Mammals are divided into three groups according to how they breed. The egg-laying mammals is the smallest group. It contains the duck-billed platypus and two species of spiny anteater. They live in Australia and some of its surrounding islands.

The pouched mammals are also found in Australia and some are also found in North and South America. The earliest stages of the young develop in an organ called the womb inside the mother then the later stages crawl into a pouch and complete their development there.

The largest group of mammals is called the placental mammals. In this group the young stay in the womb all the time to complete their development. While they are there they are attached to the womb wall by an organ called the placenta. The name of this organ is used to give the group its name. Placental animals are found on all continents and even in the sea. Seals, whales and dolphins are placental mammals.