to the southward or to perish. The Isthmus of Panama and the broad expanse of Central America offered a refuge for the tender forms which could not withstand the rigors of the Ice Age when North America was covered to the Ohio Valley with a great continental glacier. In the milder climes many of them survived, and as the ice retreated they slowly followed it—how, I shall take occasion to show in a moment. But between the ice-bound continent of Europe and the warmer lands to the southward intervened then as now the broad expanse of the Mediterranean, which could be traversed but by few animals and by still fewer plants. Many forms perished, and their places are empty to this day. Hence we see one reason for the greater number of plant species on our own continent.
The botanical geologist finds other things to study, for there are many plants of bygone ages preserved to this day as fossils in the rocks of various horizons. The science of paleobotany grows more slowly than the science of paleozoölogy, which greedily usurps the name of paleontology, as if plants were not quite as important, had not contributed quite as much of value to the earth's crust, as animals. The reasons why paleobotany is such a slow-growing and fragmentary science are two. In the first place, as I have suggested, paleontologists devote themselves in their investigations and teaching too exclusively to animal remains, and hence he who will know more of the plants of past ages must study and learn largely unaided. But the second reason is more fundamental—namely, that plant structures are less easily preserved than those of animals (whose shells or other hard parts are very resistant), and hence many have been destroyed during the various changes that the rocks in which their remains were imbedded have undergone. The fossil remains which are now known give confirmation of the fact mentioned a moment ago that plants are ever changing. The plants of the Carboniferous Age were very different from those of to-day. The aspect of field and forest then must have been mysterious indeed; the heavy atmosphere, the intense light, the moisture, contributing to a vegetation of more than tropical luxuriance. Between the lofty stems of tree ferns, in the deep shade cast by their great fronds, wandered animals hideous to the eye, though perhaps no more dangerous than our mild-eyed cows. But we find to-day, growing here and there, plants which greatly resemble those of the coal measures, not only the ferns, but the horsetails (scouring rushes of our ancestors, Equisetum), the Lycopodiums, without which no northern Christmas festival is quite complete, and others less conspicuous. These the paleontologist convinces us are the direct descendants of those plants which compose our coal. He thereby adds his facts to that history of life which shows that plants are related,
