Variation and Classification

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Get to Know About Variation and Classification

Biologists sort almost ten million living things into groups using classification. This approach starts by placing each organism in one of two kingdoms. One kingdom holds all animals and the other holds all plants.

Each kingdom splits life into creatures with a backbone and creatures without one. Organisms with a backbone join the vertebrate group. Creatures without a backbone go into the invertebrate group. Vertebrates break into five classes. Fish, amphibians, reptiles, birds and mammals form these classes.

Mammals keep a steady internal heat. They use lungs to breathe. They have fur or hair. Most mammals give birth to live young. Mothers feed each newborn with milk. The activity tool above shows how each group fits into this system.