Mammals are the dominant life forms on Earth today and have been for many millions of years, even though the number of living species (somewhat more than 4,000) is small when compared to the 750,000 species of insects. Over much of the world, the character of the landscape has been transformed by the activities of humans and the other mammals that they have introduced into new environments.

The class Mammalia includes about 19 living orders of mammals and an equivalent number of extinct ones. The individual species are widely distributed, from the edges of the Arctic ice cap in the north, down through temperate, desert, and tropical areas, to the edge of the Antarctic continent in the south; they also occupy the oceans and major river systems of the world. Mammals span an incredible range of sizes, from shrews and bats that weigh but a few grams (a fraction of an ounce) to the blue whale, weighing more than 100 tons, the largest mammal that has ever lived.
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