Evolution of Life/Introduction
INTRODUCTION.
By the Evolution of Life we mean the slow and gradual development of life as distinguished from its special and sudden creation; that plants and animals are the modified descendants of pre-existing organisms, not the unchanged posterity of similar forms of life originally specially created. Let us illustrate our meaning by considering the origin of a common animal like the Horse. According to the creation hypothesis, all horses are the descendants of a pair of horses, originally, specially created. Supposing the Evolution theory, however, to be true, the Horse is the modified descendant of an extinct species of Horse, the Hipparion. Preceding the Hipparion there lived the Anchitherium, whose organization bears the same relation to the Hipparion that the Hipparion's does to that of the Horse; while in a still earlier period we find in the Paleotherium the ancestor of the Anchitherium. But the Rhinoceros and the Tapir are also nearly related to the Paleotherium. We see, therefore, why all naturalists are agreed in regarding the Horse, Rhinoceros, and Tapir as the representatives of one group. For, if these animals are the posterity of a common ancestor, it is natural that their organization should have much in common. Through extinct forms, like the Xiphodon and Anthracotherium, the Ruminating animals, the Pig, and the Hippopotamus, are linked with the Anoplotherium; while glancing at Tree VII. we see that the Paleotherium and Anoplotherium are regarded as the descendants of a common stock, represented by extinct forms, like Coryphodon and Lophiodon. Basing the investigation on the facts of Anatomy, Embryology, and Geology, the genealogy of the animal and vegetal kingdoms has, in this manner, been more or less made out, the indefinitely remote ancestors of all plant and animal life being represented by the Monera, structureless, infinitely small, jelly-like beings, belonging neither to the animal nor to the vegetal kingdom. This view of the gradual development of existing forms of life from pre-existing ones is in harmony with the conclusions of other sciences. Most ethnologists are agreed that the different races of men have descended from a common stock, notwithstanding the great differences exhibited in color, shape of the head, and character of the hair. Philologists derive the various languages from one of three or four roots. The history of Art offers us interesting illustrations of the doctrine of Evolution. Thus, the present perfection of music has been attained only through very gradual additions from time to time. Modern orchestration is so complicated that one would hardly believe that it could have been developed out of the simple jingle of barbarians. Astronomers think it highly probable that our solar system was once a chaotic mass, and that from this the planets were thrown off, the central body becoming later the Sun. This theory, which is commonly known as the Nebular Hypothesis of La Place, naturally suggests the name of Kant, the famous philosopher of Königsberg, who first distinctly enunciated the view of the gradual development of the solar system, and the doctrine of Evolution in general. But as the differentiation of the simple into the complex, of the homogeneous into the heterogeneous, of which the development of race and language is an example, has been fully discussed by Mr. Herbert Spencer in his different works, we therefore pass on to the consideration of the objection, that while the origin of races, of languages, of the stars, etc. is a legitimate object of study, the origin of plants and animals is an inquiry of an entirely different nature,—a subject about which man can learn nothing. Those who are continually referring to the mysteriousness of Life as an objection to its study, seem to forget that the ultimate causes of all other phenomena, such as the falling of an apple, the combining of elements, the crystallization of a salt, etc., are equally mysterious. The forces by which these phenomena are brought about are studied in their effects; and the laws according to which these effects are produced belong to Astronomy, Chemistry, Crystallography. But of the cause or essence of gravitation we know nothing. We can only say that bodies attract one another according to a certain law. Equally unknown is the ultimate cause of crystallization. We can only say that in saline solutions, under favorable conditions, according to law, geometrical forms are produced. Compare now the growth of a crystal with that of a plant or an animal. If a seed be sown and it attract certain elements from the soil, a definite form, a plant, is produced; in the same way the chick results from the embryo attracting the material of its future body from the yelk. The laws governing the forces by which these effects are brought about, the phenomenaof the growth of a plant or of an animal, are as legitimate objects of study as the growth of a crystal. The nature of the inquiry is the same: the study of the growth of a crystal differing from that of a plant or an animal not in kind but only in degree; the investigation being in each case the