---
title: "Classification and Kinship"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 44
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship
---

# Chapter 44 — Classification and Kinship

Why does the [classification](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-classification) of [Chapter 21](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#ch-g4-classifying-animals) work so well? Sort [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) by [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), and a thousand other features fall obediently into line — milk-feeders have [backbones](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), feather-wearers lay shelled [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), and no one has to check. [Remark 21.10](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#rem-g4-classifying-animals-tree) left the question hanging. This chapter answers it with one word — [kinship](#prop-g6-classification-kinship-kinship) — and rebuilds the boxes as a family [tree](#def-g6-classification-kinship-tree).

## 44.1 Attributes, shared and nested

**Definition 44.1 (Attribute).**

An *attribute* is a feature an organism *possesses*, stated so it can be checked: has a [skeleton](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) with a [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate); has four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts); has [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and milk-feeding; has [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering). [Classification](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-classification) is built on attributes — [Definition 14.1](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#def-g3-sorting-living-things-criterion)’s good criteria, formalized.

**Proposition 44.2 (Attributes nest).**

Checked across many [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species), [attributes](#def-g6-classification-kinship-attribute) fall into *nested* sets — every [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) with [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) also has four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts); every four-limbed [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) also has a [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate); never the reverse pattern ([fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) without [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate)) and never crossing sets ([fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) together). That nesting is why groups-inside-groups ([Definition 21.1](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-classification)) fits the living world at all.

**Proof.** *Admitted at this level.* ∎

![Attributes nest: fur-bearers sit inside the four-limbed, the four-limbed inside the backboned. The trout has the backbone but stands outside the four-limbed box.](https://one-course.com/images/onecourse/chapters/biology-1/g6-classification-kinship/fig-aa2b62c63889.svg)

*[Attributes](#def-g6-classification-kinship-attribute) nest: fur-bearers sit inside the four-limbed, the four-limbed inside the backboned. The trout has the [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) but stands outside the four-limbed box.*

**Example 44.3 (Placing three animals).**

Check the boxes for the whale, the trout and the blackbird. Whale: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) yes, four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) yes (front flippers, and hidden vestiges behind), fur-and-milk yes — innermost box, with the cat. Trout: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) yes, stop — outer box only. Blackbird: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) (two [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), two wings), [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) — the four-limbed box, in [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering)’ own compartment. No [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) ever demands two crossing boxes at once.

## 44.2 Kinship: the reason behind the nesting

**Proposition 44.4 (Shared attributes mean shared ancestry).**

The explanation of the nesting is inheritance ([Remark 23.3](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#rem-g4-animal-reproduction-mix): young receive from their parents). [Species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) sharing an [attribute](#def-g6-classification-kinship-attribute) share it because they *inherited it from a common ancestor* — a distant grandparent [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species). The more [attributes](#def-g6-classification-kinship-attribute) two [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) share, the closer their *kinship*: the cat and the whale, both furred milk-feeders, are closer cousins to each other than either is to the trout. A [classification](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-classification) group is, at bottom, a *family*: an ancestor and all its descendants.

**Proof.** *Admitted at this level.* ∎

**Example 44.5 (Reading the whale correctly at last).**

This is why the whale puzzle ([Exercise 21.9](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#exo-g4-classifying-animals-9)) had to resolve as it did: fur-and-milk marks the whale’s family line — its ancestors were milk-feeding land [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) — while its fish-like shape is the water’s doing ([Exercise 28.11](https://one-course.com/books/biology/1/en/chapter/28-adapted-to-their-world#exo-g4-adapted-to-their-world-11)), stamped on an unrelated family by the same way of life. [Attributes](#def-g6-classification-kinship-attribute) of ancestry sort families; resemblances of [habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat) can be borrowed by strangers. [Classification](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-classification) trusts the first and distrusts the second.

**Definition 44.6 (The tree of kinship).**

[Kinship](#prop-g6-classification-kinship-kinship) is drawn as a *tree*: [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) points stand for common ancestors, and each [attribute](#def-g6-classification-kinship-attribute) appears once, at the [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) where it arose — every [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) beyond that fork inherits it. The nested boxes and the tree carry the same information: a box is a [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) with all its twigs.

