Sir Archibald Geikie tells us (p. 837), and in the main every one agrees with him, that at the beginning of the Tertiary era in which we now live, the climate was of a tropical and subtropical character in Europe. Gradually it became more temperate, and eventually it passed into a phase of extreme cold, but since that time the cold has again gradually diminished. It is quite evident, therefore, that from a purely geological point of view our south-western flora must have migrated northward before the cold came on, and survived in sheltered localities under the influence of the mild coast climate. Some, however, suppose that there occurred a phase of extreme mildness immediately after the Glacial period, and that it was during that time that the Lusitanian fauna and flora became established in the British Islands. To this Professor James Geikie replies (_b_, p. 169), "there are few points we can be more sure of than this, that since the close of the Glacial epoch--since the deposition of the clays with Arctic shells and the Saxicava sands--there have been no great oscillations, but only a gradual amelioration of climate. It is quite impossible to believe that any warm period could have intervened between the last Arctic and the present temperate conditions without leaving some notable evidence in the superficial deposits of Scotland, Scandinavia, and North America." Thus it appears that on the whole the assumption that the Lusitanian fauna and flora are very ancient and pre-glacial is also supported on geological evidence. [Illustration: Fig. 5.--Map of Europe, with arrows indicating approximately the course taken by the different streams of migration towards the British Islands.]