Sūta said: The five types of subjects (i.e. human beings, quadrupeds, birds, reptiles and trees) are remembered as the sons of Ṛtu. They are characterised by their seasonal change. Since the mobile and the immobile beings are born through the Ārtavas (seasonal changes, menstruation etc.) the Ārtavas are fathers and the Ṛtus are grandfathers. When they come together, the subjects of the Prajāpati are born.