![A pocket tree of kinship. Each colored dot is an attribute appearing once; every branch beyond it inherits. Cat and whale share the newest fork — the closest cousins drawn here.](https://one-course.com/images/onecourse/chapters/biology-1/g6-classification-kinship/fig-acb87b08764d.svg)

*A pocket [tree of kinship](#def-g6-classification-kinship-tree). Each colored dot is an [attribute](#def-g6-classification-kinship-attribute) appearing once; every [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) beyond it inherits. Cat and whale share the newest fork — the closest cousins drawn here.*

**Method 44.7 (Building a small kinship tree).**

Given a handful of [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species):

1. table their [attributes](#def-g6-classification-kinship-attribute) — [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) in rows, checkable [attributes](#def-g6-classification-kinship-attribute) in columns, ticks where possessed;
2. group by shared ticks, widest [attribute](#def-g6-classification-kinship-attribute) first: the nesting appears in the table;
3. draw the [tree](#def-g6-classification-kinship-tree) : one fork per [attribute](#def-g6-classification-kinship-attribute) , the widest nearest the trunk; place each [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) past exactly the forks whose [attributes](#def-g6-classification-kinship-attribute) it possesses;
4. read [kinship](#prop-g6-classification-kinship-kinship) : two [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) ’ closeness is the last fork they share.

**Remark 44.8 (What the tree quietly implies).**

Take the [tree](#def-g6-classification-kinship-tree) seriously and it says something enormous: follow any two [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) back and they *meet*. Cat and trout share a grandparent [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species); so, further back, must cat and snail, daisy and child. All [living things](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) would then be one family, its unity ([Proposition 29.2](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#prop-g5-first-look-at-cells-principle)) a family resemblance — and [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) must be able to *change* across generations, or the branching could never have happened. Whether that is true, and how it could work, is the great question of [Chapter 67](https://one-course.com/books/biology/1/en/chapter/67-evidence-of-evolution#ch-g9-evidence-of-evolution)’s year — this [tree](#def-g6-classification-kinship-tree) is the diagram that forces it.

## 44.3 Exercises

**Exercise 44.1 ★.**

What is an [attribute](#def-g6-classification-kinship-attribute)? Give three, stated checkably.

**Solution of Exercise 44.1.**

A feature an organism possesses, stated checkably: has a [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate); has four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts); 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) (or: has [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering)).

**Exercise 44.2 ★.**

What does it mean that [attributes](#def-g6-classification-kinship-attribute) *nest*? Give the chapter’s three-box example.

**Solution of Exercise 44.2.**

Each [attribute](#def-g6-classification-kinship-attribute)’s set of owners sits wholly inside another’s: all fur-bearers have four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), all four-limbed [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) have [backbones](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) — never the reverse, never crossing. Boxes inside boxes, with no straddlers.

**Exercise 44.3 ★.**

Why do [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) share [attributes](#def-g6-classification-kinship-attribute), by [Proposition 44.4](#prop-g6-classification-kinship-kinship)?

**Solution of Exercise 44.3.**

Because they inherited them from a common ancestor: an [attribute](#def-g6-classification-kinship-attribute) travels down a family line, so its owners are that ancestor’s descendants — a family.

**Exercise 44.4 ★.**

On the pocket [tree](#def-g6-classification-kinship-tree), which [attribute](#def-g6-classification-kinship-attribute) dot does the trout stand beyond? Which dots does the whale stand beyond?

**Solution of Exercise 44.4.**

The trout stands beyond only the [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) dot. The whale stands beyond [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), and fur-and-milk — all three.

**Exercise 44.5 ★.**

Who are closer cousins: whale and cat, or whale and trout? Read the answer from the [tree](#def-g6-classification-kinship-tree)’s forks.

**Solution of Exercise 44.5.**

Whale and cat: they share the fur-and-milk fork, the newest one drawn; the trout parted company at the oldest fork.

**Exercise 44.6 ★.**

State the relation between a nested box and a [tree](#def-g6-classification-kinship-tree) [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree).

**Solution of Exercise 44.6.**

They are the same information twice: a box is a [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) with all its twigs — everything past a fork inherits the fork’s [attribute](#def-g6-classification-kinship-attribute) and sits in its box.

**Exercise 44.7 ★★.**

Why does [classification](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-classification) trust fur-and-milk but distrust “shaped like a [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)”? Use [Example 44.5](#ex-g6-classification-kinship-whale).

**Solution of Exercise 44.7.**

Fur-and-milk is inherited down one family line — it marks ancestry. “Shaped like a [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)” can be stamped on strangers by the same watery way of life, as the whale shows: shape borrowed, family elsewhere. [Classification](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-classification) sorts families, so it trusts the heirloom and distrusts the borrowed coat.

