Animal kingdom Millions of different types of animals inhabit the Earth - more even than we know about. Animals are grouped according to features they have in common. Mammals, amphibians, birds, reptiles and fish are all vertebrates.
Mammals are the most advanced class of animal life.
They are warm-blooded, which means their body temperature is constant. They are the only animals to produce milk, and only mammals possess true hair or fur. Most birds can fly. They have streamlined, feathered bodies and wings. Their legs and feet are scaly and they have beaks for feeding, but no teeth.