The differentiation of the human from the anthropoid stock must have begun a long time ago; as to when it began there is no direct evidence; and even if fossil remains of the earlier stages of our evolution had been discovered, we could only judge from the strata in which they occurred what must have been their relative antiquity. When it comes to reducing the chronology of past ages to figures, geologists either decline to make any estimate, or the results of their calculations may differ as 1 to 10. Since my own studies give me no claim to an opinion on such matters, whilst it is helpful to have clear ideas, however tentative, I shall adopt the views of Dr. Arthur Keith in his work on _The Antiquity of Man_, based on estimates published by Prof. Sollas.[53] On turning to p. 509 of that work, a genealogical tree will be found, showing the probable lines of descent of the higher Primates. The separation of the human from the great anthropoid stock is represented as having happened at about the last third of the Oligocene period—say 2,000,000 years ago (or, according to the later estimate, 3,500,000). Pithecanthropus (of Java) branched off as a distinct genus about the middle of the Miocene. Neanderthal man (_Homo Neanderthalensis_) and Piltdown man (_Eoanthropus Dawsoni_) separated as distinct species (or genera) from the stock of modern man (absurdly named _Homo sapiens_) early in the Pliocene, and became extinct respectively (say) 20,000 and 300,000 years ago. The races of modern man began to differentiate near the end of the Pliocene (say) 500,000 years from the present time. Such is the “working hypothesis.” The skull capacity of the great anthropoids averages 500 c.c.; that of Pithecanthropus is estimated at 900 c.c.; the Australian native average is 1200 c.c.; Eoanthropus, according to Dr. Keith, rises to 1400;[54] a Neanderthal skull has been measured at 1600 c.c.; the modern English average is under 1500 c.c. Of course, mental power depends not on size of the brain only, but also on its differentiation, which may have recently advanced.