VIII. The possibility of our ever entering into direct communication with such inhabitants of other planets seems to be excluded by the immense distance of our earth from the other heavenly bodies, and the absence of the requisite atmosphere in the intervening space, which contains only ether.
But while many of the stars are probably in a similar stage of biogenetic development to that of our earth (for the last one hundred million years at least), others have advanced far beyond this stage, and, in their planetary old age, are hastening towards their end—the same end that inevitably awaits our own globe. The radiation of heat into space gradually lowers the temperature until all the water is turned into ice; that is the end of all organic life. The substance of the rotating mass contracts more and more; the rapidity of its motion gradually falls off. The orbits of the planets and of their moons grow narrower. At length the moons fall upon the planets, and the planets are drawn into the sun that gave them birth. The collision again produces an enormous quantity of heat. The pulverized mass of the colliding bodies is distributed freely through infinite space, and the eternal drama of sun-birth begins afresh.
The sublime picture which modern astrophysics thus unveils before the mind’s eye shows us an eternal birth and death of countless heavenly bodies, a periodic change from one to the other of the different cosmogenetic conditions, which we observe side by side in the universe. While the embryo of a new world is being formed from a nebula in one corner of the vast stage of the universe, another has already condensed into a rotating sphere of liquid fire in some far distant spot; a third has already cast off rings at its equator, which round themselves into planets; a fourth has become a vast sun whose planets have formed a secondary retinue of moons, and so on. And between them are floating about in space myriads of smaller bodies, meteorites, or shooting-stars, which cross and recross the paths of the planets apparently like lawless vagabonds, and of which a great number fall onto the planets every day. Thus there is a continuous but slow change in the velocities and the orbits of the revolving spheres. The frozen moons fall onto the planets, the planets onto their suns. Two distant suns, perhaps already stark and cold, rush together with inconceivable force and melt away into nebulous clouds. And such prodigious heat is generated by the collision that the nebula is once more raised to incandescence, and the old drama begins again. Yet in this “perpetual motion” the infinite substance of the universe, the sum total of its matter and energy, remains eternally unchanged, and we have an eternal repetition in infinite time of the periodic dance of the worlds, the metamorphosis of the cosmos that ever returns to its starting-point. Over all rules the law of substance.
II.—PROGRESS OF GEOLOGY
The earth and its origin were much later than the heavens in becoming the object of scientific investigation. The numerous ancient and modern cosmogonies do, indeed, profess to give us as good an insight into the origin of the earth as into that of the heavens; but the mythological raiment, in which all alike are clothed, betrays their origin in poetic fancy. Among the countless legends of creation which we find in the history of religions and of thought there is one that soon took precedence of all the rest—the Mosaic story of creation as told in the first book of the Hexateuch. It did not exist in its present form until long after the death of Moses (probably not until eight hundred years afterwards); but its sources are much older, and are to be found for the most part in Assyrian, Babylonian, and Hindoo legends. This Hebrew legend of creation obtained its great influence through its adoption into the Christian faith and its consecration as the “Word of God.” Greek philosophers had already, five hundred years before Christ, explained the natural origin of the earth in the same way as that of other cosmic bodies. Xenophanes of Colophon had even recognized the true character of the fossils which were afterwards to prove of such moment; the great painter, Leonardo da Vinci, of the fifteenth century, also explained the fossils as the petrified remains of animals which had lived in earlier periods of the earth’s history. But the authority of the Bible, especially the myth of the deluge, prevented any further progress in this direction, and insured the triumph of the Mosaic legend until about the middle of the last century. It survives even at the present day among orthodox theologians. However, in the second half of the eighteenth century, scientific inquiry into the structure of the crust of the earth set to work independently of the Mosaic story, and it soon led to certain conclusions as to the origin of the earth. The founder of geology, Werner of Freiberg, thought that all the rocks were formed in water, while Voigt and Hutton (1788) rightly contended that only the stratified, fossil-bearing rocks had had an aquatic origin, and that the Vulcanic or Plutonic mountain ranges had been formed by the cooling down of molten matter.
The heated conflict of these “Neptunian” and “Plutonic” schools was still going on during the first three decades of the present century; it was only settled when Karl Hoff (1822) established the principle of “actualism,” and Sir Charles Lyell applied it with signal success to the entire natural evolution of the earth. The Principles of Geology of Lyell (1830) secured the full recognition of the supremely important theory of continuity in the formation of the earth’s crust, as opposed to the catastrophic theory of Cuvier.[34] Palæontology, which had been founded by Cuvier’s work on fossil bones (1812), was of the greatest service to geology; by the middle of the present century it had advanced so far that the chief periods in the history of the earth and its inhabitants could be established. The comparatively thin crust of the earth was now recognized with certainty to be the hard surface formed by the cooling of an incandescent fluid planet, which still continues its slow, unbroken course of refrigeration and condensation. The crumpling of the stiffened crust, “the reaction of the molten fiery contents on the cool surface,” and especially the unceasing geological action of water, are the natural causes which are daily at work in the secular formation of the crust of the earth and its mountains.
To the brilliant progress of modern geology we owe three extremely important results of general import. In the first place, it has excluded from the story of the earth all questions of miracle, all questions of supernatural agencies, in the building of the mountains and the shaping of the continents. In the second place, our idea of the length of the vast period of time which had been absorbed in their formation has been considerably enlarged. We now know that the huge mountains of the palæozoic, mesozoic, and cenozoic formations have taken, not thousands, but millions of years in their growth. In the third place, we now know that all the countless fossils that are found in those formations are not “sports of nature,” as was believed one hundred and fifty years ago, but the petrified remains of organisms that lived in earlier periods of the earth’s history, and arose by gradual transformation from a long series of ancestors.
III.—PROGRESS OF PHYSICS AND CHEMISTRY
The many important discoveries which these fundamental sciences have made during the nineteenth century are so well known, and their practical application in every branch of modern life is so obvious, that we need not discuss them in detail here. In particular, the application of steam and electricity has given to our nineteenth century its characteristic “machinist-stamp.” But the colossal progress of inorganic and organic chemistry is not less important. All branches of modern civilization—medicine and technology, industry and agriculture, mining and forestry, land and water transport—have been so much improved in the course of the century, especially in the second half, that our ancestors of the eighteenth century would find themselves in a new world, could they return. But more valuable and important still is the great theoretical expansion of our knowledge of nature, which we owe to the establishment of the law of substance. Once Lavoisier (1789) had established the law of the persistence of matter, and Dalton (1808) had founded his new atomic theory with its assistance, a way was open to modern chemistry along which it has advanced with a rapidity and success beyond all anticipation. The same must be said of physics in respect of the law of the conservation of energy. Its discovery by Robert Mayer (1842) and Hermann Helmholtz (1847) inaugurated for this science also a new epoch of the most fruitful development; for it put physics in a position to grasp the universal unity of the forces of nature and the eternal play of natural processes, in which one force may be converted into another at any moment.