My challenging toxic subject is the molecule. Not any particular nanoscale unit of potentially hazardous substance, nor any particular substance in the aggregate. Rather, I am interested in the material and historical grounds for the peculiar idea that molecules and only molecules may be the proximate causes of (chemical) toxicity. This principle that a toxin is a chemical is a molecule defines the stage on which most modern dramas of toxic chemical perceptibility and epistemology play out. It sustains, among other things, the perverse pattern of "regrettable substitution" (whence this essay's title), also known as "chemical whack-a-mole"—the replacement of a hazardous substance by a substitute that turns out to be just as toxic. The origins of this history lie in visualization: the late 19th century convention (much the same today) of representing chemical substances using doodles of letters, representing atoms, and lines, representing bonds. The principle of toxin-as-chemical-as-molecule came about through the transformation of such doodles into fixed chemical names and notation. Deployed within the authoritative information technologies of chemistry, from many-volume print reference works to present-day digital databases, this molecular vocabulary ordered the chemical world for the convenience of chemists and, especially, the synthetic chemicals industry. I would like to reverse this process. By foregrounding the contingency of the molecule as toxic agent, I wish to open space for remapping environmental toxicity along dimensions new and forgotten, such as that of 19th century Alsatian chemist Charles Gerhardt, for whom chemicals were beings “defined by their metamorphoses, that is, by their past or by their future."