The New Student's Reference Work/Evolution
Evolution. Although evolution is a word in constant use, there still is great vagueness in the minds of most people as to what it stands for. In its broad sense it has come to mean the development of all nature from the past. If we think of the long train of events in the formation of the world and peopling it with life, we may look upon this story, figuratively, as written on a scroll that is being unrolled. Everything that has come to pass is on that part so far unrolled, and everything in the future is still covered but will appear in due course of time. Thus, evolution in its broadest sense may mean the unrolling of the scroll of the universe, including the formation of stars, solar systems and the elements of the inorganic world as well as of all living nature. But the word as usually employed is limited to organic evolution, or the formation of life upon our planet, and in this sense it is used throughout this article. It is a common mistake to suppose that Darwinism and evolution are the same, and there also is much misunderstanding as to the nature of the entire question. Hence we should first get a clear idea of what evolution is, then of the basis upon which it rests, and finally trace the growth of evolutionary thought, especially in the 19th century. Evolution, as used in biology, is a history of the steps by which animals and plants came to be what they are. The great variety of animals and plants is amazing. The water, the earth, the air teem with life; the fishes of the sea are almost innumerable; and in a single group of the insect world—the beetles—there are upwards of 50,000 species known and described. In addition to the living animals, there is entombed in the rocks a great multitude of forms that lived eons ago and became extinct. In view of all these forms the question comes at once to the mind: How shall we account for this great diversity of organic life? Have the great variety of forms existed unchanged from the days of their creation to the present, or have they, perchance, undergone modifications, so that one original form may have, through its descendants, emerged into different kinds? This is not merely an idle question, insoluble from the very nature of the case, for the present races of animals and plants have a long lineage reaching into the past, and it is a historical question to be answered by trying to discover their parentage. We shall see that there is good reason to believe that the higher animals have been derived gradually from simpler ones, including with the higher animals, also, the body of man. The very general acceptance of this view in the latter part of the 19th century, under the title of the doctrine of evolution, has made an epoch. It has had great influence, perhaps the greatest of any one doctrine, in modifying the point of view in many fields of thought. It is, therefore, so important that all intelligent people ought to know what the doctrine is. By way of clearing the ground it may be repeated that it is not a question of creation through Divine agencies or non-creation, but a question of method of creation. It is not a theological question, but an historical one. Nevertheless, when the doctrine arose with new force about 1860, it found itself in conflict with theological ideas and especially with the doctrine of creation called special creation. It is natural to think that the latter was a view held by the early theologians and Fathers of the church, but it is instructive to find that some of them, such as St. Augustine, openly expressed the opinion that God created all things “in the germ.” The doctrine of special creation was introduced into English Protestant thought through the epics of John Milton, to whose forceful and picturesque language we have been indebted for many of our accepted dogmas, without having paused to examine into them.
The question of the parentage of animals involves the distant past. The very center of it is this: Are species fixed and permanent, as Linnæus, the great Swedish botanist and naturalist, supposed; or do they change? If it can be shown that species (or particular kinds of animals and plants) change so much as to be recognized as different kinds, then there has been evolution. If, on the other hand, the evidence shows that species are fixed, there has been no evolution. It is well-known that by cultivation we can produce varieties of flowers, and by breeding we can produce different kinds of pigeons, fowl, and stock. Therefore, living beings may change through changes in their surroundings. But we must bring the history of past ages to bear upon the question. Fortunately there are preserved in the rocks the petrified remains of animals, showing their history for many thousands of years, and we may use them to test the question. It is plain that rocks of a lower level were deposited before those that cover them. Now, in Slavonia we have some fresh-water lakes that have been drying up from geological times. All through the ages these waters were inhabited by snails, and the first formed ones were parents of the later broods. As the animals died, their shells fell to the bottom and were covered by mud and débris, and held there like currants in a pudding. In the course of ages these layers thickened and were changed into rock, and thereby