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 The work is a historical document, not a modern textbook.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.

Plot or Argument

• Book I, Chs. 1–16: The four causes and the organs of generation

  Having treated the other three causes in Parts of Animals, Aristotle turns to the moving cause. He surveys which animals have sexes, the position and purpose of testes and uterus in different groups, and modes of copulation in fish, birds, insects and cephalopods.

• Book I, Chs. 17–18: Against pangenesis

  He sets out the four arguments for the view that semen comes from the whole body and refutes each: resemblances skip generations, children resemble parents in voice and gait from which nothing can be 'drawn', and the theory would produce two complete animals rather than one.

• Book I, Chs. 19–23: Semen, menstrual fluid and the male as craftsman

  Semen is a residue of useful nourishment in its final stage; the catamenia are its less-concocted female analogue. The male contributes form and the principle of movement, the female the material, on the models of rennet and milk and of the carpenter and his wood. Book I closes with plants, in which the sexes are not separated.

• Book II, Chs. 1–3: Why sexes, and how soul arrives

  Because individuals perish, generation lets the species share in eternity. Animals are ranked by the perfection of their offspring at birth. The embryo has nutritive soul first, then sensitive soul; reason 'alone' enters from outside. Semen is a foam of water and pneuma, carrying vital heat.

• Book II, Chs. 4–8: Epigenesis and its failures

  The heart is formed first and governs the rest; parts are outlined before they are coloured and hardened. The umbilical cord roots the embryo like a plant. Book II ends with sterility in humans and the sterility of mules, criticising Empedocles and Democritus for explanations that are 'too general and empty'.

• Book III, Chs. 1–8: Eggs

  Birds' eggs, including wind-eggs laid without mating, the formation of yolk and white, and the hatching chick; cartilaginous fish that bear live young from internal eggs; the folk belief that fish conceive by swallowing milt, which Aristotle rejects; and the eggs of cephalopods and crustaceans.

• Book III, Chs. 9–11: Insects, bees and spontaneous generation

  Larvae are imperfect eggs. The generation of bees is examined through competing theories, with Aristotle's own tentative account and his warning that the facts are not yet grasped. Testacea and some plants arise, he argues, where vital heat is enclosed in earth and water.

• Book IV, Chs. 1–6: Sex, resemblance and deformity

  Against Anaxagoras, Empedocles, Democritus and Leophanes, he explains sex as the result of the male principle mastering or failing to master the material. Resemblance to father, mother or remote ancestors depends on which 'movements' prevail. Monstrosities, extra parts, twins, superfetation and premature 'seven-months children' are explained as departures from type.

• Book IV, Chs. 7–10: The mola, milk, birth and gestation

  The mola uteri, milk as concocted blood, head-first birth, and the rule that the length of gestation tends to follow the length of life.

• Book V: Features that serve no purpose

  Eye colour, keenness of sight and hearing, the texture and colour of hair, baldness and greying, the colour of skins and coats, depth of voice and the shedding of teeth are explained by material necessity, not final cause, closing the treatise with a reminder that not everything in an animal is 'for the sake of' something.

Characters

• Aristotle, author and investigator

  Lecturing at the Lyceum, he speaks in the first person plural, cites his own dissections and the History of Animals, and repeatedly marks where observations are still insufficient.

• Empedocles, chief rival theorist

  The poet-philosopher of Acragas, quoted on sex determination by the heat of the womb and on milk as 'white pus'; Aristotle answers him with evidence from twins found together in one womb.

• Democritus, rival theorist

  The atomist, criticised for holding that external parts form before internal ones, that the embryo is moulded on the mother, and that monstrosities arise from two emissions of semen.

• Anaxagoras, rival theorist

  Credited with the view that sex is fixed by which side of the body the semen comes from; Aristotle links his cosmology to the pangenesis theory.

• Leophanes, cited authority

  An otherwise obscure writer who claimed that tying one testicle determined the sex of the child, a claim Aristotle calls guessing 'from probabilities'.

• Herodotus and Ctesias, criticised reporters

  Herodotus 'the storyteller' is faulted for tales about black semen and fish conceiving by swallowing milt; Ctesias for claiming elephant semen hardens like amber.

• The woman of Elis, case cited

  A woman whose grandson, but not daughter, resembled her Ethiopian partner; Aristotle uses the case to show that resemblance can skip a generation.

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.

  • Find two places where Aristotle says the evidence is not yet adequate. What does he do next?
  • Why does he call Herodotus 'the storyteller' when rejecting the fish-and-milt tale?
  • Where does Aristotle's own theory about females outrun what he could observe?

• 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.

  • What does the carpenter analogy say about whether anything material passes from father to child?
  • How does the rennet image differ from the carpenter image?
  • Modern biology shows both parents contribute genetic material. Which part of Aristotle's picture does that overturn, and is anything left?

• 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.

  • What evidence does Aristotle give that the heart forms first?
  • Why does he reject the Orphic image of an animal knitted like a net?
  • What does Aristotle mean by saying reason alone comes 'from outside'?

• 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.

  • Restate Aristotle's argument for why generation exists. What premises does it assume?
  • Does the argument make death less important or more?
  • How does Psalm 103:15–17 compare the passing individual with what endures?

