But the cause of this diversity lies deeper. New cultures and new waves of migration of peoples were pouring into Europe, especially into the Baltic region now left free of ice, enjoying a mild climate, and offering an abundance of food along the shores of its rivers, lakes, and seas. The Tardenoisian culture had spread northward from the Mediterranean. The broad-headed people of Furfooz, Grenelle, and Ofret had apparently crossed Europe from the east and had settled in a long zone extending northward and southward through Belgium and France and probably southward into Spain, for we remember the broad-heads found at Mugem, in Portugal. But their distribution was far wider than this strip of territory. New Neolithic types of culture had already entered Italy, perhaps as early as Magdalenian times. Series of waves appear to have passed into Poland, Russia, and Siberia, and to have moved northward until they reached the coast in Scandinavia and to the eastward. In all these cases we may probably imagine a gradual and perhaps slow infiltration or "seeping" in of the new population rather than an invasion in crowds or masses, such as we are likely to imagine. Vast stretches of habitable land had been newly opened, and there was plenty of room for all comers. In many regions the old population may have remained comparatively undisturbed until a much later date. But even they slowly came under the influence of the new and improved technique and mode of life. All this collision of culture and conflict of peoples meant stimuli, awakening, the jogging of dull minds, a veritable spur of necessity. A new day was beginning to break. The dawn was dim and cloudy, but there was the possibility and prospect of clear shining. CHAPTER III LAND HABITATIONS Our history of Paleolithic times is drawn very largely from the successive strata of remains found in rock-shelters and near the mouths of caves, where the succession of epochs is clear and indubitable. We naturally look for similar reliable testimony concerning the chronological succession of Neolithic utensils, pottery and other remains. Here, however, we have been disappointed to a large degree. Paleolithic layers were generally or frequently overlaid by beds of stalagmite or fallen rocks, which have saved them from disturbance. But the Neolithic and Bronze layers are superficial, usually of no great thickness; they have been less solidified and protected, and far more exposed to the disturbing work of burrowing mammals and of men digging for buried treasures. These circumstances, combined with far less continuity of occupation, have greatly diminished the chronological value of their study.