That some of these examples are of great antiquity is inferred from the circumstances under which they were discovered. The evidence relates either to their association with extinct animals such as the Mammoth, or again the bones may have been found at great depths from the surface, in strata judged to have been undisturbed since the remains were deposited. One of the earliest discoveries was that of the Engis skull; the differences separating this skull from those of modern Europeans are so extraordinarily slight that doubt has been expressed as to the antiquity assigned to the specimen, and indeed this doubt has not been finally dispelled. The bones from Denise (now rehabilitated in respect of their antiquity by Professor Boule) present similar features. But on the other hand the jaws found at La Naulette and Malarnaud suggest the former existence of a lowlier and more bestial form of humanity. Support is provided by the famous skull of the Neanderthal, but in regard to the latter, conclusive evidence (as distinct from presumption) is unfortunately lacking. Further confirmation is given by the Forbes Quarry skull from Gibraltar, but although its resemblance to that of the Neanderthal was clearly noted by Dr Busk and Sir William Turner[10] as long ago as 1864, the specimen was long neglected. In this case, as in that of the Neanderthal, corroborative evidence as to the geological or archaeological horizon is lamentably defective. After a lapse of some twenty years, the discoveries of human skeletons at Spy in Belgium, undoubtedly associated as they were with remains of Mammoth, threw a flood of light on the subject, and enormously enhanced the significance of the earlier discoveries. The former existence in Europe of a human type, different from all other known inhabitants of that continent, and presenting no small resemblance to the lowliest modern representatives of mankind, may be said to have been finally established by the results of the excavations at Spy. Moreover the differences thus recognised are such as to lend strong support to the evolutionary view as to the origin of the more recent human stocks from an ancestral series including representatives of a simian phase. Yet the co-existence of a higher type represented by the Engis skull must not be overlooked, nor indeed has this been the case. The significance of so remarkable a phenomenon is more fully discussed in the sequel; but no detailed account of the earlier discoveries need be given. A bibliography is appended and here references (H[oe]rnes[44], 1908; Schwalbe[55]) will be found to the more important sources of information upon those specimens.