the novel, that Jacques’s urge to kill is an atavistic impulse whose origins are lost in the remote past. ‘Was this the swollen legacy of a grudge that had passed from man to man since the first infidelity in the dark recesses of some primeval cave?’ Jacques asks himself (II). The question is never resolved. It returns insistently throughout the novel, tormenting Jacques’s conscience and calling for the reader’s attention. Yet however remote its possible origins, Jacques’s urge to kill is also perceived as the legacy of a more recent past, an unwanted bequest from the family of drunkards and delinquents into which he has been born and whose tainted blood he has inherited. Jacques gloomily reflects on the generations of Macquarts and Lantiers that have preceded him. ‘It couldn’t really be called a normal family. So many of them had some flaw, and he often thought he must have inherited this family flaw himself’ (II). It is this hereditary ‘flaw’ (the French ‘fêlure’ literally means ‘crack’) that allows resentments and obsessions which no longer find room in the conscious memory to seep through the walls of social conditioning into a supposedly more civilized modern world. The notion of heredity acting as a mediating agent for primitive impulse or as a catalyst for some latent genetic disorder is close to the thinking of Tarde.

Some of the physical features which Lombroso had identified as distinguishing marks of the criminal also find their way into Zola’s description of characters in the novel. Roubaud is given a low forehead and short, hairy fingers; Jacques’s otherwise handsome appearance is marred by a pronounced lower jaw. This should not be taken to imply that the novel simply endorses Lombroso’s ideas and reduces the physical appearance of characters to a series of ready-made hallmarks of criminal types. Zola does not allow these supposedly ‘criminal’ features to go unchallenged. When Cabuche stands before the judge at the final trial, accused, according to the examining magistrate’s interpretation of events, not only of murder but of an act of necrophilia, Zola provides him, in a manner that verges on caricature, with the ‘enormous fists and carnivorous jaws’ appropriate to the crime committed. The well-dressed ladies crowding the reserved balcony of the courtroom eager to catch a glimpse of this monster of sexual depravity need no further convincing. Zola ensures, however, that the reader of the novel knows that Cabuche is perfectly innocent. More than this, Cabuche is the one character in the novel who displays any true generosity of spirit. His physical appearance thus belies his true nature; if he lives as a recluse in a hovel in the woods, it is partly because social prejudice against a man of his ‘type’ forces him to do so.

If the novel places a question mark against Lombroso’s criminal stereotyping, it appears more readily to embrace Tarde’s thinking on the ability of the criminal instinct to adapt itself to changing social conditions. This is seen in the way in which murderous intent exploits opportunities afforded by the new technology of the age, namely the railway. Both Roubaud and Flore are railway employees, and their lives are to a large extent regulated by the demands of their employment. Flore, it is true, manages from time to time to escape her duties and discovers a form of liberation and independence by roaming the nearby countryside. Yet she must always return to her job. It therefore comes as no surprise that when she and Roubaud plan their separate murders, they both think spontaneously of the railway as a means of achieving their end. Roubaud kills his victim in a reserved compartment on board an express train. Flore, considerably more resourceful and more calculating than Roubaud, thinks of three different ways of causing a train crash before chance provides her with an even better one. Pecqueux’s plan to murder Jacques also exploits the opportunity provided by his job with the railway company; he flings Jacques from the footplate of a locomotive travelling at speed.