--- title: "Sorting Living Things" book: "Primary & Middle School Biology" subject: biology language: en chapter: 14 exercises: 10 source: https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things --- # Chapter 14 — Sorting Living Things Tip a box of mixed buttons on the table and your hands start [sorting](#def-g3-sorting-living-things-criterion) by themselves: big with big, red with red, four holes with four holes. Naturalists felt the same urge in front of the living world’s jumble of beetles, [birds](#prop-g3-sorting-living-things-groups), ferns and [fish](#prop-g3-sorting-living-things-groups) — and [sorting](#def-g3-sorting-living-things-criterion) [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) carefully turned out to be one of [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living)’s most powerful ideas. ## 14.1 Sorting needs a rule **Definition 14.1 (Sorting and criteria).** To *sort* a collection is to put together the things that go together. A *criterion* is the rule used for the sorting — “has six [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts)”, “has [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering)”. A good criterion for [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) is a feature of the body you can *observe*: something the [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) or [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) *has*, not the place it lives or what it happens to be doing. **Example 14.2 (Two sortings of the same animals).** Take the cat, the duck, the trout and the ladybug. Sorted by “lives near water”: duck and trout together. Sorted by “has [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering)”: the duck stands alone. Both are [sortings](#def-g3-sorting-living-things-criterion) — but only the second uses the [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal)’s own body, and that is the kind biologists trust: an [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) can move house, but it cannot change its [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) for scales. **Remark 14.3 (Why not sort by home or menu?).** [Habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) and menu change with the seasons and with luck — the blackbird of [Chapter 10](https://one-course.com/books/biology/1/en/chapter/10-what-animals-eat#ch-g2-what-animals-eat) switches from worms to berries without becoming a different [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal). Body features stay. That is why “what it has” beats “what it does” as a [sorting](#def-g3-sorting-living-things-criterion) rule. ## 14.2 The great groups of animals **Proposition 14.4 (Some big groups).** [Sorting](#def-g3-sorting-living-things-criterion) by body features gathers [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) into big groups, including: - *insects* : six [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) , a body in three parts — the ant, the ladybug, the bee, the butterfly; - *fish* : fins and scales, life spent in water — the trout, the goldfish; - *birds* : [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) , wings and a beak — the duck, the blackbird, the ostrich; - *mammals* : [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) or hair, and mothers that feed their young [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) — the cat, the cow, the bat, the whale — and humans; - *amphibians* : moist [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) , [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) laid in water, a tadpole childhood — the frog, the newt. **Proof.** *Admitted at this level.* ∎ ![Five big groups, each named by what its members have.](https://one-course.com/images/onecourse/chapters/biology-1/g3-sorting-living-things/fig-dbe680701148.svg) *Five big groups, each named by what its members *have*.* **Example 14.5 (Surprises done right).** Criteria save us from lazy guesses. The bat flies like a bird — but it has [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), no [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), and feeds its young [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born): a mammal. The whale swims like a [fish](#prop-g3-sorting-living-things-groups) — [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) reduced to almost nothing, but [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) for its calf and no scales: a mammal too. The ostrich cannot fly at all, yet [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and beak say: bird. The [criterion](#def-g3-sorting-living-things-criterion), not the impression, decides. ![It flies like a bird — and wears fur on naked wings: the bat, the classic test of sorting by features rather than impressions.](https://one-course.com/images/onecourse/chapters/biology-1/g3-sorting-living-things/img-f2312970fb1d.jpg) *It flies like a bird — and wears [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) on naked wings: the bat, the classic test of [sorting](#def-g3-sorting-living-things-criterion) by features rather than impressions.* **Method 14.6 (Sorting an unknown animal).** 1. Observe the body: [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) , [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) , [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) or scales? how many [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) ? wings? a beak? 2. Ask about the young if you can: [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) ? [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) ? a tadpole stage? 3. Compare with the groups of [Proposition 14.4](#prop-g3-sorting-living-things-groups) and place the [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) where its *features* fit; 4. if an impression (“it flies!”) disagrees with a feature (“it has [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) ”), trust the feature. ## 14.3 Plants get sorted too **Proposition 14.7 (Sorting plants).** [Plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) are sorted by their features just like [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal). Useful criteria: does it make *[flowers](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts)* and [seeds](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) (the daisy, the chestnut) or never any (the fern, the moss)? Is its [stem](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) soft and green, or woody like a *tree*’s trunk? Are the [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) broad, or needles like the pine’s? **Proof.** *Admitted at this level.* ∎ **Example 14.8 (Three corners of the park).** In one park: the rose bush — [flowers](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts), woody [stems](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts); the moss on the north wall — no [flowers](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) ever, no real [stem](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) at all; the pine — a tree with needles and cones. Three [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), three different answers to the same criteria. **Remark 14.9 (Sorting is the first step of a science).