Such being the case, definite dates do not begin to appear in Indian literary history till about 500 A.D. The chronology of the Vedic period is altogether conjectural, being based entirely on internal evidence. Three main literary strata can be clearly distinguished in it by differences in language and style, as well as in religious and social views. For the development of each of these strata a reasonable length of time must be allowed; but all we can here hope to do is to approximate to the truth by centuries. The lower limit of the second Vedic stratum cannot, however, be fixed later than 500 B.C., because its latest doctrines are presupposed by Buddhism, and the date of the death of Buddha has been with a high degree of probability calculated, from the recorded dates of the various Buddhist councils, to be 480 B.C. With regard to the commencement of the Vedic age, there seems to have been a decided tendency among Sanskrit scholars to place it too high. 2000 B.C. is commonly represented as its starting-point. Supposing this to be correct, the truly vast period of 1500 years is required to account for a development of language and thought hardly greater than that between the Homeric and the Attic age of Greece. Professor Max Müller's earlier estimate of 1200 B.C., formed forty years ago, appears to be much nearer the mark. A lapse of three centuries, say from 1300-1000 B.C., would amply account for the difference between what is oldest and newest in Vedic hymn poetry. Considering that the affinity of the oldest form of the Avestan language with the dialect of the Vedas is already so great that, by the mere application of phonetic laws, whole Avestan stanzas may be translated word for word into Vedic, so as to produce verses correct not only in form but in poetic spirit; considering further, that if we knew the Avestan language at as early a stage as we know the Vedic, the former would necessarily be almost identical with the latter, it is impossible to avoid the conclusion that the Indian branch must have separated from the Iranian only a very short time before the beginnings of Vedic literature, and can therefore have hardly entered the North-West of India even as early as 1500 B.C. All previous estimates of the antiquity of the Vedic period have been outdone by the recent theory of Professor Jacobi of Bonn, who supposes that period goes back to at least 4000 B.C. This theory is based on astronomical calculations connected with a change in the beginning of the seasons, which Professor Jacobi thinks has taken place since the time of the Rigveda. The whole estimate is, however, invalidated by the assumption of a doubtful, and even improbable, meaning in a Vedic word, which forms the very starting-point of the theory. Meanwhile we must rest content with the certainty that Vedic literature in any case is of considerably higher antiquity than that of Greece.