redistribution of matter, for there is nothing in the crystal that did not pre-exist in the saline solution, nothing in the plant that was not derived from the seed or the soil, nothing in the chicken that did not pre-exist in the egg or in the air. The ultimate cause of the so-called Vital Force is as unknowable as the cause of all other kinds of Force. The laws, however, by which the effects of the so-called Vital Force are brought about, will be worked out exactly as the laws of other Forces have been. And as the difficulties experienced in the study of the so-called vital phenomena are due to their complexity as compared with the simplicity of the so-called physical ones, it is quite natural that the organic sciences should be less advanced than the inorganic. These terms, however, Organic and Inorganic, Vital and Physical, Animate and Inanimate, Living and Dead, are very unphilosophical, since their use implies an entirely false view of Nature, The classification of objects into Animal, Vegetal, and Mineral, is a good arrangement for study; but it is a purely artificial one, no such distinction existing in Nature. The usual tests for distinguishing animals from plants, plants from minerals, have been rendered perfectly worthless by the discoveries of late years. Living beings like the Monera, representatives of a kingdom intermediate between the animal and the vegetal, are so structureless, so absolutely homogeneous, that crystals are complex bodies as compared with them. Products like sugar and alcohol, supposed at one time to be purely organic in their origin, to be produced only by the so-called Vital Force, are now made in laboratories by the combination of their inorganic elements. There are no substances in the Organic world whose elements are not resolvable into those of the Inorganic, no Vital Forces which are not convertible into Physical ones. In a word, no one can show where the Inorganic world ends and the Organic begins, the transition being so gradual. The objection is therefore groundless that the study of life is entirely different from that of all other phenomena, and that one can learn little about it. On the contrary, we have every reason to expect that in time we shall have a Science of Life, or Biology, a Science which will bear the same relation to Zoology and Botany that History does to mere Chronicle; that will tell us not only what plants and animals live and have lived, but why some died out and others survived; whether these survivors were modified, and, if so, by what means; why certain plants and animals are found in places not best suited to their structure; why similar forms of life are not always found under similar physical conditions, and why dissimilar plants and animals are often found under similar physical conditions; why animals have organs of no use to them; why some animals are protected, why others are not, etc. etc. We consider that these questions are answered by the theory of the Evolution of Life, and that this theory may be regarded as the fundamental truth of Biology. We propose in this essay to bring together, in as popular a manner as possible, some of the evidence in favor of this theory, endeavoring to show that the different plants and animals are linked together by transitional forms; that there has been a progress from the simple, lowly organized, to the highly complex forms of life; that the transitional stages through which a plant or an animal passes in the development from its primitive to its adult condition are permanently retained in the lower forms of life; that the development of the higher forms of life from the lower has been brought about by Natural Selection; and that Man has descended from a lower extinct form, of which the Gorilla and the Chimpanzee are the nearest living representatives.
Before discussing these different subjects, as there seems to be still some misunderstanding in reference to the views of the predecessors of Mr. Darwin, the most distinguished of the advocates of the Evolution of Life, it may be perhaps not superfluous to glance at the literature of the subject. Here and there among the writings of the ancients, one meets with passages, and even works, like those of Lucretius, from which it is evident that at different times some doctrine like that of the Evolution of Life was held by the thinkers of antiquity. Passing by the speculations of De Maillet, the first attempt in modern times to bring together the evidence in favor of the Evolution of Life, with the causes sufficient to produce it, is to be found in the writings of Lamarck—the Philosophical Zoology (1809), and the History of Animals without Vertebrae (1815). Lamarck was Professor of Zoology at the Garden of Plants, in Paris, and, far from being a mere dreamer, was an eminent naturalist, as every one admits, whatever may be thought of his speculations. Many of these speculations, however, are regarded by distinguished living Biologists as profound truths, such as, that "there is no distinct vital principle," that "life is only a physical phenomenon," that "the nervous system produces ideas, and all the acts of the intelligence," etc.
