Zola’s reason for writing this novel, and many others, was to explore what it means to understand that we evolved from wild and savage animals. He makes no mention of evolution per se, but no reader at the time could miss his message. Speakers such as Herbert Spencer, and even preachers like Henry Ward Beecher, went around saying evolution was God’s way of ennobling us. Zola had a different view: evolution condemned us, rather than ennobling us.
This view of the human beast also meant that if someone was in the way of what you wanted to have, whether money or sex, you simply got them out of the way. One of the main characters, the woman Séverine, with whom nearly all of the men were in love and who took advantage of their weaknesses, thought to herself (about killing her husband), “He was in the way, so you got rid of him; what could be more natural?”
But these views were based on an understanding of evolution that we now know to be incorrect. The human beast was not just violent and aggressive; when Zola wrote, “The woman belonged to the [cave]man who could kill his rivals,” he was only partly right. Human nature also includes love and altruism. It is instinctual for us to be bad; it is also instinctual for us to be good. I have written frequently about altruism. Zola was therefore wrong, but so were most scientists. His characters thought, evolution has made me evil; but, in fact, evolution had made them both evil and good at the same time. (One of the characters had a psychiatric disorder; for him, perhaps, being good was impossible.)
The final scene is unforgettable. The only two main characters left alive, two train operators, hated each other because they both loved a woman whom one of them had just murdered. As the train moved at full speed, the men struggled and both fell off the train. The uncontrollable train, full of soldiers headed off to the front in the Franco-Prussian War, flew off to an inevitable catastrophe that even Zola did not dare to write about. This is the picture Zola leaves us with about what the world is like: it is a train on which selfish and clueless humans ride to their destruction.The real “bête humaine” was society. That is, what some scientists today call the superorganism. No matter what happened, whether murders or train wrecks, the trains were back on their normal schedules with a delay of, at most, one day. The trains, the machine of life, all of society, continue on as if nothing good or (usually) bad has happened in any individual lives.