If we glance for a moment at the various fields in which the idea of evolution is scientifically applied we find that, firstly, the whole universe is conceived as a unity; secondly, our earth; thirdly, organic life on the earth; fourthly, man, as its highest product; and fifthly, the soul, as a special immaterial entity. Thus we have, in historical succession, the evolutionary research of cosmology, geology, biology, anthropology, and psychology.
The first comprehensive idea of cosmological evolution was put forth by the famous critical philosopher Immanuel Kant, in 1755, in the great work of his earlier years, General Natural History of the Heavens, or an Attempt to Conceive and to Explain the Origin of the Universe mechanically, according to the Newtonian Laws. This remarkable work appeared anonymously, and was dedicated to Frederick the Great, who, however, never saw it. It was little noticed, and was soon entirely forgotten, until it was exhumed ninety years afterwards by Alexander von Humboldt. Note particularly that on the title-page stress is laid on the mechanical origin of the world and its explanation on Newtonian principles; in this way the strictly Monistic character of the whole cosmogony and the absolutely universal rule of natural law are clearly expressed. It is true that Kant speaks much in it of God and his wisdom and omnipotence; but this is limited to the affirmation that God created once for all the unchangeable laws of nature, and was henceforward bound by them and only able to work through them. The Dualism which became so pronounced subsequently in the philosopher of Koenigsberg counts for very little here.
The idea of a natural development of the world occurs in a clearer and more consistent form, and is provided with a firm mathematical basis, forty years afterwards, in the remarkable Mécanique Céleste of Pierre Laplace. His popular Exposition du Système du Monde (1796) destroyed at its roots the legend of creation that had hitherto prevailed, or the Mosaic narrative in the Bible. Laplace, who had become Minister of the Interior, Count, and Chancellor of the Senate, under Napoleon, was merely honourable and consistent when he replied to the emperor's question, "What room there was for God in his system?": "Sire, I had no need for that unfounded hypothesis." What strange ministers there are sometimes![2] The shrewdness of the Church soon recognised that the personal Creator was dethroned, and the creation-myth destroyed, by this Monistic and now generally received theory of cosmic development. Nevertheless it maintained towards it the attitude which it had taken up 250 years earlier in regard to the closely related and irrefutable system of Copernicus. It endeavoured to conceal the truth as long as possible, or to oppose it with Jesuitical methods, and finally it yielded. If the Churches now silently admit the Copernican system and the cosmogony of Laplace and have ceased to oppose them, we must attribute the fact, partly to a feeling of their spiritual impotence, partly to an astute calculation that the ignorant masses do not reflect on these great problems.
In order to obtain a clear idea and a firm conviction of this cosmic evolution by natural law, the eternal birth and death of millions of suns and stars, one needs some mathematical training and a lively imagination, as well as a certain competence in astronomy and physics. The evolutionary process is much simpler, and more readily grasped in geology. Every shower of rain or wave of the sea, every volcanic eruption and every pebble, gives us a direct proof of the changes that are constantly taking place on the surface of our planet. However, the historical significance of these changes was not properly appreciated until 1822, by Karl von Hoff of Gotha, and modern geology was only founded in 1830 by Charles Lyell, who explained the whole origin and composition of the solid crust of the earth, the formation of the mountains, and the periods of the earth's development, in a connected system by natural laws. From the immense thickness of the stratified rocks, which contain the fossilised remains of extinct organisms, we discovered the enormous length—running into millions of years—of the periods during which these sedimentary rocks were deposited in water. Even the duration of the organic history of the earth—that is to say, the period during which the plant and animal population of our planet was developing—must itself be put at more than a hundred million years. These results of geology and paleontology destroyed the current legend of the six days' work of a personal Creator. Many attempts were made, it is true, and are still being made, to reconcile the Mosaic supernatural story of creation with modern geology.[3] All these efforts of believers are in vain. We may say, in fact, that it is precisely the study of geology, the reflection it entails on the enormous periods of evolution, and the habit of seeking the simple mechanical causes of their constant changes, that contribute very considerably to the advance of enlightenment. Yet in spite of this (or, possibly, because of this), geological instruction is either greatly neglected or entirely suppressed in most schools. It is certainly eminently calculated (in connection with geography) to enlarge the mind, and acquaint the child with the idea of evolution. An educated person who knows the elements of geology will never experience ennui. He will find everywhere in surrounding nature, in the rocks and in the water, in the desert and on the mountains, the most instructive stimuli to reflection.
