Mammals are the highest group of vertebrates, which evolved from reptiles. Their main characteristics are: the body surface has hair, which generally consists of five parts: head, neck, trunk, limbs and tail; Breathe with the lungs; The body temperature is constant, which is warm-blooded animals; The brain is large and developed; Breastfeeding; Viviparous. Breast-feeding and viviparous are the most remarkable characteristics of mammals. Embryos develop in the mother's body, and the mother beast directly gives birth to the fetus. Mothers have mammary glands, which can secrete milk to feed their young.