Foucault observes the contrast between two models of heredity in the nineteenth century: aristocratic genealogy that tracked bloodlines and bourgeois genealogy that attended to the health and vitality of the body.

[T]he aristocracy had . . . asserted the special character of its body . . . in the form of blood, that is, in the form of the antiquity of its ancestry and of the value of its alliances; the bourgeoisie on the contrary looked to its progeny and the health of its organism when it laid claim to a specific body. The bourgeoisie’s “blood” was its sex.

To put the contrast crudely, the bourgeoisie sought hereditary distinction in producing strapping lads and lovely lasses whose vital bodies would improve the stock, much as breeders of horses sought to produce racing champions. Aristocrats, on the other hand, risked bleeding out from hemophilia for the sake of reproducing a pure (and politically advantageous) bloodline. Never mind the health of the organism when the power of the family line was at stake.

The bourgeois model of heredity was Darwinian in its understanding of species-level change through variation over time. As Foucault remarked in his “Candidacy Presentation,” it was no accident that Darwin relied on the research of agricultural breeding to come to some of his most important insights. When applied to human heritability, the bourgeois model inclined to optimism about improvement of the “race” through careful breeding—as well as to anxiety about “degenerescence” through incest or miscegenation. Not to be confused with the political breeding of the aristocrats, bourgeois breeding extended to “body hygiene, the art of longevity, ways of having healthy children . . . and methods for improving the human lineage.” The bourgeois model of heredity thus lent itself to “a dynamic racism, a racism of expansion,” in which humanity takes control of its own evolution to better the race.

Purity of bloodline was only a secondary concern for the aristocracy. According to Foucault, purity derived its importance from political concerns. Aristocratic genealogies interpreted heredity according to political opportunity. Genealogy in this sense is always hermeneutical and always inventive. Nobility is expressed primarily in the politically opportune marriage. An aristocratic stemma, or family tree, exists to reveal and validate these opportunities.

This background allows us to make two observations about Foucault’s terminology. First, when Foucault says “genealogy,” he does not mean the natural sciences’ method of studying heritability. And he is more interested in the aristocratic, rather than the bourgeois approach, to heredity. This first observation affords a second: Foucault contrasts the science of genetics to aristocratic genealogy. And he adopts the aristocratic version of genealogy—concerned with sanguinity for political purposes—as the model for his analogical use of the term to name his own historical method. When Foucault says “genealogy,” he is thinking of aristocratic genealogy.

These observations enable us to parse this mature statement of Foucault’s genealogical method:

This kind of method entails going behind the institution and trying to discover in a wider and more overall perspective what we can broadly call a technology of power. In the same way, this analysis allows us to replace a genetic analysis through filiation with a genealogical analysis—genealogy should not be confused with genesis and filiation—which reconstructs a whole network of alliances, communications, and points of support.

(missing reference)