Morphological studies of various organ systems of vertebrates indicate that these are constructed on the same basic plan. The minor differences seen in some forms are the adaptive modifications to the diverse mode of living. This similarity is known as homology. Homology is the similarity between organs of different animals based on common ancestry or common embryonic origin. Therefore, the homologous organs have common origin and are built on the same fundamental pattern, but perform varied functions and have different appearance. Homology is seen in every organ system from fish to man.