Ranging in size from a shrew to the blue whale, mammals are found in the air, on and under the ground, and in the oceans, rivers, and lakes of the world. A total of 151 families are divided into 1226 genera and represent 5416 species. The mammalian middle ear is a key diagnostic character and has evolved from three small bones found in the jaw of primitive vertebrates. Mammals regulate their internal temperature (endothermy). Diagnostic features include: sweat glands, hair, a complex lung structure, and mammary glands: the production of milk to feed and nurture young is a shared feature of all mammals.
Encyclopedia of Biodiversity (Second Edition), 2013, Pages 681-707,