As we search among the fragmentary herbaria scattered through the sedimentary rocks in that comparatively small portion of the earth's crust which is accessible to investigation, we discover evidence of a shifting of the balance of power among different classes of plants in the course of our survey of successive floras. Plants now insignificant and few in number are found to be descendants of a long line of ancestors stretching back to a remote antiquity when they formed the dominant class. Others which flourished in a former period no longer survive, either themselves or in direct descendants. 'The extinction of species has been involved in the most gratuitous mystery.' We can only speculate vaguely as to the cause of success or failure. Certain types were better armed for the struggle for life, and produced descendants able to hold their own and to perpetuate the race through the ages in an unbroken line. Others had a shorter life and fell out of the ranks of the advancing and ever changing army. To quote Darwin's words:'We need not marvel at extinction; if we must marvel, let it be at our own presumption in imagining for a moment that we understand the many complex contingencies on which the existence of each species depends.' BIBLIOGRAPHY Many of the books and papers dealing with subjects touched upon in this volume are not included in the following list. For reference to a more complete bibliography the reader should consult more technical treatises. 1. Holmes, T. Rice. Ancient Britain and the invasions of Julius Caesar. Oxford, 1907. 2. Mitchell, A. The Past in the Present; what is Civilisation? Edinburgh, 1880. 3. Weismann, A. Essays upon Heredity and Kindred Biological Problems, (I. The Duration of Life.) Vol. I. Edited by E. B. Poulton, S. Schönland, and A. E. Shipley. (Second Edition.) Oxford, 1891. 4. The Historie of the World, commonly called the Naturall Historic of the C. Plinius Secundus. Translated into English by Philemon Holland. London, 1634. 5. Hooker, J. D. On Three Oaks of Palestine. _Trans. Linnean Society_, Vol. XXIII. p. 381. 1862. 6. Holtermann, Carl. Der Einfluss des Klimas auf den Bau der Pflanzengewebe. Leipzig, 1907. 7. Atkinson, A. Notes on an Ancient Boat found at Brigg. _Archaeologia_, Vol. I. p. 361. 1887. 8. Huxley, T. H. Man's Place in Nature and other Anthropological Essays. _Collected Essays_, Vol. VII. (On the methods and results of Ethnology.) London, 1901. 9. Lewis, F. J. The sequence of Plant Remains in the British Peat Mosses. _Science Progress_, No. 6. October, 1907. 10. Strahan, A. On submerged Land-surfaces at Barry, Glamorganshire. With notes on the Fauna and Flora by Clement Reid; etc. _Quart. Journ. Geological Society_, Vol. LII. p. 474. 1896.