Mammals are organisms that give birth to live young, contain mammary glands which females use to produce milk to nourish their young. Mammals have hair or fur and are warm-blooded. Most mammals have a placenta to nourish their young in the uterus, although some primitive Monotremes are egg-layers and Marsupials have their young spend most of their time developing in the pouch because they give birth to very undeveloped young. Mammals first appeared in the time line of evolution approximately 200 million years ago. The divisions of the Class Mammalia are subclass Prototheria which are called monotremes. Examples are platypuses and echidnas. An interesting fact is that they sweat out milk on their bellies to nourish their young and lay eggs. These are very primitive mammals. Subclass Theria are the live bearing mammals. Subclass Theria includes Metatheria, or marsupials like oppossums and kangaroos--these have underdeveloped placentas and give birth to very tiny undeveloped young which continue developing in the mother's pouch. Eutheria includes placentals, which are animals that give birth to live young after a period of gestation spent in the uterus, while nourished by the placenta. Examples are dogs, cats, humans, whales, mice, bats, etc.