**Exercise 44.8 ★★.**

Build the [attribute](#def-g6-classification-kinship-attribute) table of [Method 44.7](#met-g6-classification-kinship-build) for: trout, frog, cat, blackbird — [attributes](#def-g6-classification-kinship-attribute): [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), fur-and-milk, [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering).

**Solution of Exercise 44.8.**

Trout: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) only. Frog: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts). Cat: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), fur-and-milk. Blackbird: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering). The nesting shows in the ticks: every row’s ticks include the [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate); every coat-tick row also ticks four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts).

**Exercise 44.9 ★★.**

Add the lizard to the pocket [tree](#def-g6-classification-kinship-tree): which forks does it pass, and where does its [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) leave?

**Solution of Exercise 44.9.**

It passes [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) and four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), and [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) on its own [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) with its dry scaly [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) — beside, not beyond, the [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and [feather](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) forks.

**Exercise 44.10 ★★.**

The bat flies like a blackbird but nurses like a cat. Which resemblance marks [kinship](#prop-g6-classification-kinship-kinship) and which is borrowed by way of life? Decide with the [tree](#def-g6-classification-kinship-tree)’s logic.

**Solution of Exercise 44.10.**

Nursing marks [kinship](#prop-g6-classification-kinship-kinship): fur-and-milk is the mammal family’s heirloom, so the bat [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) with the cat. Flying is borrowed by way of life — wings arose separately on the bird [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) and the bat [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree), as streamlining did in whale and trout.

**Exercise 44.11 ★★.**

“A group is an ancestor and all its descendants.” Test the sentence on the [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) and on [Exercise 21.11](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#exo-g4-classifying-animals-11)’s platypus: does the egg-laying oddity break the family, or belong to it?

**Solution of Exercise 44.11.**

It belongs. The platypus descends from the [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)’ common ancestor and carries the family heirlooms ([fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)); its [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) is an old feature its early [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) kept while the others lost it. The family is the ancestor and *all* descendants — oddities included; the bundle ([Exercise 21.11](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#exo-g4-classifying-animals-11)) already voted it in.

**Exercise 44.12 ★★★.**

Follow [Remark 44.8](#rem-g6-classification-kinship-implies) to its edge: what must be true of [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) over long time, if every pair of [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) really meets? State the implication in one sentence — and name what kind of evidence you would demand for it.

**Solution of Exercise 44.12.**

If every pair of [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) meets, then [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) must change across generations over long time — new [attributes](#def-g6-classification-kinship-attribute) arising and being inherited — so that one ancestral line could [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) into trout, cat and daisy. Implication in one sentence: all [living things](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) descend, with change, from common ancestors. Evidence to demand: traces of the change — intermediate forms, preserved remains in old rocks, and family resemblances placed exactly where the [tree](#def-g6-classification-kinship-tree) predicts (the business of a later year).

## 44.4 Problem: The Museum Drawer

**Problem 44.1.**

Weekend problem — six specimens, one attribute table, one tree

A museum lends the class a drawer of six specimens: a trout, a frog, a lizard, a pigeon, a mouse, and a dolphin’s [skull](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) with flipper casts. The task: classify them by [attributes](#def-g6-classification-kinship-attribute) and draw their [kinship](#prop-g6-classification-kinship-kinship) [tree](#def-g6-classification-kinship-tree).

**Part I — The table.**

1. Build the table for the six, with [attributes](#def-g6-classification-kinship-attribute) : [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) ; four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) ; dry scaly [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) ; [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) ; [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) . (For the dolphin, near-hairless, the museum label vouches: milk-fed young — [Example 14.5](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#ex-g3-sorting-living-things-surprises) explains why that testimony settles it.)
2. Which [attribute](#def-g6-classification-kinship-attribute) do all six share? What does the widest sharing mean, in [kinship](#prop-g6-classification-kinship-kinship) terms?
3. Which specimens tick “four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) ”? Explain the pigeon’s and the dolphin’s ticks — what are wings and flippers, on the [tree](#def-g6-classification-kinship-tree) ’s logic?
4. Why does “lives in water” appear nowhere in the table, though trout and dolphin both do? Cite the chapter’s rule.

