In Wings of the Night, Howard anachronistically referred to evolution in the story line. On page 318 of the Del Rey edition titled The Savage Tales of Solomon Kane, Howard writes:
Perhaps they [the harpies] were the offspring of a forbidden and obscene mating of man and beast; more likely they were a freakish offshoot on the branch of evolution—for Kane had long ago dimly sensed the truth in the heretical theories of the ancient philosophers, that man is but a higher beast.At best this is unusual, but perhaps can be easily explained. While Howard does not elude to Darwin, his use of the term evolution is odd. He should have known better, especially since he has the Solomon Kane stories set around the turn of the 16th to 17th century. This clearly makes the use of the term anachronistic since evolution was not used until the late 19th century (a few decades after Darwin published his now famous work titled Origin of the Species).
By definition an anachronism is a literary error in chronology, or placing an event, item, ideal, or language expression in the wrong period. The most common anachronism occurs with language expressions. The lack of research regarding idioms and their time frames is a constant error in fiction. However, a philosophical or scientific concept is not as common.
Notice in the quote how Howard uses the term evolution but refers back to ancient philosophers. He's correct in this assessment—Aristotle and Plato both refer to mankind as higher animals (or as Howard puts it, "beasts.") but why use the term evolution? I think he did so because evolution was a hot topic in the 20s. The Scopes Monkey trial was a landmark trial in 1925, just a few years prior to Howard writing Wings in the Night. The fuss and talk about evolution and it's ramifications were a hot topic during this period.
Most, if not all, evangelical fundamentalists rejected the theory of evolution during this time. Howard obviously recognizes this and uses the language of his day, commonly used by these same fundamentalists: "for Kane had long ago dimly sensed the truth in the heretical theories." Even though this is a fairly large literary error in the story line, I do not think it was accidental. I think Howard was well aware of what he was writing and used it anyway due to current debate and events. Moreover, I think this makes the story more interesting because it let's readers of his day and today realize that Howard was using theories, ideals, and philosophies of his day to grapple over in his stories.
I actually love it when I find things like this in fiction, especially the obvious anachronisms. You know, the ones that we can be certain that the author clearly would have never used if not for a particular purpose.