A note for parents: Aristotle describes dissection and animal breeding plainly. The lesson keeps to observation and classification.
For young readers
About ages 8 to 12. Read it together, or on your own.
Aristotle loved animals. While living on the Greek island of Lesbos, he studied the sea creatures of a big lagoon there: fish, octopus, crabs, sea urchins. He asked fishermen, farmers and beekeepers what they knew, and he cut animals open to see how they were made inside.
He described about five hundred kinds of animals. He noticed that a dolphin breathes air and feeds its babies milk, like a cow, even though it lives in the sea and looks like a fish. Many people after him called dolphins and whales fish. Aristotle had already seen the difference.
Try this
Watch like Aristotle. Choose one small animal you can watch safely without touching it: an ant, a bird at a feeder, a snail after rain, a pet. Watch for ten minutes and write down everything it does. How does it move? What does it eat? Draw it. Next time, see if you notice something new.Aristotle also watched a chicken grow inside its egg. He opened eggs day after day and saw that after about three days a tiny red speck appears, beating: the chick's heart. Scientists still study chick eggs this way.
For teens and adults
The full lesson.
Who, when and why
Aristotle (384–322 BC) was born in Stagira in northern Greece, the son of a doctor. At seventeen he went to Athens and studied in Plato's Academy for about twenty years. After Plato died he lived on the island of Lesbos and elsewhere, tutored the young Alexander the Great for a few years, and in 335 BC returned to Athens and founded his own school, the Lyceum. Much of his work on animals seems to come from his years on Lesbos (c. 345–343 BC) with his student Theophrastus, who went on to write the first great books on plants. Many of his sea creatures can be identified with those of the island's lagoon at Pyrrha.
Before Aristotle, Greek writing about animals was mostly fable, medicine or farming advice. Nobody had tried to describe all the kinds of animals and explain why their bodies are built as they are. The animal books are about a quarter of his surviving writings.
| Work | What it does |
|---|---|
| History of Animals | collects facts: parts, ways of life, food, breeding and behavior of hundreds of animals ("history" here means inquiry) |
| Parts of Animals | explains why animals have the parts they do, chiefly by what each part is for |
| Motion of Animals and Gait of Animals | how animals move: legs, wings, fins, and the reason for walking on four, two or many legs |
| Generation of Animals | how animals reproduce and develop |
Why study humble animals?
For if some have no graces to charm the sense, yet even these, by disclosing to intellectual perception the artistic spirit that designed them, give immense pleasure to all who can trace links of causation, and are inclined to philosophy. … We therefore must not recoil with childish aversion from the examination of the humbler animals. Every realm of nature is marvellous.
Careful observation
All creatures that have a blow-hole respire and inspire, for they are provided with lungs. … The dolphin and the porpoise are provided with milk, and suckle their young.
With the common hen after three days and three nights there is the first indication of the embryo; with larger birds the interval being longer, with smaller birds shorter. … the heart appears, like a speck of blood, in the white of the egg. This point beats and moves as though endowed with life.
Modern embryology agrees closely: a chick's heart begins to beat in the second day of incubation and is easy to see by the third. Aristotle also described the octopus's special mating arm, and how a kind of dogfish nourishes its young through a cord like a mammal's. Both reports were doubted for centuries and confirmed in the 1800s.
Sorting the animals
Aristotle did not build a single tree like modern biologists. He sorted animals by many features at once: blood or no blood, live birth or eggs, lungs or gills, how they move. His main split, animals with red blood and without, matches our split between vertebrates and invertebrates almost exactly. Among the bloodless animals he separated the soft-bodied cephalopods, the crustaceans, the shelled animals and the insects, groups biologists still recognize.
Worked example: Sort by two features
- Red blood: horse, dolphin, eagle, bat. The octopus has none (its blood is blue).
- Live young: horse, dolphin, bat. The eagle lays eggs.
Answer: Horse, dolphin and bat
Common mistake
What later science kept, and what it changed
- Kept: many of his observations, his main groups of animals, and the method of comparing the same part across many animals (comparative anatomy).
- Changed: Aristotle thought each kind of animal is fixed forever. Darwin (1859) showed that species change and share common ancestors, which explains why their parts fit their ways of life.
- Admired: after reading Ogle's translation of Parts of Animals in 1882, Darwin wrote to Ogle that Linnaeus and Cuvier had been his "two gods", but "they were mere school-boys to old Aristotle".
Going deeper
Context, the big questions, and how later books answered.
Why are animals built as they are?
Parts of Animals explains features chiefly by their purpose: birds have beaks of different shapes for different foods, animals with horns do not also have tusks, because nature does not give double defenses. Aristotle also recognized "necessity": some features are simply the result of what bodies are made of. This mix of purpose and necessity is still how biologists think, though Darwin changed what "purpose" means: a part is "for" something because ancestors with that part left more offspring.
Aristotle also saw a "ladder of nature" running from plants up to people, with no sharp breaks: sponges and sea anemones, he said, are hard to place as plant or animal. Later writers made this into a rigid ranking; Aristotle treated it as a gradual series. Modern biology replaces the ladder with a branching tree.
Big questions
- Is the dolphin's resemblance to a fish or to a cow more important? How do you decide which features matter for sorting?
- Aristotle trusted fishermen's reports. When should a scientist trust other people's observations?
- Why might studying "humble" animals, like worms or snails, be as important as studying lions?
Talk about it together
- Do the ten-minute animal watch. What surprised you?
- Is a dolphin more like a fish or more like a cow? What makes you say so?
- Aristotle thought some of his ideas were right because they seemed sensible, like the heart doing our thinking. How could he have tested that?
- Why do you think Aristotle said even humble animals are worth studying?
- Make your own way to sort ten animals. Do you sort by where they live, what they eat or how they're built? What changes?