Page:Popular Science Monthly Volume 27.djvu/626

606 that the history of the succession of life is recorded with the same regularity, and may be read by those who will bestow the necessary labor upon it. Those who have, during the last ten years, devoted themselves to this study have been rewarded by the discovery of the course of development of many lines of animals, so that it is now possible to show the kind of changes in structure which have resulted in the species of animals with which we are familiar as living on the surface of the earth at the present time. Not that this continent has given us the parentage of all forms of animal life, or all forms of animals with skeletons, or vertebrata, but it has given us many of them. To take the vertebrata, we have obtained the long-since extinct ancestor of the very lowest vertebrates. Then we have discovered the ancestor of the true fishes. We have the ancestor of all the reptiles, of the birds, and of the mammals. If we considered the mammals, or milk-givers, separately, we have traced up a great many lines to their points of departure from very primitive things. Thus we have obtained the genealogical trees of the deer, the camels, the musk, the horse, the tapir, and the rhinoceros, of the cats and dogs, of the lemurs and monkeys, and have important evidence as to the origin of man. We have the primitive mammals from which all these kinds that I have mentioned drew their descent, and from which, no doubt, many other lines were derived which we have not yet discovered in North America. Such are the lines of the elephants, the hyenas, the bears, the hogs, and the oxen. The ancestors of the strange, pouch bearing marsupialia, have been found in part. These creatures, now confined (except the opossums) to Australia and the adjacent islands, were, at an early period, widely distributed over the earth. Some of these are found in the fossiliferous deposits of our plains and Rocky Mountains.

So soon as the possibility of learning the manner of creation of animals is admitted, curiosity and speculation are awakened. Many alternatives naturally occur to the mind. Were any of the living kinds of animals descended from any other living species, or have the ancestral animals all disappeared from the earth? Have the giants of ancient periods become reduced in size and strength, or have the giants of to-day grown from weak and insignificant beginnings? Have things grown more and more perfect with the lapse of the ages, or have they degenerated from more perfect ancestors? Have these changes advanced alike in all continents, or have they proceeded differently in different parts of the earth? Such are the questions that confronted the student of North American vertebrate paleontology fifteen years ago, and some of them could only be answered by North American material, not only because its record is the most complete, but because, as the second continent studied, it furnished the first opportunity in the history of the science for a comparison with the record already placed before us by the paleontologists of Europe.