we have shells preserved in their proper order of birth and life, the most ancient ones at the bottom and the newest at the top. We can sink a shaft or dig a trench, and collect the shells and preserve them in proper order. Those nearest the top are descended from those near the bottom, but are very different. No one would hesitate to name them different species, in fact naturalists made six or seven different species when these shells were first examined. If, however, the whole collection were laid out in a long row in proper order, while those at the ends would be very different, if we begin at one end and pass to the other, the shells all grade into one another by such slight changes that there is no line showing where one kind leaves off and another begins. This shows their history for thousands of years: The species have not remained constant, but have changed into other species. We have other examples of similar conditions in Württemberg, which show that shells may change not only into different species but into different genera so gradually that one cannot tell where to separate one kind absolutely from another. But we have more perfect and more remarkable series than those mentioned, involving higher animals. For example, there is contained in the rocks of our western states the complete history of the horse-family—written as it were on tablets of stone—and extending over a period of more than 2,000,000 years. Geologists can, of course, measure the thickness of rocks and form some estimate of the time it took to have them deposited, for the rocks that lie in layers or strata were deposited layer by layer, usually by water. In the deposits near the surface we can find remains of the immediate ancestors of the horse of to-day, and there is but little change. In lower rocks we can come upon the forms from which these were derived, and so on, as long as the record holds out. If in this way we go into the past a half-million years, we find the ancestors of the horse reduced in size and with three toes on the fore and hind feet. The horse has now only a single toe on each foot, but small splint-like bones that represent the rudiments of two more. If we go back 1,000,000 years, we find three toes and the rudiment of a fourth, and, going back 2,000,000 years, we find four fully developed toes and bones in the feet to support them. It is believed that in still older rocks a five-toed form will be discovered, which was the parent of the four-toed form. To sum up this evidence: The parentage of the horse is surely from animals with four (and possibly five) toes, not like the horse of the present day. Therefore, horses have been evolved, not originally created as we see them to-day. We have treated our question as an historical question, and have gone searching for clues in the past as an archæologist explores the past of buried cities, by digging, and collecting coins, implements, records, etc. and putting the facts together into a history. In the museums at Yale University are preserved upward of 30 steps or stages in the history of the horse-family, showing that it arose by evolution or gradual changes from a four or five-toed ancestor about the size of a fox and that it passed through many changes besides increase in size in the 2,000,000 years in which we can get facts as to its history. These facts, taken in connection with a multitude of others pointing in the same direction, give us the answer to the question: Were the immense number of living forms created just as we find them, or were they created by a process of evolution?
The series of shells and horses mentioned above are called evolutionary series. They were not known at the time of the publication of Darwin's great book on The Origin of Species.
The most interesting evidences of evolution bearing on all animal life are found in the various stages through which animals pass on their way from the egg to the fully formed animal. Every animal, above the Protozoa, begins its life as a single cell and passes through every gradation from that condition upward. As animals develop in this way, they become successively more and more complex, and many rudimentary organs arise and disappear. For example, in the young chick, developing within the hen's egg, there appear, after four days, gill-slits or openings into the throat, like the gills of lower fishes. These organs belong entirely to water-life, and are not of direct use to the chick. The heart and blood-vessels at this stage are also of the fish-like type, but all this disappears in a few days, long before the chicken is hatched. Similar gill-slits appear also very early in the development of the young rabbit and of all higher animals. The best way of explaining their presence—which certainly means something—is to say that they are inherited from remote parents. Animals in the course of their development repeat certain stages of their past histories. The presence of gill-clefts in the embryos of birds and rabbits probably means that both these animals sprang from ancestors that had gill-clefts, or water breathing animals. Such traces are like hieroglyphics and inscriptions on temples and columns—they are clues to ancient history and they weigh heavily on the side of evolution. If space permitted, the evidences in favor of evolution might be very much multiplied.
