Soma Pavamāna ("the clearly flowing") are composed by authors of the same families as produced Books II.—VII., a fact, apart from other evidence, sufficiently indicated by their having the characteristic refrains of those families. The Pavamāna hymns have affinities to the first and eighth books also. When the hymns of the different families were combined into books, and clearly not till then, all their Pavamāna hymns were taken out and gathered into a single collection. This of course does not imply that the Pavamāna hymns themselves were of recent origin. On the contrary, though some of them may date from the time when the tenth book came into existence, there is good reason to suppose that the poetry of the Soma hymns, which has many points in common with the Avesta, and deals with a ritual going back to the Indo-Iranian period, reached its conclusion as a whole in early times among the Vedic singers. Differences of age in the hymns of the ninth book have been almost entirely effaced; at any rate, research has as yet hardly succeeded in distinguishing chronological stages in this collection.
With regard to the tenth book, there can be no doubt that its hymns came into being at a time when the first nine already existed. Its composers grew up in the knowledge of the older books, with which they betray their familiarity at every turn. The fact that the author of one of its groups (20—26) begins with the opening words (agnim īḷe) of the first stanza of the Rigveda, is probably an indication that Books I.—IX. already existed in his day even as a combined collection. That the tenth book is indeed an aggregate of supplementary hymns is shown by its position after the Soma book, and by the number of its hymns being made up to that of