How do you define a mammal?

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The mammals also arose from reptiles in the Jurassic period and are now the dominant form of terrestrial vertebrate life. Like the birds, they have a four-chambered heart and a double-circuit circulatory system and are able to regulate body temperature. In the case of mammals the insulating covering is provided by hair, a feature unique to the class, although in a few forms (particularly in marine species) nearly all the hair is lost, and insulation is provided by fat. A second distinguishing characteristic of mammals is the production of milk by the females for the nourishment of the young. All mammals have internal fertilization, and all but the most primitive (the egg-laying monotremes of Australia) bear live young. The mammalian egg contains little yolk. In the marsupials the young are born at an extremely undeveloped stage and continue to develop in a milk-supplied pouch. In the vastly more numerous placental mammals nourishment is passed from the circulatory system of the mother to that of the embryo by means of a placenta, and the young are born well-developed. Most mammals have highly evolved sense organs and larger brains than other vertebrates. As a group they display great adaptability to a variety of conditions and have spread to all regions of the world.