The Gene: An Intimate History - Siddhartha Mukherjee Page 0,10

the “law of births,” would ensure that only such harmonious “fortunate” unions would occur in the future. A political utopia would develop as a consequence of genetic utopia.

It took a mind as precise and analytical as Aristotle’s to systematically dismantle Pythagoras’s theory of heredity. Aristotle was not a particularly ardent champion of women, but he nevertheless believed in using evidence as the basis of theory building. He set about dissecting the merits and problems of “spermism” using experimental data from the biological world. The result, a compact treatise titled Generation of Animals, would serve as a foundational text for human genetics just as Plato’s Republic was a founding text for political philosophy.

Aristotle rejected the notion that heredity was carried exclusively in male semen or sperm. He noted, astutely, that children can inherit features from their mothers and grandmothers (just as they inherit features from their fathers and grandfathers), and that these features can even skip generations, disappearing for one generation and reappearing in the next. “And from deformed [parents] deformed [offspring] comes to be,” he wrote, “just as lame come to be from lame and blind from blind, and in general they resemble often the features that are against nature, and have inborn signs such as growths and scars. Some of such features have even been transmitted through three [generations]: for instance, someone who had a mark on his arm and his son was born without it, but his grandson had black in the same place, but in a blurred way. . . . In Sicily a woman committed adultery with a man from Ethiopia; the daughter did not become an Ethiopian, but her [grand]daughter did.” A grandson could be born with his grandmother’s nose or her skin color, without that feature being visible in either parent—a phenomenon virtually impossible to explain in terms of Pythagoras’s scheme of purely patrilineal heredity.

Aristotle challenged Pythagoras’s “traveling library” notion that semen collected hereditary information by coursing through the body and obtaining secret “instructions” from each individual part. “Men generate before they yet have certain characters, such as a beard or grey hair,” Aristotle wrote perceptively—but they pass on those features to their children. Occasionally, the feature transmitted through heredity was not even corporeal: a manner of walking, say, or a way of staring into space, or even a state of mind. Aristotle argued that such traits—not material to start with—could not materialize into semen. And finally, and perhaps more obviously, he attacked Pythagoras’s scheme with the most self-evident of arguments: it could not possibly account for female anatomy. How could a father’s sperm “absorb” the instructions to produce his daughter’s “generative parts,” Aristotle asked, when none of these parts was to be found anywhere in the father’s body? Pythagoras’s theory could explain every aspect of genesis except the most crucial one: genitals.

Aristotle offered an alternative theory that was strikingly radical for its time: perhaps females, like males, contribute actual material to the fetus—a form of female semen. And perhaps the fetus is formed by the mutual contributions of male and female parts. Grasping for analogies, Aristotle called the male contribution a “principle of movement.” “Movement,” here, was not literally motion, but instruction, or information—code, to use a modern formulation. The actual material exchanged during intercourse was merely a stand-in for a more obscure and mysterious exchange. Matter, in fact, didn’t really matter; what passed from man to woman was not matter, but message. Like an architectural plan for a building, or like a carpenter’s handiwork to a piece of wood, male semen carried the instructions to build a child. “[Just as] no material part comes from the carpenter to the wood in which he works,” Aristotle wrote, “but the shape and the form are imparted from him to the material by means of the motion he sets up. . . . In like manner, Nature uses the semen as a tool.”

Female semen, in contrast, contributed the physical raw material for the fetus—wood for the carpenter, or mortar for the building: the stuff and the stuffing of life. Aristotle argued that the actual material provided by females was menstrual blood. Male semen sculpted menstrual blood into the shape of a child (the claim might sound outlandish today, but here too Aristotle’s meticulous logic was at work. Since the disappearance of menstrual blood is coincident with conception, Aristotle assumed that the fetus must be made from it).

Aristotle was wrong in his partitioning of male and female contributions into “material” and “message,” but abstractly, he had