The general character of the ten books is not identical in all cases. Six of them (ii.-vii.) are homogeneous. Each of these, in the first place, is the work of a different seer or his descendants according to the ancient tradition, which is borne out by internal evidence. They were doubtless long handed down separately in the families to which they owed their being. Moreover, the hymns contained in these "family books," as they are usually called, are arranged on a uniform plan differing from that of the rest. The first, eighth, and tenth books are not the productions of a single family of seers respectively, but consist of a number of groups based on identity of authorship. The arrangement of the ninth book is in no way connected with its composers; its unity is due to all its hymns being addressed to the single deity Soma, while its groups depend on identity of metre. The family books also contain groups; but each of these is formed of hymns addressed to one and the same deity. Turning to the principle on which the entire books of the Rigveda are arranged in relation to one another, we find that Books II.-VII., if allowance is made for later additions, form a series of collections which contain a successively increasing number of hymns. This fact, combined with the uniformity of these books in general character and internal arrangement, renders it probable that they formed the nucleus of the Rigveda, to which the remaining books were successively added. It further seems likely that the nine shorter collections, which form the second part of Book I., as being similarly based on identity of authorship, were subsequently combined and prefixed to the family books, which served as the model for their internal arrangement. The hymns of the eighth book in general show a mutual affinity hardly less pronounced than that to be found in the family books. For they are connected by numerous repetitions of similar phrases and lines running through the whole book. The latter, however, does not form a parallel to the family books. For though a single family, that of the Kanvas, at least predominates among its authors, the prevalence in it of the strophic form of composition impresses upon it a character of its own. Moreover, the fact that the eighth book contains fewer hymns than the seventh, in itself shows that the former did not constitute one of the family series.