At the beginning of the Tertiary period there were still many tropical forms, such as Palms, Cycads, _Nipa_, various _Artocarpaceæ_, _Lauraceæ_, _Araliaceæ_, and others, growing side by side with such temperate forms as _Quercus_, _Alnus_, _Betula_, _Populus_, _Viburnum_, and others of the same kind. Before the middle of the Tertiary was reached the last Cycads died in what is now known as Europe; and soon after the middle Tertiary all the tropical types died out of this zone. At the same time those plants whose leaves appear to have fallen at the end of the warm season began to become common, which is taken as an indication of a climatic influence at work. Some writers consider that in the Cretaceous times there was no cold season, and therefore no regular period of leaf fall, but as the climate became temperate the deciduous trees increased in numbers; yet the Gymnospermic and Angiospermic woods which are found with petrified structure show well-marked annual rings and seem to contradict this view. Toward the end of the Tertiary times there were practically no more tropical forms in the European flora, though there still remained a number of plants which are now found either only in America or only in Asia. The Glacial epoch at the close of the Tertiary appears to have driven all the plants before it, and afterwards, when its glaciers retreated, shrinking up to the North and up the sides of the high mountains, the plant species that returned to take possession of the land in the Quaternary or present period were those which are still inhabiting it, and the floras of the tropics, Asia, and America were no longer mixed with that of Europe.[9] CHAPTER IX PAST HISTORIES OF PLANT FAMILIES II. Higher Gymnosperms The more recent history of the higher Gymnosperms, in the Upper Cretaceous and Tertiary periods, much resembles that of the flowering plants as sketched in the previous chapter. Many of the genera appear to have been those still living, and some of the species even may have come very close to or have been identical with those of to-day. The forms now characteristic of the different continents were growing together, and appear to have been widely distributed over the globe. For example, _Sequoia_ and _Taxodium_, two types now characteristic of America, and _Glyptostrobus_, at present found in Asia, were still growing with the other European types in Europe so late as middle Tertiary times.