Closely related to foxes and dogs. Range: Formerly over most of North America. Habitat Group in American Museum of Natural History.] [Illustration: BABY OPOSSUMS RIDING ON THEIR MOTHER'S BACK For the first few weeks after they are born the mother carries her babies in her pocket; later they ride on her back holding on by clinging to her fur with their paws and by wrapping their tails about that of their mother. Range: Middle and Southern States. From Group in American Museum of Natural History.] 3. ANIMALS Mammals Mammals differ from birds in that they have hair instead of feathers, and that they are first fed upon milk produced by the mother. Unfortunately the mammals are usually called simply _animals_, but the latter is obviously too inclusive a term and should not be used in this way. There is no reason why the name _mammal_ should not be commonly used, just as _birds_, _reptiles_, _amphibians_, and _fishes_ are used for the other groups of backboned animals. [Illustration: NEW YORK WEASEL IN SUMMER PELAGE] [Illustration: OTTER WITH ITS FAVORITE FOOD The Otter belongs to the Weasel family, and feeds almost entirely upon fish. Range: This and related varieties over Northern and Eastern North America. From Group in American Museum of Natural History.] In the United States the lowest or most primitive mammal is the Opossum. The baby Opossums--from six to a dozen of them--are born when very small and undeveloped and are immediately placed by the mother in an external pouch, where they continue to grow until they are too large to get into their mother's pocket; then they frequently ride upon their mother's back, clinging to her fur with their finger-like toes and wrapping their tails about their mother's tail. The Opossum is the only animal in this country the young of which are carried around in the mother's pocket, and the only one which has a prehensile tail; that is, one used for coiling around and clinging to branches, and the like. Its food is various, consisting of both animal and plant material--insects, young birds, pawpaws, persimmons, etc. In the food devoured the Opossum probably does more good than harm. [Illustration: NEW YORK WEASEL IN WINTER A blood-thirsty cousin of the Otter and the Mink. Range: This and related species found all over United States and Canada. Group in American Museum of Natural History.] [Illustration: RACCOON AT ENTRANCE TO ITS DEN IN A HOLLOW TREE A near relative of the bears. Note the black face-mark and the ringed tail. Range: This or a related variety occurs in all parts of United States. Photograph from American Museum of Natural History.] In their food habits many mammals are decidedly injurious. Rats, Weasels, Minks, and Foxes destroy poultry; Wolves and Pumas kill domestic and game animals; Woodchucks or Groundhogs eat clover and various garden plants; Moles damage the lawns; Rats, Mice, and Gophers spoil and devour grain; Mice and Rabbits girdle fruit trees, thus killing them.