It's always seemed to me (from Tom and Huck, at least) that Twain's respect for nature was so premordial and primitive as to be animist--all those superstitions about the owl and the snakeskin etc. From there, eventually, you could come full circle to the nearly mystic environmentalism you can see in Earth IN the Balance. Twain's doesn't get very far along that arc, maybe, but his descriptions of the river fog, thunderstorms, etc, in Huck, plus his late misanthropy, that there was a potential environmentalist at work there! Dan Walker