Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Vice and virtue are chemical products like vitriol and sugar.
You may have noticed that there is no epigraph in any of the translations of Thérèse Raquin, but there is one in the original French. That's because Zola used the epigraph only in the very first printing of the novel, way back in 1867.
By the time the second edition of the novel was released in 1868, Zola had replaced this epigraph with his infamous Preface, which was written in response to hostile criticism of the novel. But we digress.
A few words on the epigraph itself. It was taken from the Introduction to English Literature, written by the philosopher Hippolyte Taine. This quote exemplifies the objective, morally-detached stance of naturalist writers like Zola, who believed that moral values (like vice and virtue) are nothing more than bodily reactions.
In Thérèse Raquin, this quote is especially relevant, since the two main protagonists commit a murder. And from the naturalist standpoint, they're not responsible for that murder; they're just at the mercy of biology and circumstance.
Terrifying.