Generation of Animals

Summary

Generation of Animals (Greek Peri zōiōn geneseōs, Latin De Generatione Animalium) is Aristotle's treatise on how animals come into being, and the most ambitious work of embryology before the seventeenth century. It completes the sequence that begins with the History of Animals (the collected facts) and Parts of Animals (the purposes of organs). Where Parts of Animals explained the final cause of each part, this book turns to the efficient cause: what starts and steers the development of a new animal. Book I describes the reproductive organs across the animal kingdom and then asks what semen is. Aristotle rejects the theory, found in Hippocratic writers and in Democritus, that semen is drawn from every part of the body (later called pangenesis), using arguments from children who resemble remote ancestors, from parents who lack a limb, and from traits such as grey hair that parents do not yet have. Semen, he concludes, is a residue of the most refined nourishment, made from blood; the menstrual fluid is the female counterpart, less 'concocted' because the female has less vital heat. The male supplies form and movement, the female supplies matter, as rennet sets milk or a carpenter shapes wood without any part of him entering the bed. Book II asks why there are sexes at all, answering that individuals cannot be eternal but the species can, and classifies animals by how perfect their offspring are at birth. It then explains how soul arises in the embryo (nutritive first, then sensitive, with reason alone coming 'from outside'), and argues against preformation that the parts form one after another, beginning with the heart, like a painter sketching outlines before adding colour. Book III treats the eggs of birds, fish, cephalopods and crustaceans, the larvae of insects, the much-disputed generation of bees, and shellfish and other creatures he believed arose spontaneously from mud and decay. Book IV takes up sex determination, family resemblance, birth defects, multiple births, superfetation, milk and the length of gestation. Book V explains features such as eye colour, hair, greying, voice and teeth, which he assigns to material necessity rather than purpose. The work is a historical document, not a modern textbook. Its account of the female as a less-heated, 'as it were, mutilated' male, and its theory of spontaneous generation, were mistaken, and students should read them as Aristotle's arguments rather than as facts. What remains impressive is the method: comparative observation across hundreds of species, repeated dissection, the discipline of distinguishing what is seen from what is merely said, and the famous rule in the bee chapter that 'credit must be given rather to observation than to theories'. Aristotle's picture of gradual development (epigenesis) shaped medieval and Renaissance embryology, and William Harvey still took it as his model in 1651.

Top Lessons

• Observation before theory

  At the end of the bee chapter Aristotle admits the facts 'have not yet been sufficiently grasped' and states that observation should be trusted over theory, and theory only when it agrees with the facts. The book repeatedly distinguishes what has been seen, what is reported, and what fishermen or historians merely repeat. Students can admire the principle while noticing that Aristotle himself sometimes reasoned beyond his evidence.

• Carpenter, wood and rennet

  Aristotle explains generation by analogies from craft and kitchen: the male principle shapes the material as a carpenter shapes timber, or sets it as rennet curdles milk. These images carry his whole theory of form and matter into biology. Students should test what each analogy captures and where it misleads.

• Building an embryo part by part

  Against those who thought all parts are present from the start, Aristotle argues that the embryo forms in sequence, beginning with the heart, and that nature outlines before it colours, like a painter. This view, later called epigenesis, was argued over until the nineteenth century. It also shaped long debates in Christian thought about when the soul is present in the unborn child, debates the book itself does not settle.

• Eternal in the only way possible

  Book II opens with a philosophical reason for reproduction: since individual animals must die, they share in the eternal by producing others like themselves. For Aristotle this explains why there are always men, animals and plants. Christian readers can compare this natural longing for permanence with Scripture's account of mortality and of a life that does not depend on the species alone.

• Reading an ancient error historically

  Aristotle calls the female 'as it were, a mutilated male' and a 'natural deficiency' of heat. These claims follow from his theory that females concoct less and supply only matter, and they were repeated for centuries. Students should state the argument accurately, identify the evidence it lacked, and notice that Aristotle also insists the female is a natural necessity, not an accident, for the preservation of the kind.

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Medical Lens

Generation of Animals is the foundation text of Western embryology and was read by physicians from Galen to Harvey. It contains the first sustained critique of pangenesis, a theory of heredity based on competing 'movements' from each parent, accounts of conception, gestation, multiple births, premature birth, the mola uteri, sterility and lactation, and a theory of congenital malformation as departure from type. Its physiology rests on vital heat and 'concoction' rather than on the four humours of Hippocratic medicine, though the two systems were later blended. Its account of the female as an imperfect male was deeply influential and wrong; present it as part of the history of medicine. Aristotle's case reports (for example the woman with a mola who carried it for years) are anecdotes he records, not clinical observations in a modern sense.

• Book I, Chs. 17–18 (721b–726a)

  Refutation of the theory that seed is drawn from all parts of the body, using family resemblances that skip generations (the woman of Elis), parents with mutilations, and the inheritance of voice and gait.

• Book I, Chs. 19–20 (726a–729a)

  Menstrual fluid identified as the female analogue of semen, a less-concocted residue of blood; the female contributes matter and the male form.

• Book II, Chs. 4–6 (738b–745b)

  Embryonic development: the heart forms first, the umbilical cord roots the embryo like a plant, and parts are outlined before they are completed. Based in part on dissection of embryos and incubated eggs.

• Book II, Ch. 7 (746b–747a)

  Sterility in men and women from imperfection of the generative parts or from age and constitution, with the fertility tests he reports: semen tested in water, and women tested by pessaries and by colours smeared on the eyes.

• Book IV, Chs. 3–4 (767a–773a)

  Theory of family resemblance and of monstrosities, extra or missing parts and twins, including a reference to physicians opening an imperforate passage at puberty.

• Book IV, Chs. 6–8 (774b–777a)

  Premature 'seven-months children' who survive, the claim that sedentary women have more painful labour than those who work, the mola uteri, and milk as concocted blood.

• Book V, Chs. 3–5 (781b–785a)

  Baldness, greying and the hair of eunuchs, explained by heat, moisture and concoction; greying from age distinguished from greying from disease.