444 Ulisse Aldrovandi

...we shall investigate the procreation of various types of heads in fetuses. Among these causes, the first place is held by the weakness of the progenitor. Since this feebleness has a wide range, it can take on various degrees. Therefore, a weak parent, whose formative power is feeble while the matter remains dominant, will indeed procreate a male human, but one not entirely similar in appearance. However, if the exhaustion of both the power and the individual increases regarding sex, then a fetus similar to the parent in form is not produced, nor is it a male, but a dissimilar female. But if the power of the sex is weaker while the strength of the individual remains robust, a female will be born with dissimilarities in the body's extremities. Finally, from a weak seminal power, a fetus emerges that resembles not a human, but an animal, either in part or in its entirety.

From this, learned men rightly conclude that when the power of the male falters, monstrous births issue from women; this defect varies according to the degree of weakness. This error or flaw is observed sometimes in the head and sometimes in other parts, depending on which portion of the semen—dedicated to the constitution of the head or another part—is "sickly." When this exhaustion reaches its ultimate degree, no animal is procreated at all; instead, a *mola* (a mass of flesh) or pure, infertile seed is discharged.

Furthermore, when the power of the seed takes on a foreign nature, monstrous fetuses that reflect the species of an animal different from the nature of the parents are brought into the light. Thus, a brute animal may be born from a legitimate union. But if only a portion of the seed takes on a foreign nature, then either the head alone or another part of the fetus will represent the head or part of a beast. What we affirm regarding humans should also be understood to apply to other living creatures.

This clearly explains why a human fetus is sometimes born with the head of a monkey or a dog, and why a dog is sometimes born with the head of a hare. This is the teaching of Aristotle, who taught in his *Problems* that monsters are produced in the womb from "corrupted" seed. By "corrupted seed," he did not mean putrefied, but rather seed that has taken on a foreign nature due to a lack of vital power; consequently, an animal similar to the vital seed is not generated. However, it is also true that animals—albeit imperfect ones like worms, shapeless serpents, and frogs—can be generated from seed that has actually putrefied in the womb. Thus, the seed in the womb degenerates just as wheat sown in the earth by farmers sometimes degenerates into darnel, much to the loss of the country folk.

Furthermore, the material principle of the fetus can be a cause of the monstrous births described above. Indeed, Aristotle assigns this to a poor disposition of the matter of the fetuses. This is because the actions of active forces are only completed in a perfectly disposed "patient" (the receiving matter), and everything that is received is always received according to the mode of the recipient. Therefore, an agent that is not at all weak, nor deviating from its own nature, produces an effect that is most similar—formed according to the conditions of its generic, specific, and individual nature—as long as it engages with perfectly disposed matter. Conversely, if that same agent encounters material that is unsuitable or less fit to receive individual qualities, it produces something similar in its specific essence but not in its individual form.

When the matter is filled with a greater unsuitability, lacking any inclination to receive the conditions of its specific nature, it takes on a foreign form, and an entire animal of a different species results. If only a portion of this matter lacks these prerequisites, then a monster emerges with a part—for example, the head—that is foreign to its own species, provided that portion of matter was destined for the head. When many portions of matter lack these prerequisites, then many parts of the fetus will take on the form of limbs from different species of animals. Ultimately, from matter destitute of all suitability, only lumps of flesh and *molae* are produced in the womb, as has been explained in its proper place.

Again, according to Aristotle's teaching, this monstrous defect should be attributed to the nourishing power. When this power is unable to perform its proper duty in assimilating food into the parts, the human form of those parts is undoubtedly corrupted. For if we follow the same line of reasoning, we can say that just as the formative power of the conceptus, when overwhelmed by weakness, cannot imprint the human image onto some part of the matter (and thus a foreign figure is born there), so too the nourishing power and the assimilating food, while