The evolutionary process in organic nature is much more difficult to grasp. Here we must distinguish two different series of biological development, which have only been brought into proper causal connection by means of our biogenetic law (1866); one series is found in embryology (or ontogeny), the other in phylogeny (or race-development). In Germany "evolution" always meant embryology, or a part of the whole, until forty years ago. It stood for a microscopic examination of the wonderful processes by means of which the elaborate structure of the plant or animal body is formed from the simple seed of the plant or the egg of the bird. Until the beginning of the nineteenth century the erroneous view was generally received that this marvellously complicated structure existed, completely formed, in the simple ovum, and that the various organs had merely to grow and to shape themselves independently by a process of "evolution" (or unfolding), before they entered into activity. An able German scientist, Caspar Friedrich Wolff (son of a Berlin tailor), had already shown the error of this "pre-formation theory" in 1759. He had proved, in his dissertation for the doctorate, that no trace of the later body, of its bones, muscles, nerves, and feathers, can be found in the hen's egg (the commonest and most convenient object for study), but merely a small round disk, consisting of two thin superimposed layers. He had further showed that the various organs are only built up gradually out of these simple elements, and that we can trace, step by step, a series of real new growths. However, these momentous discoveries, and the sound "theory of epigenesis" that he based on them, were wholly ignored for fifty years, and even rejected by the leading authorities. It was not until Oken had re-discovered these important facts at Jena (1806), Pander had more carefully distinguished the germinal layers (1817), and finally Carl Ernst von Baer had happily combined observation and reflection in his classical Animal Embryology (1828), that embryology attained the rank of an independent science with a sound empirical base.
A little later it secured a well-merited recognition in botany also, especially owing to the efforts of Matthias Schleiden of Jena, the distinguished student who provided biology with a new foundation in the "cell theory" (1838). But it was not until the middle of the nineteenth century that people generally recognised that the ovum of the plant or animal is itself only a simple cell, and that the later tissues and organs gradually develop from this "elementary organism" by a repeated cleavage of, and division of labour in, the cells. The most important step was then made of recognising that our human organism also develops from an ovum (first discovered by Baer in 1827), in virtue of the same laws, and that its embryonic development resembles that of the other mammals, especially that of the ape. Each of us was, at the beginning of his existence, a simple globule of protoplasm, surrounded by a membrane, about 1 120 of an inch in diameter, with a firmer nucleus inside it. These important embryological discoveries confirmed the rational conception of the human organism that had been attained much earlier by comparative anatomy: the conviction that the human frame is built in the same way, and develops similarly from a simple ovum, as the body of all other mammals. Even Linné had already (1735) given man a place in the mammal class in his famous System of Nature.
Differently from these embryological facts, which can be directly observed, the phenomena of phylogeny (the development of species), which are needed to set the former in their true light, are usually outside the range of immediate observation. What was the origin of the countless species of animals and plants? How can we explain the remarkable relationships which unite similar species into genera and these into classes? Linné answers the question very simply with the belief in creation, relying on the generally accepted Mosaic narrative: "There are as many different species of animals and plants as there were different forms created by God in the beginning." The first scientific answer was given in 1809 by the great French scientist, Lamarck. He taught, in his suggestive Philosophie Zoologique, that the resemblances in form and structure of groups of species are due to real affinity, and that all organisms descend from a few very simple primitive forms (or, possibly, from a single one). These primitive forms were developed out of lifeless matter by spontaneous generation. The resemblances of related groups of species are explained by inheritance from common stem-forms; their dissimilarities are due to adaptation to different environments, and to variety in the action of the modifiable organs. The human race has arisen in the same way, by transformation of a series of mammal ancestors, the nearest of which are ape-like primates.
These great ideas of Lamarck, which threw light on the whole field of organic life, and were closely approached by Goethe in his own speculations, gave rise to the theory that we now know as transformism, or the theory of evolution or descent. But the far-seeing Lamarck was—as Caspar Friedrich Wolff had been fifty years before—half a century before his time. His theory obtained no recognition, and was soon wholly forgotten.
It was brought into the light once more in 1859 by the genius of Charles Darwin, who had been born in the very year that the Philosophie Zoologique was published. The substance and the success of his system, which has gone by the name of Darwinism (in the wider sense) for forty-six years, are so generally known that I need not dwell on them. I will only point out that the great success of Darwin's epoch-making works is due to two causes: firstly, to the fact that the English scientist most ingeniously worked up the empirical material that had accumulated during fifty years into a systematic proof of the theory of descent; and secondly, to the fact that he gave it the support of a second theory of his own, the theory of natural selection. This theory, which gives a causal explanation of the transformation of species, is what we ought to call "Darwinism" in the strict sense. We cannot go here into the question how far this theory is justified, or how far it is corrected by more recent theories, such as Weismann's theory of germ-plasm (1844), or De Vries's theory of mutations (1900). Our concern is rather with the unparalleled influence that Darwinism, and its application to man, have had during the last forty years on the whole province of science; and at the same time, with its irreconcilable opposition to the dogmas of the Churches.