Thus the tenth book represents a definitely later stratum of composition in the Rigveda. Individual hymns in the earlier books have also been proved by various recognised criteria to be of later origin than others, and some advance has been made towards assigning them to three or even five literary epochs. Research has, however, not yet arrived at any certain results as to the age of whole groups in the earlier books. For it must be borne in mind that posteriority of collection and incorporation does not necessarily prove a later date of composition. Some hundreds of years must have been needed for all the hymns found in the Rigveda to come into being. There was also, doubtless, after the separation of the Indians from the Iranians, an intermediate period, though it was probably of no great length. In this transitional age must have been composed the more ancient poems which are lost, and in which the style of the earliest preserved hymns, already composed with much skill, was developed. The poets of the older part of the Rigveda themselves mention predecessors, in whose wise they sing, whose songs they desire to renew, and speak of ancestral hymns produced in days of yore. As far as linguistic evidence is concerned, it affords little help in discriminating periods within the Rigveda except with regard to the tenth book. For throughout the hymns, in spite of the number of authors, essentially the same language prevails. It is quite possible to distinguish differences of thought, style, and poetical ability, but hardly any differences of dialect. Nevertheless, patient and minute linguistic research, combined with the indications derived from arrangement, metre, and subject-matter, is beginning to yield evidence which may lead to the recognition of chronological strata in the older books of the Rigveda. Though the aid of MSS. for this early period entirely fails, we yet happily possess for the Rigveda an abundant mass of various readings over 2000 years old. These are contained in the other Vedas, which are largely composed of hymns, stanzas, and lines borrowed from the Rigveda. The other Vedas are, in fact, for the criticism of the Rigveda, what manuscripts are for other literary monuments. We are thus enabled to collate with the text of the Rigveda directly handed down, various readings considerably older than even the testimony of Yaska and of the Pratiçakhyas.