The third method is that of (3) _anatomy_, which is a study of the comparative size and shape of the bones of man and other animals as a method of showing relative periods of existence. Also, just as the structure of the bones of a child, as compared with that of a man, would determine their relative ages, so the bones of the species that have been preserved through fossilization may show the relative ages of different types of animals. The study of the skeletons of animals, including those of man, has led to the science of anthropometry. The fourth method is to study the procession of man by (4) _cultures_, or the industrial and ornamental implements that have been preserved in the river drift, rocks, and caves of the earth from the time that man used them until they were discovered. Just as we have to-day models of the improvement of the sewing-machine, the reaper, or the flying-machine, each one a little more perfect, so we shall find in the relics of prehistoric times this same gradual development--first a stone in its natural state used for cutting, then chipped to make it more perfect, and finally beautified in form and perfected by polishing. Thus we shall find progress from the natural stone boulder used for throwing and hammering, the developed product made by chipping and polishing the natural boulder, making it more useful and more beautiful, and so for all the {62} multitude of implements used in the hunt and in domestic affairs. Not only do we have here an illustration of continuous progress in invention and use, but also an adaptation of new material, for we pass from the use of stone to that of metals, probably in the prehistoric period, although the beginnings of the use of bronze and iron come mainly within the periods of historical records. It is not possible here to follow the interesting history of the glacial movement, but a few words of explanation seem necessary. The Ice Age, or the glacial period, refers to a span of time ranging from 500,000 years ago, at the beginning of the first glaciation, to the close of the post-glacial period, about 25,000 years ago. During this period great ice caps, ranging in the valleys and spreading out on the plains over a broad area, proceeded from the north of Europe to the south, covering at the extreme stages nearly the entire surface of the continent. This great movement consists of four distinct forward movements and their return movements. There is evidence to show that before the south movement of the first great ice cap, a temperate climate extended very far toward the pole and gave opportunity for vegetation now extinct in that region.