Actually, a university professor who specialises in science history once told me that Darwin was referring to human ethnicities in the subtitle, and she's a far more qualified source than TalkOrigins or any other biased propaganda website that claims otherwise. Still, the title of the book doesn't invalidate the content.
Since Darwin doesn't speak of human evolution at all in Origin of Species, the word "races" in the subtitle of that book probably refers to varieties and sub-species of plant and animal populations. I don't know why your science historian would say otherwise, given the content of the book.
In Descent of Man, where he does discuss human evolution, he does use "race" to refer to ethnicities including Maori, Tasmanian, Scots, French and Irish as well as several others.
Interestingly, Darwin did not consider skin colour to be a significant indicator of race and almost always refers to ethnicities geographically rather than in terms of colour. Probably the terms he uses that moderns would find most objectionable are "civilized races" (note the plural) and "savage races". It is pretty clear he thinks the European races "civilized" and all others "savage" or at best partially "civilized". Even so, he still draws distinctions among the various European races, clearly considering the Irish a good deal less civilized than the Scots.
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