• 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.

  • Trace the steps from 'less heat' to Aristotle's conclusion about the female. Which premise fails?
  • Why does Aristotle say producing females is necessary even though he calls it a departure from type?
  • How should a careful reader treat a great thinker's mistaken conclusions?

Context

Generation of Animals belongs to Aristotle's biological corpus, which together makes up about a quarter of his surviving work. Its examples come largely from the eastern Aegean and Macedonia, and many scholars connect the field observations with Aristotle's years at Assos and on Lesbos (c. 347–343 BC) after leaving Plato's Academy, though the treatise as we have it reflects later lecturing at the Lyceum in Athens. It enters an existing debate: Hippocratic treatises such as On Generation and On the Nature of the Child had argued that both parents emit seed drawn from the whole body.

Composition

The text is a set of lecture treatises rather than a polished book, with cross-references to the History of Animals, Parts of Animals and De Anima. The usual date is the 330s–320s BC, but the internal chronology of the biological works is disputed. It survived through Byzantine manuscripts and Arabic translation, entered Latin through Michael Scot's translation from Arabic (early thirteenth century) and William of Moerbeke's from Greek (c. 1260), and was printed in Greek in the Aldine edition of 1495–98. Arthur Platt's English translation, used here, appeared in the Oxford series in 1910.

Reception

Galen accepted much of Aristotle's embryology but argued that the female also contributes seed. Medieval scholastics, including Albert the Great and Thomas Aquinas, built on the account of successive souls in the embryo. William Harvey's Exercitationes de Generatione Animalium (1651) kept Aristotle's epigenesis while overturning many details, and Darwin, in an 1882 letter, ranked Aristotle above Linnaeus and Cuvier as a naturalist. Its errors on sex and spontaneous generation were refuted by experimental biology in the seventeenth to nineteenth centuries.

Comparisons

• History of Animals

  The History collects the observations (on copulation, gestation, eggs and young) that Generation of Animals sets out to explain; Aristotle refers readers back to it repeatedly for anatomical detail. Read the History for the facts and this treatise for the causes.

• Parts of Animals

  Parts of Animals explains organs by their final cause and ends where this book begins; the opening sentence of Generation of Animals presents itself as the continuation. Together they show Aristotle applying all four causes to living things.

• De Anima

  De Anima defines soul as the form of a living body and distinguishes nutritive, sensitive and rational powers; Generation of Animals applies that scheme to the embryo, adding the claim that reason alone comes 'from outside'.

• Hippocratic Writings

  The Hippocratic treatises on generation hold that both parents produce seed drawn from the whole body; Aristotle's Book I is in large part a refutation of that view. Comparing them shows a medical and a philosophical tradition arguing over the same questions.

• Galen: Selected Works

  Galen, five centuries later, accepted Aristotle's account of the embryo's development but defended a female seed against him; their disagreement framed medical embryology until the Renaissance.

• On the Motion of the Heart and Blood in Animals (De Motu Cordis)

  Harvey's 1628 treatise on circulation honours Aristotle's view of the heart as the first principle of the body, which Generation of Animals supports with the observation that the heart forms first. Harvey later wrote his own De Generatione Animalium (1651), testing Aristotle's embryology on hens' eggs and the deer of Charles I's parks.

• On the Origin of Species

  Darwin's account of species as changing over time contrasts sharply with Aristotle's claim that generation keeps each kind eternal 'as a species'. Both, however, explain animal features by comparing many forms, and Darwin admired Aristotle's observations.

Quotes

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.

  Heredity Degeneration Ancient Medicine
• 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.

  Women and Medicine Ancient Medicine
• 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.

  Anatomy Dissection Childbirth Maternal
• 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.

  Ancient Medicine Women and Medicine
• 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.

  Heredity Degeneration Disability Deformity Childbirth Maternal
• 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.

  Childbirth Maternal Women and Medicine
• 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.

  Aging Senescence Humoral Medicine

Creative Prompts

• Classroom

  Read Book III, Chapter 10 on bees and make a two-column chart: what Aristotle says was observed, and what he infers or reports from others. Then decide which column his conclusion rests on.

• Essay

  Aristotle compares nature to a carpenter, a cook using rennet and a painter. Choose one analogy and argue whether it clarifies or distorts how an embryo develops.

• Dialogue

  Write a conversation at the Lyceum in which a Hippocratic physician defends the view that seed comes from the whole body and Aristotle answers with the case of the woman of Elis.

Further Reading

• Generation of Animals, Aristotle, trans. A. L. Peck

  The Loeb Classical Library edition (1942), with Greek text and a long introduction on Aristotle's physiology.

• History of Animals, Aristotle

  The collection of observations on which this treatise draws.

• Exercitationes de Generatione Animalium, William Harvey

  Harvey's 1651 work on generation, written in dialogue with Aristotle.

• A History of Embryology, Joseph Needham

  Classic survey (1934; 2nd ed. 1959) with a long chapter on Aristotle.

• The Lagoon: How Aristotle Invented Science, Armand Marie Leroi

  A biologist's account (2014) of Aristotle's biology, including generation and heredity.