**Part II — The [tree](#def-g6-classification-kinship-tree).**

5. Draw the [tree](#def-g6-classification-kinship-tree) : order the forks — [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) , four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) , then the coats (scales, [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) , [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) ) on their own [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) — and place all six.
6. Read from your [tree](#def-g6-classification-kinship-tree) : the mouse’s closest cousin in the drawer, and the trout’s.
7. The frog stands past “four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) ” but before every coat fork. What does its bare, moist [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) ( [Remark 21.6](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#rem-g4-classifying-animals-confusion) ) mark it as, and where does its [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) leave?
8. A label falls off a seventh specimen: dry scaly [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) , four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) , [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) . Place it on the [tree](#def-g6-classification-kinship-tree) and name its class — without seeing the label.

**Part III — The meaning.**

9. The dolphin and the trout share streamlining; the dolphin and the mouse share fur-and-milk ancestry. Which pair sits closer on the [tree](#def-g6-classification-kinship-tree) , and why must the [tree](#def-g6-classification-kinship-tree) ignore the streamlining?
10. State what the fork “four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) ” claims historically: what single thing, long ago, do frog, lizard, pigeon, mouse and dolphin all descend from?
11. The class’s [tree](#def-g6-classification-kinship-tree) makes every [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) meet, eventually, at the trunk. Say in one sentence what that drawing quietly claims about all six specimens — and about the class members drawing it.
12. A skeptic says: “your [tree](#def-g6-classification-kinship-tree) is just a filing system.” Give the two-part reply: what the nesting of [attributes](#def-g6-classification-kinship-attribute) is a *fact* about, and what the [tree](#def-g6-classification-kinship-tree) *explains* that mere filing would leave unexplained.

**Solution of Problem 44.1.**

**1.** Trout: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate). Frog: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts). Lizard: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), dry scaly [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense). Pigeon: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering). Mouse: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), fur-and-milk. Dolphin: [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate), four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), fur-and-milk (the label’s testimony on [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born); features decide, and [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) is the deciding feature).

**2.** The [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) — all six are [vertebrates](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate). The widest sharing marks the oldest common ancestor: all six are one family at that depth.

**3.** Frog, lizard, pigeon, mouse, dolphin. The pigeon’s wings and the dolphin’s flippers are four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) in working dress: the same inherited [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), shaped by different lives — the [tree](#def-g6-classification-kinship-tree) counts the inheritance, not the dress.

**4.** Because a shared address is not a shared family: “lives in water” is a [habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat), borrowable by strangers, not an [attribute](#def-g6-classification-kinship-attribute) of the body’s ancestry — [Definition 14.1](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#def-g3-sorting-living-things-criterion)’s old rule, now with its reason.

**5.** Trunk $\rightarrow$ [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) fork (trout [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) after it) $\rightarrow$ [four-limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) fork $\rightarrow$ three coat [branches](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree): dry scales (lizard), [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) (pigeon), fur-and-milk (mouse and dolphin together on the newest fork); the frog [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) after [four-limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), before every coat.

**6.** The mouse’s closest drawer-cousin is the dolphin (they share the fur-and-milk fork). The trout’s closest is everyone equally — it parted at the first fork, so all five others are its equally distant cousins.

**7.** An amphibian — four-limbed, bare moist [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), no coat fork of its own; its [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) between the [four-limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) fork and the coats.

**8.** Past [backbone](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) and four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), onto the dry scaly [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree): a reptile — classified sight unseen, because [attributes](#def-g6-classification-kinship-attribute), not labels, decide.

**9.** Dolphin and mouse sit closer: fur-and-milk is an inherited fork. The streamlining is the water’s uniform, issued alike to unrelated swimmers ([Exercise 28.11](https://one-course.com/books/biology/1/en/chapter/28-adapted-to-their-world#exo-g4-adapted-to-their-world-11)); counted, it would file borrowed dress as family and wreck the nesting.

**10.** A single common ancestor [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) — an early four-limbed [vertebrate](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#def-g4-classifying-animals-vertebrate) — from which frog, lizard, pigeon, mouse and dolphin all descend, each carrying its four [limbs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) in different dress.

**11.** That all six — and the class drawing them — belong to one branching family, descended with change from common ancestors, humans on the fur-and-milk [branch](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-tree) among the rest.

**12.** The nesting is a fact about the living world — [attributes](#def-g6-classification-kinship-attribute) really do fall into boxes-in-boxes, which no filing clerk decreed. The [tree](#def-g6-classification-kinship-tree) explains the fact: inheritance from common ancestors *must* produce nested [attributes](#def-g6-classification-kinship-attribute), while mere filing would leave the world’s obliging tidiness a miracle.