Ultimately different races, not a different species.
It is, BTW, no surprise that early scientist saw them as a different species. It hadn’t been that long before that that it had been a not uncommon scientific view that black persons were essentially just a related sub-species. That just being black made one substantively different from being white (a view, I might point out, that is mainly only held by Leftist and black persons these days).
In the past I’ve pointed out that this “scientific” view ultimately lay behind justification for anti-miscegenation laws, that these were the basis for persons not having the same status before the law and therefore being unable to marry. This would be similar to how slave could not marry free (no matter the race) or common not marry a royal or a legal child marry an adult or an idiot marry. I also pointed out that when the cause for different status before the Law went away, in was the case for when the view that black and white persons were substantively different fell, then the statutes based on that difference would inevitably crumble and such common law rights as the ability of a man (eligible to marry) to marry a woman (also eligible to marry) would trump the holdovers from bad science.
And, yes, I did point these things out to show how homosexual “marriages” are far different than interracial marriages, for the latter ultimately had no affect of the nature of marriage being between a man and a woman where the latter sought to completely alter the covenant by removing the logical and moral demand that each partner adds a specific diversity to the union ... whereas homosexual shackups simply lack the diversity of partners that the institution requires.