In the works just alluded to, Lamarck, basing his views on the structure of plants and animals, and their petrified remains, develops the theory of there having been a progress in the organic world from the simpler forms of life to the higher; that all organisms in the lapse of ages had descended from pre-existing ones. As causes of the transmutation of species, Lamarck held that the force of the will, as exhibited in the use and disuse of organs, exercised great influence in modifying the structure of animals; he attached also great importance to the facts of inheritance. While it is admitted that there is a great deal in the writings of Lamarck that cannot be maintained, still, he must be considered as the first who attempted to develop in detail the theory of the Evolution of Life, and one of its most distinguished advocates. Noticing that Lamarck held that the Monkey descent of Man, previously advocated by Monboddo, was a necessary consequence of his theory, we pass on to Geoffrey St.-Hilaire, the distinguished and constant opponent of Cuvier in the discussions on the Origin of Species at the Garden of Plants. Although for a long time St.-Hilaire had thought as Lamarck, it was not till 1828 that in his essay "On the Principle of the Unity of Organic Composition" he openly defended the doctrine of the transmutation of species. While in France Lamarck and St-Hilaire were studying the transmutation of species, Goethe and Oken were investigating the same subject in Germany. Goethe is famous as a poet, but is not so well known as a man of science. He, however, made the capital discovery of the intermaxillary bone in Man, which gives him rank as an anatomist, while his theory of the "Metamorphosis of Plants" has always been regarded as a most important contribution to philosophical Botany. In this work on Plants, Goethe develops the view of the different parts of the flower being modified leaves. This theory, which IS a beautiful illustration of Evolution, had been previously promulgated by Wolff, but had fallen into perfect oblivion. Goethe was always pointing out the "unity of Nature," and advocating the doctrine of Development, and must be considered one of the most distinguished of the German Biologists. Oken was one of the most remarkable men Germany has ever seen, not only for the extent of his knowledge, but for the originality of his views. Although his ideas are very often mystified by obscure language, nevertheless it is certain that he had a clear perception of some of the most important modern truths, such as the mechanical theory of Pleat, the doctrine of Cells, etc. His Idea of a "primordial mucosity" as the basis of life is very much like that of the "protoplasmic" doctrine of the present day. However this may be, there is no doubt that Oken was a firm believer in the Development theory of Lamarck.
Notwithstanding that the theory of the Transmutation of Species was defended by men of such ability as Lamarck, St.-Hilaire, Goethe, and Oken, it fell entirely into disrepute after 1830, the year of the famous discussion between Cuvier and St.-Hilaire at the Academy of Sciences. From that time the question of the origin of species was considered transcendental, not a subject for inquiry. It seems proper now to mention the influence of the "Principles of Geology," by Sir Charles Lyell, published in 1832. The doctrine of Catastrophes, or the supposition that at different times all life had been destroyed by the convulsions through which the earth had passed, and that a new life had been created from time to time, was supported by the high authority of Cuvier. Lyell, in the work just mentioned, put forward the view that these catastrophes had been only local, and that they had been brought about by the same forces that are now modifying the earth; that life has always existed,—new forms appearing, old forms passing away; that disturbances have taken place at different times in different places, just as at present we have earthquakes, volcanic eruptions, etc. This view is at present accepted by most Geologists, very few believing any longer in the Cuvierian theory of the "Revolutions of the Globe." The effect of the "Principles of Geology" on the progress of the Development theory was very great, since one of the objections to it, of life having been often extinct all over the globe, was therein shown to be groundless. Although the doctrine of Development was opposed by naturalists, nevertheless from time to time it was advocated, as in 1837 by Dean Herbert, in 1844 by the anonymous author of the Vestiges of Creation, in 1846 by D'Omalius d'Halloy, in 1852 by Naudin, in 1855 by the Rev. Baden Powell and by Büchner, and from 1852 to 1858 by Mr. Herbert Spencer, Remembering the vast discoveries that had been made since the days of Lamarck in Zoology, Botany, Geology, that the study of Embryology had been raised to a science, that of the Geographical Distribution of plants and animals more was known, etc., let us now call attention to the famous book on the Origin of Species, by Mr. Darwin. The two great merits of this work are its bringing together in a condensedform the evidences in favor of the Evolution of Life, and its offering Natural Selection as a cause of this Evolution. We will not dwell now on Natural Selection, as we endeavor to explain it in a chapter devoted to that subject. It seems proper, however, to mention that the discovery of Natural Selection was made independently by Mr. Wallace, who, having spent seven years in the Malay Archipelago, sent a paper to London containing his views on the Origin of Species. Following the advice of mutual friends, Mr. Darwin brought forward an abstract of his views, and the two papers appeared simultaneously in the publications of the Linnaean Society. Since the publication of the Origin of Species, many works have appeared in which this subject is discussed more or less in detail, among which may be mentioned the later ones of Messrs. Darwin and Wallace; the General Morphology and Natural History of Creation, by Prof. Haeckel; the Principles of Biology, by Mr. Herbert Spencer; the various works of Dr. Büchner; the Origin of Species, etc., by Prof. Huxley; the Introduction to the Flora of Tasmania, by Sir William Hooker; the Comparative Anatomy of Prof. Gegenbauer; the Crustacea of Fritz Müller; the different papers by Prof. Cope, etc. etc.
Hoping now to have made clear the general object of our essay, to have shown how gradual has been the development of the theory of the Evolution of Life, and having merely noticed some of the important literature on the subject, we pass on to the consideration of the Evidences, the Causes, and the Consequences of this Evolution.