Explain about traits of mammals.

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.