It is covered in Darwins writings. More than that it is simple logic. If there is no creator than there has to be spontaneous generation. No he doesn’t state specifically that a rock came to life but again before life existed rocks are about all there is. The biggest problem you and other evolutionist have is you don’t want to defend this simple fact of the theory, you would rather get past the hard part and argue the part that is logical put still unproven. Just as you wish to state evolution as a proven fact, which it is not. Science by consensus is not science. Perfectly legitimate scientist express doubt in the TOE and are denied tenure and otherwise persecuted professionally, not what I would called honest scientific debate. What it is is the same science practiced by the perpetrators of global warming one either subscribes to it or he faces the unprofessional dogma of these bullies masquerading as scientists.
It’s interesting the way creationists, if they can’t argue against what someone actually said, will argue against what they “must have meant.” It’s one of the traits that distinguishes creationism from real science.
But I think I’ve found what you’re referring to. Darwin did once speculate in a letter to a friend about life forming “in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present”—but he was doing so in the context of saying why such life wouldn’t survive today. He was not saying that that’s how he thought life began originally.
And besides, it was just speculation in a personal letter. What he was willing to say in print was “[how life itself first originated] are problems for the distant future, if they are ever to be solved by man” (Descent of Man, chapter 3) and “our ignorance is as profound on the origin of life as on the origin of force or matter” (essay to the Atheneum, 1863). In other words, he didn’t know, and he didn’t think it was necessary to his theory.
Note the part about the origin of force or matter. Do you require physicists and chemists to state where those come from before you accept their theories about how they work?
And evolution is a fact. Living things once existed that don’t exist any more, and exist now that didn’t use to exist. The theory of evolution explains why that’s so.