Broca, Paul, “The Troglodytes or Cave-Dwellers of the Valley of the Vézère,” _Smithsonian Report_, 1872, pp. 310-347. “Man as the Contemporary of the Mammoth and the Reindeer in Middle Europe,” translated by C. A. Alexander, for the Smithsonian Institution, from “Aus der Natur: die neuesten Entdeckungen auf dem Gebiete der Naturwissenschaften,” Leipzig, 1867, _Smithsonian Report_, 1867, pp. 335-362. Wilson, Thomas, “Anthropology at the Paris Exposition in 1889,” _Smithsonian Report_, 1890, pp. 641-680. _Classifications._ In reading the above books for the purpose of supplementing the first three numbers of this series, it must be remembered that no uniform system of classification has been adopted and that the reader must become familiar with several in order to be able to use the reference books most advantageously. To help the reader in getting a working idea of the different classifications the following summaries may be of value: The progress of mankind is classified by Professor W. Boyd Dawkins into the following periods: I. The Pleistocene period. 1. The mid-Pleistocene period. 2. The late Pleistocene period. II. The Prehistoric period. III. The Historic period. Others classify the period with reference to the materials used in making implements and weapons and give us the following: I. The Stone Age. 1. The Paleolithic or Rough Stone Age. 2. The Neolithic or Polished Stone Age. II. The Age of Metals. 1. The Age of Copper. 2. The Age of Bronze. 3. The Age of Iron. Still others classify with reference to the dominant industry of the period, giving: I. The House Industries, or the Period of Domestic Economy. 1. The Hunting Stage. 2. The Fishing Stage. 3. The Pastoral Stage. 4. The Agricultural Stage, etc. II. The Handicrafts, or the Period of Town Economy. III. The Factory System, or the Period of National Economy. The Pleistocene period is the one that is treated in the first three volumes of this series. It is the period that is frequently designated as the Paleolithic or Rough Stone Age in contrast to the Neolithic or Polished Stone Age which belongs to what Professor Dawkins calls the Prehistoric Period.