[It should be observed that the Milankovich Chronology, the alternation of core and flake cultures, and the association of flake-tools with Neanderthaloid types and of bifaces with Homo Supiens are all very much in question at the present time. But subject to this qualification, the figures here printed may give some sort of approximation to the length of the periods covered. EDITOR].
Our knowledge of man's ancestry and of the early phases of his own existence can only be increased by new discoveries of flint tools and fossil man in association. Such finds have been very rare because industrial excavations like gravel pits, which might yield the evidence if carefully watched by skilled observers, do in fact destroy it. The unearthing of the Swanscombe skull in 1935 shows what might happen if the ability to recognise flint forms became more widespread and it therefore seems worth while to give a popular summary of our present knowledge.