The science which we now call the science of evolution (in the broadest sense) is, both in its general outline and in its separate parts, a child of the nineteenth century; it is one of its most momentous and most brilliant achievements. Almost unknown in the preceding century, this theory has now become the sure foundation of our whole world-system. I have treated it exhaustively in my General Morphology (1866), more popularly in my Natural History of Creation (1868), and in its special application to man in my Anthropogeny (1874). Here I shall restrict myself to a brief survey of the chief advances which the science has made in the course of the century. It falls into four sections, according to the nature of its object; that is, it deals with the natural origin of (1) the cosmos, (2) the earth, (3) terrestrial forms of life, and (4) man.
I.—MONISTIC COSMOGONY
The first attempt to explain the constitution and the mechanical origin of the world in a simple manner by “Newtonian laws”—that is, by mathematical and physical laws—was made by Immanuel Kant in the famous work of his youth (1755), General History of the Earth and Theory of the Heavens. Unfortunately, this distinguished and daring work remained almost unknown for ninety years; it was only disinterred in 1845 by Alexander Humboldt in the first volume of his Cosmos. In the mean time the great French mathematician, Pierre Laplace, had arrived independently at similar views to those of Kant, and he gave them a mathematical foundation in his Exposition du Système du Monde (1796). His chief work, the Mécanique Céleste, appeared a hundred years ago. The analogous features of the cosmogony of Kant and Laplace consist, as is well known, in a mechanical explanation of the movements of the planets, and the conclusion which is drawn therefrom, that all the cosmic bodies were formed originally by a condensation of rotating nebulous spheres. This “nebular hypothesis” has been much improved and supplemented since, but it is still the best of all the attempts to explain the origin of the world on monistic and mechanical lines. It has recently been strongly confirmed and enlarged by the theory that this cosmogonic process did not simply take place once, but is periodically repeated. While new cosmic bodies arise and develop out of rotating masses of nebula in some parts of the universe, in other parts old, extinct, frigid suns come into collision, and are once more reduced by the heat generated to the condition of nebulæ.
Nearly all the older and the more recent cosmogonies, including most of those which were inspired by Kant and Laplace, started from the popular idea that the world had had a beginning. Hence, according to a widespread version of the nebular hypothesis, “in the beginning” was made a vast nebula of infinitely attenuated and light material, and at a certain moment (“countless ages ago”) a movement of rotation was imparted to this mass. Given this “first beginning” of the cosmogonic movement, it is easy, on mechanical principles, to deduce and mathematically establish the further phenomena of the formation of the cosmic bodies, the separation of the planets, and so forth. This first “origin of movement” is Du Bois-Reymond’s second “world-enigma”; he regards it as transcendental. Many other scientists and philosophers are equally helpless before this difficulty; they resign themselves to the notion that we have here a primary “supernatural impetus” to the scheme of things, a “miracle.”
In our opinion, this second “world-enigma” is solved by the recognition that movement is as innate and original a property of substance as is sensation. The proof of this monistic assumption is found, first, in the law of substance, and, secondly, in the discoveries which astronomy and physics have made in the latter half of the century. By the spectral analysis of Bunsen and Kirchhoff (1860) we have found, not only that the millions of bodies, which fill the infinity of space, are of the same material as our own sun and earth, but also that they are in various stages of evolution; we have obtained by its aid information as to the movements and distances of the stars, which the telescope would never have given us. Moreover, the telescope itself has been vastly improved, and has, in alliance with photography, made a host of scientific discoveries of which no one dreamed at the beginning of the century. In particular, a closer acquaintance with comets, meteorites, star-clusters, and nebulæ has helped us to realize the great significance of the smaller bodies which are found in millions in the space between the stars.
We now know that the paths of the millions of heavenly bodies are changeable, and to some extent irregular, whereas the planetary system was formerly thought to be constant, and the rotating spheres were described as pursuing their orbits in eternal regularity. Astro-physics owes much of its triumph to the immense progress of other branches of physics, of optics, and electricity, and especially of the theory of ether. And here, again, our supreme law of substance is found to be one of the most valuable achievements of modern science. We now know that it rules unconditionally in the most distant reaches of space, just as it does in our planetary system, in the most minute particle of the earth as well as in the smallest cell of our human frame. We are, moreover, justified in concluding, if we are not logically compelled to conclude, that the persistence of matter and force has held good throughout all time as it does to-day. Through all eternity the infinite universe has been, and is, subject to the law of substance.
From this great progress of astronomy and physics, which mutually elucidate and supplement each other, we draw a series of most important conclusions with regard to the constitution and evolution of the cosmos, and the persistence and transformation of substance. Let us put them briefly in the following theses:
I. The extent of the universe is infinite and unbounded; it is empty in no part, but everywhere filled with substance.
II. The duration of the world is equally infinite and unbounded; it has no beginning and no end: it is eternity.
III. Substance is everywhere and always in uninterrupted movement and transformation: nowhere is there perfect repose and rigidity; yet the infinite quantity of matter and of eternally changing force remains constant.