The strongest proof of the unity of heredity as displayed in the dominant Crô-Magnon race in Europe from early Aurignacian until the close of Magdalenian times is the unity of their art impulse. This indicates a unity of mind and of spirit. It is something which could not pass to them from another race, like an industrial invention, but was inborn and creative. These people were the Palæolithic Greeks; artistic observation and representation and a true sense of proportion and of beauty were instinct with them from the beginning. Their stone and bone industry may show vicissitudes and the influence of invasion and of trade and the bringing in of new inventions, but their art shows a continuous evolution and development from first to last, animated by a single motive, namely, the appreciation of the beauty of form and the realistic representation of it. [Illustration: FIG. 155. Outline of a mammoth painted in red ochre in the cavern of Pindal, and attributed by Breuil to the Aurignacian. Only two limbs are represented. After Breuil.] This art, as first discovered by Lartet and further made known through the brilliant studies of Piette and Breuil, is industrial (_l'art mobilier_), consisting of the decoration of small personal belongings, ornaments, and implements of stone, bone, and ivory. According to the later researches of Sautuola, Rivière, Cartailhac, Capitan, and Breuil it is also mural or parietal (_l'art pariétal_), consisting of drawings, engravings, paintings, and bas-reliefs on the walls of caverns and grottos. It remained for Breuil especially to demonstrate that the mobile and the parietal art are identical, the work of the same artistic race, developing along closely similar lines, step by step. Thus the art becomes a new means not only of interpreting the psychology of the race but of establishing the prehistoric chronology. DATING OF THE ART One of the first questions which rises in our mind is this--how is this art dated; how can these steps be positively determined? The age of these engraved or painted designs on the walls of the caverns is determined in a number of ways described by Breuil.(36) The simplest method is where the wall designs of one period are covered by the archæological layers of succeeding periods. This has been observed in four cases, as at Pair-non-Pair, Gironde, where primitive engravings of horses, caprids, and bovids are buried under flints characteristic of the late Aurignacian mingled with bones of the mammoth, rhinoceros, lion, hyæna, bison, and reindeer. Again, the deeply engraved bison on the wall of the grotto of La Grèze, Dordogne, is found beneath a talus of Solutrean flints associated with remains of the bison, reindeer, and rhinoceros. In the Grotte de la Mairie, Dordogne, are found several finely engraved middle Magdalenian figures of animals buried beneath late Magdalenian implements associated with the reindeer fauna.