** Well-sorted groups let people ask better questions: whatever is true of [insects](#prop-g3-sorting-living-things-groups) in general — six [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), a body in three parts, often wings — is suddenly known for hundreds of thousands of species at once. Later chapters will turn [sorting](#def-g3-sorting-living-things-criterion) into a family tree of all [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) — for now, learn to sort by what you can see. ## 14.4 Exercises **Exercise 14.1 ★.** What is a [criterion](#def-g3-sorting-living-things-criterion)? Give one good [criterion](#def-g3-sorting-living-things-criterion) for [sorting](#def-g3-sorting-living-things-criterion) [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal), and one that biologists avoid. **Solution of Exercise 14.1.** A [criterion](#def-g3-sorting-living-things-criterion) is the rule used for [sorting](#def-g3-sorting-living-things-criterion). Good: a body feature you can observe — “has [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering)”, “has six [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts)”. Avoided: where it lives or what it happens to be doing — “lives near water”, “flies”. **Exercise 14.2 ★.** Give the feature (or features) that defines each group: [insects](#prop-g3-sorting-living-things-groups), [fish](#prop-g3-sorting-living-things-groups), [birds](#prop-g3-sorting-living-things-groups), [mammals](#prop-g3-sorting-living-things-groups). **Solution of Exercise 14.2.** [Insects](#prop-g3-sorting-living-things-groups): six [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) and a body in three parts. [Fish](#prop-g3-sorting-living-things-groups): fins and scales, living in water. [Birds](#prop-g3-sorting-living-things-groups): [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), wings and a beak. [Mammals](#prop-g3-sorting-living-things-groups): [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) or hair, and mothers that feed their young [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). **Exercise 14.3 ★.** Place each [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) in its group: the bee, the goldfish, the ostrich, the cow, the frog. **Solution of Exercise 14.3.** Bee: insect. Goldfish: [fish](#prop-g3-sorting-living-things-groups). Ostrich: bird. Cow: mammal. Frog: amphibian. **Exercise 14.4 ★.** The bat flies. Why is it a mammal and not a bird? Which features decide? **Solution of Exercise 14.4.** It has [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and feeds its young [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), and it has no [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and no beak. Flying is something it does, not something that defines a bird — the features say mammal. **Exercise 14.5 ★.** The whale lives in the sea. Why is it not a [fish](#prop-g3-sorting-living-things-groups)? Which features decide? **Solution of Exercise 14.5.** It has no scales and no gills-and-fins body plan of a [fish](#prop-g3-sorting-living-things-groups) — and it feeds its calf [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). Swimming is its habit; its features say mammal. **Exercise 14.6 ★.** A spider has eight [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts). Can it be an insect? What does the [criterion](#def-g3-sorting-living-things-criterion) answer? **Solution of Exercise 14.6.** No. The insect [criterion](#def-g3-sorting-living-things-criterion) is six [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) (and a three-part body); the spider’s eight [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) fail it. The spider belongs to another group, whatever it may look like at first glance. **Exercise 14.7 ★.** Sort these [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) with the criteria of [Proposition 14.7](#prop-g3-sorting-living-things-plants): the daisy, the moss, the pine, the rose bush. **Solution of Exercise 14.7.** Daisy: [flowers](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) and [seeds](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed), soft green [stem](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts). Moss: no [flowers](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) ever, no real [stem](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts). Pine: a tree (woody trunk), [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) as needles, no [flowers](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) but cones with [seeds](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed). Rose bush: [flowers](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) and [seeds](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed), woody [stems](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts). **Exercise 14.8 ★.** Try it: sort the [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) of your street or garden — every one you can spot in a walk — into the groups of this chapter. Which group wins by number? **Solution of Exercise 14.8.** Open answer. In most streets the [insects](#prop-g3-sorting-living-things-groups) win by far — ants, flies, bees, beetles, butterflies — with [birds](#prop-g3-sorting-living-things-groups) second. **Exercise 14.9 ★★.** Lea sorts [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) into “pets” and “wild [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal)”. Her friend sorts the same [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) into “[mammals](#prop-g3-sorting-living-things-groups)” and “[birds](#prop-g3-sorting-living-things-groups)”. Whose [sorting](#def-g3-sorting-living-things-criterion) is the biologist’s kind, and why? Use [Remark 14.3](#rem-g3-sorting-living-things-why). **Solution of Exercise 14.9.** The friend’s. “Pets versus wild” sorts by how [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) live with people, which can change — a stray cat is the same [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) indoors and out. “[Mammals](#prop-g3-sorting-living-things-groups) versus [birds](#prop-g3-sorting-living-things-groups)” sorts by body features that stay for life: the biologist’s kind of [criterion](#def-g3-sorting-living-things-criterion). **Exercise 14.10 ★★.** A penguin swims superbly, cannot fly, and is covered in what look like shiny tiles. A naturalist looks closer: the “tiles” are tightly packed [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), and there is a beak. Run [Method 14.6](#met-g3-sorting-living-things-sort) on the penguin, step by step. **Solution of Exercise 14.10.** Step 1: covering — [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) (tightly packed), a beak, two [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), wings (as flippers). Step 2: young hatch from [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). Step 3: the features match the [birds](#prop-g3-sorting-living-things-groups). Step 4: the impressions “swims like a [fish](#prop-g3-sorting-living-things-groups), cannot fly” disagree — and lose. The penguin is a bird.