CH. XL. BIOLOGY. 419 CHAPTER XL. SCIENCE OF THE NINETEENTH CENTURY (CONTINUED). Facts which led Naturalists to believe that the Different Kinds of Animals are descended from Common Ancestors - All Animals of each Class formed on one Plan-Embryological StructureLiving and Fossil Animals of a Country resemble each other — Gradual Succession of Animals on the Globe-Links between Different Species-Darwin's Theory of Natural Selection-Wallace worked out the same Theory independently-Sketch of the Theory of Natural Selection-Selection of Animals by Man-Selection by Natural Causes-Difficulties in Natural History which are explained by this Theory-Foolish Prejudices against it-Concluding Remarks on the History of Science. Facts which have led Naturalists to believe that the different kinds of Animals are descended from common Ancestors. We now come to the first attempt of any value which has ever been made, to explain how the different kinds of animals and plants have been produced. This question is so very difficult, and seems so much beyond our grasp, that we find very few people throughout the history of science who even tried to answer it. Aristotle, it is true, remarked that we can trace such a close resemblance between the different species, from the lowest plant up to the highest animal, as would seem to show they are related to each other (p. 16). Bonnet, too, thought that animals were developed from lower into higher forms (p. 202); and Lamarck, as we have seen, boldly suggested the same explanation (p. 391). But people in general treated these as mere wild speculations, and were content to say that God had created animals, just in the same way as they said that the stars were created by Him, without pausing to consider how He has created them. Since the time of Buffon and Linnæus, however, many new facts had gradually been brought to light about living animals; and fossil species had been dug out of the earth, showing that many different forms had lived upon our globe, one after the other; and these new discoveries led naturalists to speculate whether some clue might not be found to explain this long succession of living beings. Then again, as naturalists spread all over the world and many new forms of animals and plants became known, it was found to be more and more difficult to separate the different species and to say which are and which are not descendants of one parent. Linnæus, as we have seen (p. 392), pointed this out in the case of plants, and wild roses are a very good example of it; for the different kinds run so much into each other that while one of our best botanists has divided them into seventeen species, another thinks that many of these must have come from the same parent, and that only five species can be distinguished. Again, among insects, the well-known naturalist, Mr. Bates, has shown that on the Amazons in South America it is often impossible to tell, among some families of butterflies, which are the same species and which keep apart from each other. Facts like these, of the relationship of living beings, had long been forcing themselves upon naturalists, and this was one of the reasons given by Lamarck for supposing animals to be all descended from a few simple forms. All the Animals of each Class formed on the same DESCENT OF ANIMALS. 421 Plan. Another reason was that curious agreement in the bones of different animals which had become more and more noticed ever since the time of John Hunter, and which Geoffroy St.-Hilaire insisted upon so strongly. Why should the animals of one class (such as the vertebrate or backboned class) be formed all on one plan even to the most minute bones; so that the wing of a bat, the front leg of a horse, the hand of a man, and the flapper of a porpoise, are all made of the same bones, which have either grown together, or lengthened and spread apart, according to the purpose they serve? And, more curious still, why should some animals have parts which are of no use to them, but only seem to be there because other animals of the same class also have them. Thus the whale has teeth like the other mammalia, but they never pierce through the gum; and the boa-constrictor has the beginnings of hind legs hidden under its skin, though they never grow out. Here again it seemed extraordinary, if a boa-constrictor and a whale were created separately, that they should be made with organs which are quite useless; while, on the other hand, if they were descended from the same ancestor as other reptiles and mammalia who have teeth and hind legs, they might be supposed to have inherited these organs; just as, for example, a child sometimes has a mole or other mark upon its body in exactly the same place as its great grandfather had before it. Embryos of Animals alike in Structure.-Another still more remarkable fact was that pointed out by Von Baer, that the higher animals, such as quadrupeds, before they are perfectly formed, cannot be distinguished from the embryos of other and lower animals, such as fish and reptiles. If animals were created separately why should a dog begin like a fish, a lizard, and a bird, and have at first parts which it loses as it grows into its own peculiar form? Living Animals of a Country agree with the Fossil ones. These were facts entirely belonging to living creatures, but now others sprang up about fossil species which were equally puzzling. We know that certain animals are only found in particular countries; kangaroos and pouched animals, for example, in Australia; and sloths and armadillos in South America. Now it is remarkable that all the fossil quadrupeds in Australia are also pouched animals, though they are of different kinds and larger in size than those now living; and in the same way different species of sloths and armadillos are found fossil in South America; while in the rocks of Europe fossil mammalia are found, only slightly different in form from those which are living there now. Naturalists therefore asked themselves again-'Would it not seem likely that the living pouched animals of Australia and the sloths and armadillos of America are the descendants of the dead ones in the rocks, although they have in the course of long ages become rather different from them; while oxen, bears, wolves, &c., are also the descendants of those which are found buried in the rocks of Europe? Gradual succession of Animals which have appeared upon the Globe.-This seemed still more likely as the study of geology advanced, and it became clear that a gradual succession of higher and higher animals had appeared upon the globe. Thus, in the oldest rocks containing fossils, we find no monkeys, no quadrupeds, no reptiles, no amphibians such as our frogs, but only shells of marine animals, and a few bones of fishes, of kinds quite different from those now living. Then in rocks above these we find the fish becoming very abundant and varied, and higher still we meet with CH. XL. SUCCESSION OF ANIMALS. 423 footprints of some animal with feet; and the bones of an amphibian, somewhat like a frog, are next found. In these times the fish began to cease to be monarchs of the water, for a little higher up huge swimming reptiles, like our crocodiles and lizards, but much larger, have left their bones in the rocks. Next come reptiles with wings, which measure sixteen feet across from tip to tip, and we must picture these huge flying lizards, with wings like bats, roaming over the globe with no higher animals to persecute them. But they were only to have their turn, for in rocks formed a little later there appear two skeletons, one of a small creature half reptile half bird, about the size of a pigeon, and the other of a real bird with some of its feathers still remaining; and in beds of about the same age there occurs the jaw of a small insect-eating animal something like an ant-eater. Birds and quadrupeds therefore had now begun to exist, and soon the bones of pouched animals are found, and then of mammalia, like our moles and shrews; and from this time. the reptiles become smaller, as if they were kept down and gradually destroyed by the higher animals, and the quadrupeds become larger and more powerful; till, in those beds which Cuvier studied near Paris, we find the gigantic elephant and rhinoceros-like animals we spoke of before; while in beds of about the same age occur the first bones of monkeys. This is a very rough sketch of the order in which animals are found in the earth's crust. The lower kinds first, and then gradually higher and higher forms as they come near to our own time; and if we could study them more closely you would see that in rocks nearly of the same age the forms are always very like each other, while the farther apart the formations are, the more different are the animals. It is true |