With regard to the vocabulary, many old words are going out of use, while others are becoming commoner. Thus the particle sīm, occurring fifty times in the rest of the Rigveda, is found only once in the tenth book. A number of words common in the later language are only to be met with in this book; for instance, labh, "to take," kāla, "time," lakshmī, "fortune," evam, "thus." Here, too, a number of conscious archaisms can be pointed out.
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
