While this period records the origin of hundreds, and there are probably thousands of unrecorded species which are very near our modern descendants of them, it was also a period when the earth’s crust was in an almost constant state of restlessness. Ice periods, huge inland seas, great volcanic upheavals, and the thrusting up of mountain chains such as the Alps, Himalayas, and some others, were only a few of the disturbances to the orderly procession of this wholly new type of vegetation that doomed the older kinds and subsequently conquered the world. The spread of this new element in the plant kingdom was greatly helped and sometimes greatly hindered by land connections between continents, now separated by the oceans. The giant redwoods, now isolated in a few localities on our Pacific Coast, were found then nearly throughout the world. Because of these changes of land areas and some others of even greater influence on plant growth, such as climate, there was a constant shuffling of floral and, of course, animal elements, so that by the end of this period the new type of flora had spread throughout the world, but with here and there very local occurrence of certain genera and families, some of which have persisted to the present day. As we shall see in the last chapter of this book, certain whole families are confined to restricted areas; the cactus and pineapple family, and the genus _Helianthus_, or sunflowers, for instance, are, with one or two trifling exceptions, wholly American. And we have already seen how many food and other useful plants were first found here by the Spaniards--chocolate, tobacco, corn, the potato, and others. It would fill the rest of this book to enumerate the plants that flourished toward the end of this period, and, in fact, it might almost be said that the flora of those days was not very different from our own, only it was distributed in different ways and mixed in very different proportions. With the disappearance or partial dwindling of more ancient groups, the rise of the plants that immediately preceded our own ushered in a new era in the history of the plant kingdom. [Illustration: FOSSIL AND LIVING ALGÆ COMPARED. C. A living algal pool colony near the Great Fountain Geyser, Yellowstone Park. (_After Walcott._) B. Fossil calcareous algæ. _Cryptozoön proliferum_ Hall, from the Cryptozoön ledge in Lester Park, near Saratoga Springs, N. Y. These algæ, which are among the oldest plants of the earth, grew in cabbage-shaped heads on the bottom of the ancient Cambrian sea and deposited lime in their tissue. The ledge has been planed down by the action of a great glacier which cut the plants across, showing their concentric interior structure. (_Photographed by H. P. Cushing. Pictures and explanations of them from “The Origin and Evolution of Life,” by Professor Henry Fairfield Osborn, who kindly permitted their reproduction here._) (_Courtesy of Brooklyn Botanic Garden._)]