A comparison of the Avesta with the Rigveda shows clearly that soma was already an important feature in the mythology and cult of the Indo-Iranian age. In both it is described as growing on the mountains, whence it is brought by birds; in both it is king of plants; in both a medicine bestowing long life and removing death. In both the sap was pressed and mixed with milk; in both its mythical home is heaven, whence it comes down to earth; in both the draught has become a mighty god; in both the celestial Soma is distinguished from the terrestrial, the god from the beverage. The similarity goes so far that Soma and Haoma have even some individual epithets in common. The evolution of thought in the Rigvedic period shows a tendency to advance from the concrete to the abstract. One result of this tendency is the creation of abstract deities, which, however, are still rare, occurring for the most part in the last book only. A few of them are deifications of abstract nouns, such as Çraddha "Faith," invoked in one short hymn, and Manyu, "Wrath," in two. These abstractions grow more numerous in the later Vedas. Thus Kama, "Desire," first appears in the Atharva-veda, where the arrows with which he pierces hearts are already referred to; he is the forerunner of the flower-arrowed god of love, familiar in classical literature. More numerous is the class of abstractions comprising deities whose names denote an agent, such as Dhatri, "Creator," or an attribute, such as Prajapati, "Lord of Creatures." These do not appear to be direct abstractions, but seem to be derived from epithets designating a particular aspect of activity or character, which at first applying to one or more of the older deities, finally acquired an independent value. Thus Prajapati, originally an epithet of such gods as Savitri and Soma, occurs in a late verse of the last book as a distinct deity possessing the attribute of a creator. This god is in the Atharva-veda and the Vajasaneyi-Samhita often, and in the Brahmanas regularly, recognised as the chief deity, the father of the gods. In the Sutras, Prajapati is identified with Brahma, his successor in the post-Vedic age. A hymn of the tenth book furnishes an interesting illustration of the curious way in which such abstractions sometimes come into being. Here is one of the stanzas:-- By whom the mighty sky, the earth so steadfast, The realm of light, heaven's vault, has been established, Who in the air the boundless space traverses: What god should we with sacrifices worship? The fourth line here is the refrain of nine successive stanzas, in which the creator is referred to as unknown, with the interrogative pronoun ka, "what?" This ka in the later Vedic literature came to be employed not only as an epithet of the creator Prajapati, but even as an independent name of the supreme god.