During the Tertiary period, or probably in the earlier days of that era. Ginkgo flourished in North America, in Alaska and in the Mackenzie River district, Greenland, Saghalien Island, and in several European regions. In [Chapter III] reference was made to the volcanic activity which characterised the north-west European area in the early Tertiary period and resulted in the formation of the thick sheets of basalt on the north-east coast of Ireland and in the Inner Hebrides. There were occasional pauses in the volcanic activity, during which vegetation established itself on the weathered surface of the lava, and left traces of its existence in the leaves and twigs preserved in the sedimentary material enclosed between successive lava-floras. At Ardtun Head in the Isle of Mull beautifully preserved leaves of Ginkgo, 2-4 inches in breadth, with the median sinus and the venation characteristic of the leaves of the existing plant, have been discovered in a bed of clay which marks the site of a lake in a depression on the lava-plateau. The resemblance of these Tertiary leaves from Mull to those of the surviving Maiden Hair tree is so close as to suggest specific identity. Mr Starkie Gardner and Baron Ettingshausen have described some seeds from the London clay (Lower Tertiary) in the Isle of Sheppey as those of Ginkgo, but this identification rests on data too insufficient to be accepted without hesitation.
The recent cultivation of Ginkgo biloba in Britain may therefore be spoken of as the re-introduction of a plant which in the earlier part or in the middle of the Tertiary period flourished in the west of Scotland, and was abundant in England in the earlier Jurassic period. It is impossible to say with any confidence where the Ginkgoales first made their appearance , whether in the far north or in the south, nor are we able to explain the gradual decline of so venerable and vigorous a race.
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.