Animals of various species found in nature have been classified into small or large groups on the basis of similarities and differences. This is an important evidence of biological evolution. All the animals were first divided by Linnaeus into different groups on the basis of binomial system. The vertebrate subfamily was divided into several classes, which according to evolution were categorized into cyclostomes, fishes, amphibians, reptiles, birds and mammals. From this sequence, it is clear that in the ancient times some vertebrates evolved from fish-like vertebrate to fishes, from fishes to amphibians, from amphibians to reptiles, from reptiles to birds and later on to mammals. Evidence of gradual evolution in all these classes can be traced by similarities found in them. By this method of classification, a family tree of all animals and plants can be prepared, which provides evidence of gradual evolution in animals of different groups, from protozoa to vertebrates.