Why do whales and dolphins belong to the same group of mammals?

Mammals (class Mammalia) are group of vertebrate animals in which the young are nourished with milk from special mammary glands of the mother. In addition to these characteristic milk glands, mammals are distinguished by several other unique features. Hair is a typical feature, although in many whales it has disappeared except in the fetal stage. This ability to feed the offspring with milk occurs in whales as well.