Donna Julia and inbreeding

“. . . 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.

3 thoughts on “Donna Julia and inbreeding”

  1. Great post, Dana. I’m pretty sure that practices with livestock helped to reveal the idea that the breeding of closely related animals produced problems. The breeding of plants (which the lines from Byron also allude to) probably revealed similar tendencies. And even before the processes of heredity were fully understood, people knew that one inherited traits from parents.

  2. On the topic of inbreeding/incest, I thought the entire dynamic of the love pentangle (?) between the five main characters seemed vaguely incestuous. Considering the narrator tells us Donna Inez at one point had some sort of romantic/sexual relationship with Don Alfonso, in a way, Don Juan is the could-have-been step-son to Don Alfonso. He then proceeds to begin his affair with Donna Julia, his psuedo step-sister. Obviously considering Byron and his personal relationship/writings to-do with incest, it seemed striking the only relationship that had a seemingly legitimate connection was the one with the most aspect of inbreeding.

  3. I know there’s something called the Westermarck Effect which is basically the theory that if you are raised from a very young age with someone (usually this theory applies to a sibling) then you won’t experience sexual attraction towards them. This is seen as a biological safeguard against incest, which can explain why even when people didn’t know about the increased possibility of birth defects, incest was seen as wrong and twisted.

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