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Length of the Prehistoric Period.—It is of course quite impossible to reduce the prehistoric period to any definite number of years. There are, however, numerous bits of evidence that enable an anthropologist to make rough estimates as to the relative lengths of the different periods into which prehistoric time is divided. Gabriel de Mortillet, one of the most industrious students of prehistoric archaeology, ventured to give a tentative estimate as to the numbers of years involved in each period. He of course claimed for this nothing more than the value of a scientific guess. It is, however, a guess based on a very careful study of all data at present available. Mortillet divides the prehistoric period, as a whole, into four epochs. The first of these is the preglacial, which he estimates as comprising seventy-eight thousand years; the second is the glacial, covering one hundred thousand years; then follows what he terms the Solutreen, which numbers eleven thousand years; and, finally, the Magdalenien, comprising thirty-three thousand years. This gives, for the prehistoric period proper, a term of about two hundred and twenty-two thousand years. Add to this perhaps twelve thousand years ushering in the civilization of Egypt, and the six thousand years of stable, sure chronology of the historical period, and we have something like two hundred and thirty thousand or two hundred and forty thousand years as the age of man.

"These figures," says Mortillet, "are certainly not exaggerated. It is even probable that they are below the truth. Constantly new discoveries are being made that tend to remove farther back the date of man's appearance." We see, then, according to this estimate, that about a quarter of a million years have elapsed since man evolved to a state that could properly be called human. This guess is as good as another, and it may advantageously be kept in mind, as it will enable us all along to understand better than we might otherwise be able to do the tremendous force of certain prejudices and preconceptions which recent man inherited from his prehistoric ancestor. Ideas which had passed current as unquestioned truths for one hundred thousand years or so are not easily cast aside.

In going back, in imagination, to the beginning of the prehistoric period, we must of course reflect, in accordance with modern ideas on the subject, that there was no year, no millennium even, when it could be said expressly: "This being was hitherto a primate, he is now a man." The transition period must have been enormously long, and the changes from generation to generation, even from century to century, must have been very slight. In speaking of the extent of the age of man this must be borne in mind: it must be recalled that, even if the period were not vague for other reasons, the vagueness of its beginning must make it indeterminate.

Bibliographical Notes.—A great mass of literature has been produced in recent years dealing with various phases of the history of prehistoric man. No single work known to the writer deals comprehensively with the scientific attainments of early man; indeed, the subject is usually ignored, except where practical phases of the mechanical arts are in question. But of course any attempt to consider the condition of primitive man talies into account, by inference at least, his knowledge and attainments. Therefore, most works on anthropology, ethnology, and primitive culture may be expected to throw some light on our present subject. Works dealing with the social and mental conditions of existing savages are also of importance, since it is now an accepted belief that the ancestors of civilized races evolved along similar lines and passed through corresponding stages of nascent culture. Herbert Spencer's Descriptive Sociology presents an unequalled mass of facts regarding existing primitive races, but, unfortunately, its inartistic method of arrangement makes it repellent to the general reader. E. B. Tyler's Primitive Culture and Anthropology; Lord Avebury's Prehistoric Times, The Origin of Civilization, and The Primitive Condition of Man; W. Boyd Dawkin's Cave-Hunting and Early Man in Britain; and Edward Clodd's Childhood of the World and Story of Primitive Man are deservedly popular. Paul Topinard's Elements d'Anthropologie Generale is one of the best-known and most comprehensive French works on the technical phases of anthropology; but Mortillet's Le Prehistorique has a more popular interest, owing to its chapters on primitive industries, though this work also contains much that is rather technical. Among periodicals, the Revue de l'Ecole d'Anthropologie de Paris, published by the professors, treats of all phases of anthropology, and the American Anthropologist, edited by F. W. Hodge for the American Anthropological Association, and intended as "a medium of communication between students of all branches of anthropology," contains much that is of interest from the present stand-point. The last-named journal devotes a good deal of space to Indian languages.