|LATE | | |Extreme | | | MESOZOIC.| | | specialization | AGE | | | | | of reptiles. | | | |-------------|------------|------------------| | | | | |Rise of flowering | OF | | |Comanchian. | | plants. | | |-----------|-------------|------------|------------------| REPTILES. | | | | |Rise of birds and | | | |Jurassic. | | flying reptiles.| | |EARLY |-------------|------------|------------------| | | MESOZOIC.| | | | | | |Triassic. | |Rise of dinosaurs.| | +-----------------------------------------------------------------------+ Since most of the skeletal and cultural remains of man can now be definitely attributed to certain glacial, interglacial, or postglacial stages, vast interest attaches to the very difficult problem of the duration of the whole Ice Age and the relative duration of its various glacial and interglacial stages. The following figures set forth the wide variations in opinion on this subject and the two opposite tendencies of speculation which lead to greatly expanded or greatly abbreviated estimates of Pleistocene time: DURATION OF THE ICE AGE +--------------------------------------------------------------------+ |1863. Charles Lyell,(46) _Principles of Geology_ 800,000 years. | |1874. James D. Dana,(47) _Manual of Geology_ 720,000 " | |1893. Charles D. Walcott,(48) _Geologic Time as | | Indicated by the Sedimentary Rocks of | | North America_ 400,000 " | |1893. W. Upham,(49) _Estimates of Geologic Times, | | Amer. Jour. Sci._, vol. XLV 100,000 " | |1894. A. Heim,(50) _Ueber das absolute Alter der | | Eiszeit_ 100,000 " | |1900. W. J. Sollas,(51) _Evolutional Geology_ 400,000 " | |1909. Albrecht Penck,(52) _Die Alpen im | | Eiszeitalter_ 520,000-840,000| |1914. James Geikie,(53) _The Antiquity of Man in | | Europe_ 620,000 (min.) | +--------------------------------------------------------------------+