[Illustration: PROBABLE LANDSCAPE IN THE CARBONIFEROUS AGE. ABOUT THE TIME COAL WAS IN THE MAKING. (_After Patonie_) 1. Tree fern 2. Giant ancestors of our horsetails. 3 and 4. Ancestors of our club mosses. 5. _Cordaites_, a primitive type, or perhaps even the ancestor of our modern evergreens. At this time no herbs and no plants with petals, were known, nor for ages after this period. (_Courtesy of Brooklyn Botanic Garden._)] At the end of this period an event of commanding interest occurred, because it happened only some 40,000 years ago. With it came the encroachment from the north and south poles of the last of the great continental glaciers. There had been many before, stretching over a past period of time, but as the last of these great ice invasions it is the most interesting to us. It crowded all these temperate and even subtropical plants that then grew up in the far north toward the equator, and scraped clear of vegetation every part of the earth which it covered. In the volume on geology you will find an account of the extent and thickness of this great ice sheet, which ultimately receded to its present home. As it went back the plants crowded forward to occupy the freshly released land, the far northern or glacial first, followed by waves of other kinds. Some of the glacial or northern plants were left on the tops of the highest mountains, where to-day they persist in complete isolation, nearly all their friends and associates of that greatest of all winters having left them for points farther north. Many students of plant geography think that wave of plant life creeping northward to occupy the region uncovered by the retreating ice is still going on, and recent studies appear to show in at least one isolated mountain in the Adirondacks that the survivals of the ice age which have been isolated on its rocky peak ever since are in considerable danger of being crowded out by invaders from the lowlands.