“. . . They bred in and in, as might be shown,
Marrying their cousins—nay, their aunts, and nieces,
Which always spoils the breed, if it increases.
This heathenish cross restored the breed again,
Ruin’d its blood, but much improved its flesh;
For from a root the ugliest in Old Spain
Sprung up a branch as beautiful as fresh;
The sons no more were short, the daughters plain” (lines 454-461)
The first canto of Don Juan, from which I have taken these lines, was published in 1819, well before Charles Darwin began his research on trait heritability. Popular knowledge is that Darwin (who married and had ten children with his first cousin; of them, three died as children and three more were infertile) was the first to posit that close familial relationships between parents might have a negative impact on their children’s health. (Ironically, he figured that out by examining some tomato plants, not his own family.)
That said, ancient cultures frequently had taboos against incest; I know the Code of Hammurabi forbids it, and I think the Bible does as well. Certain members of the Habsburg family were so affected by their consanguineous ancestry that physicians marveled that they continued to draw breath, but I cannot find any texts from the era that conclusively show that the cause was known. I wonder, then, what exactly was new in Darwin’s research. Lord Byron, writing when Darwin was all of ten years old, seems to have a fairly solid grasp of the pattern, if not the mechanics, of inbreeding, though it is interesting that he only notes cosmetic flaws in the inbred family, not health ones. This isn’t even remotely the point of the poem, but that passage jumped out at me from a history-of-science perspective, and I’d love to hear if anyone else knows more than I do.