Having illustrated what the doctrine of evolution is, the main part of our task is done, for the questions regarding the different theories of evolution and the rise of evolutionary thought must be dealt with very briefly, and the reader directed to the best books for further information. The doctrine of evolution is regarded as established beyond controversy. The controversies about evolution to-day are not as to whether it was or was not the method of creation, but as to how evolution of different forms was brought about. Therefore we must distinguish between the facts of evolution and the factors. Although several theories have been advanced to account for evolution, only three command any particular amount of attention at the present time: those of Lamarck, Darwin and Weismann. Lamarck's theory was founded in the early part of the 19th century. Being greatly impressed with the differences produced in animal-life by surroundings—climate, temperature, moisture, elevation above the sealevel, etc., he came to believe that all variation in animals had been produced by different surroundings, and the effects of use and disuse of organs. The use of the muscles of the blacksmith's arm, for example, develops it greatly; the disuse of the wings of the wingless birds has led to the disappearance of their wings. The differences thus produced are inherited. While the explanation of Lamarck has two factors, that offered by Darwin has three. He showed that animals vary in a state of nature, that these variations are inherited and, thirdly, that nature selects the fittest to survive, through the failure and death of the weaker ones in the struggle for existence. The variations that prove of advantage to the individual would be the ones preserved and handed along by heredity. This theory was arrived at, independently, by A. R. Wallace, and should be known as the Darwin-Wallace theory of natural selection. But this is not evolution; it is an attempt to explain how evolution was brought about. Weismann's theory is difficult to state briefly and clearly. It is called the theory of continuity of the germ-plasm. He accepts Darwin's natural selection, and shows that the germ-plasm, or substance from which animals arise, is continuous from generation to generation. Heredity is explained in a simple way, viz., the offspring is like the parent, because some of the same stuff enters into all its cells. He concludes, further, that the germ-plasm must be impressed by surroundings before changes in the animal can be inherited. There is at present a disposition to revive Lamarck's ideas in a modified form and to unite them with Darwin's. The theory of Weismann is losing ground.
Let us now look briefly at the rise of the doctrine of evolution. Osborn has shown that there is a continuous train of thought along this line from the Greeks to Darwin, but modern evolution began at the close of the 18th century and did not fairly take hold of the minds of people till after 1859. Linnæus (1707-78), by defining species and giving them names that were adopted all over the world, directed the attention of naturalists to species, and thus made possible a consideration of the origin of species. He considered them fixed and unchanging. His contemporary, Buffon, gradually grew to the idea that the higher forms of life were derived by modifications from the lower ones. These views were warmly supported by Erasmus Darwin, the grandfather of Charles, and by the poet-naturalist Goethe. Lamarck, however, was the first fully to grasp the idea of evolution in its entirety. His book dealing with the subject (Philosophie Zoologique) was published in 1809, but his views were not accepted, largely on account of the opposition of Cuvier. They were shared by his friend Geoffroy St. Hilaire, and, after his death, a noteworthy public debate took place at Paris in 1830 between St. Hilaire and Cuvier on the question of organic evolution. Cuvier won by weight of authority and greater brilliancy. An important step in preparing the way for Darwin was taken by Charles Lyell in his Principles of Geology (1830), in which he showed that, if we would know what occurred in the past, we must observe the agents that produce changes in the present and read the past in terms of the present. Herbert Spencer in 1852 published a remarkable article on evolution, that came near anticipating Darwin. Wallace and Darwin arrived at the same explanation independently, and in July, 1858, their views were published together. Next year Darwin published his Origin of Species. This had a compelling power, and at once attracted wide attention, partly because the time was ripe, but mainly because he had a better explanation to offer than any previous thinker. The story of the conflict of the general theory with prejudice and dogma, and its final acceptance by the scientific world, philosophers and theologians is part of the history of the past 50 years. Huxley in England and Haeckel in Germany were especially active in promoting the ideas of Darwin. What to read on the doctrine of evolution is an important question, but one should not begin with Darwin's Origin of Species, because that is voluntarily to place oneself 50 years in the past. It is better to begin with a recent statement of the theory and then go back to Darwin's books. The following books are among the best: Romanes, Darwin and after Darwin, Vol. I.; Wallace, Darwinism; Fiske, Destiny of Man; Osborn, From the Greeks to Darwin; Darwin, Origin of Species and Animals and Plants under Domestication; Weismann, The Germ-Plasm; Romanes, An Examination of Weismannism.