In the course of the year 1774 Dr. Priestley found that by heating red precipitate, or what we now call red oxide of mercury, a gas was obtained, which he called “dephlogisticated air,” or, in other words, air deprived of phlogiston, and therefore incombustible. This incombustible air was oxygen, and such was man’s first introduction to the mighty element that makes one fifth of the atmosphere in volume and eight ninths of the ocean by weight, besides forming one half of the earth’s solid crust, and supporting all fire and all life. I know of nothing which can reveal to us with such startling vividness the extent of the gulf which the human mind has traversed within little more than a hundred years. It is scarcely possible to put ourselves back into the frame of mind in which oxygen was unknown, and no man could tell what takes place when a log of wood is burned on the hearth. The language employed by Dr. Priestley carries us back to the time when chemistry was beginning to emerge from alchemy. It was Newton’s contemporary, Stahl, who invented the doctrine of phlogiston in order to account for combustion. Stahl supposed that all combustible substances contain a common element, or fire principle, which he called phlogiston, and which escapes in the process of combustion. Indeed, the act of combustion was supposed to consist in the escape of phlogiston. Whither this mysterious fire principle betook itself, after severing its connection with visible matter, was not too clearly indicated, but of course it was to that limbo far larger than purgatory, the oubliette wherein have perished men’s unsuccessful guesses at truth. Stahl’s theory, however, marked a great advance upon what had gone before, inasmuch as it stated the case in such a way as to admit of direct refutation. Little use was made of the balance in those days, but when it was observed that zinc and lead and sundry other substances grow heavier in burning, it seemed hardly correct to suppose that anything had escaped from these substances. To this objection the friends of the fire principle replied that phlogiston might weigh less than nothing, or, in other words, might be endowed with a positive attribute of levity, so that to subtract it from a body would increase the weight of the body. This was a truly shifty method of reasoning, in which your phlogiston, with its plus sign to-day and its minus sign to-morrow, exhibited a skill in facing both ways like that of an American candidate for public office.
Into the structure of false science that had been reared upon these misconceptions Dr. Priestley’s discovery of oxygen came like a bombshell. As in so many other like cases, the discovery was destined to come at about that time; it was made again three years afterward by the Swedish chemist Scheele, without knowing what Priestley had done. The study of oxygen soon pointed to the conclusion that, whatever may escape during combustion, oxygen is always united with the burning substance. Then came Lavoisier with his balance, and proved that whenever a thing burns it combines with Priestley’s oxygen, and the weight of the resulting product is equal to the weight of the substance burned plus the weight of oxygen abstracted from the air. Thus combustion is simply union with oxygen, and nothing escapes. No room was left for phlogiston. Men’s thoughts were dephlogisticated from that time forth. The balance became the ruling instrument of chemistry. One further step led to the generalization that in all chemical changes there is no such thing as increase or diminution, but only substitution, and upon this fundamental truth of the indestructibility of matter all modern chemistry rests.
When we look at the stupendous edifice of science that has been reared upon this basis, when we consider the almost limitless sweep of inorganic and organic chemistry, the myriad applications to the arts, the depth to which we have been enabled to penetrate into the innermost proclivities of matter, it seems almost incredible that a single century can have witnessed so much achievement. We must admit the fact, but our minds cannot take it in; we are staggered by it. One thing stands out prominently, as we contrast this rapid and coherent progress with the barrenness of ancient alchemy and the chaotic fumbling of the Stahl period: we see the importance of untrammeled inquiry, and of sound methods of investigation which admit of verification at every step. That humble instrument the balance, working in the service of sovereign law, has been a beneficent Jinni unlocking the portals of many a chamber wherein may be heard the secret harmonies of the world.
It is not only in chemistry, however, that the marvelous advance of science has been exhibited. In all directions the quantity of achievement has been so marked that it is worth our while to take a brief general survey of the whole, to see if haply we may seize upon the fundamental characteristics of this great progress. In the first place, a glance at astronomy will show us how much our knowledge of the world has enlarged in space since the day when Priestley set free his dephlogisticated air.
The known solar system then consisted of sun, moon, earth, and the five planets visible to the naked eye. Since the days of the Chaldæan shepherds there had been no additions except the moons of Jupiter and Saturn. Herschel’s telescope was to win its first triumph in the detection of Uranus in 1781. The Newtonian theory, promulgated in 1687, had come to be generally accepted, but there were difficulties remaining, connected with the planetary perturbations and the inequalities in the moon’s motion, which the glorious labors of Lagrange and Laplace were presently to explain and remove, — labors which bore their full fruition two generations later, in 1845, when the discovery of the planet Neptune, by purely mathematical reasoning from the observed effects of its gravitation, furnished for the Newtonian theory the grandest confirmation known in the whole history of science. In Priestley’s time, sidereal astronomy was little more than the cataloguing of such stars and nebulæ as could be seen with the telescopes then at command. Sixty years after the discovery of oxygen the distance of no star had been measured. In 1836, Auguste Comte assured his readers that such a feat was impossible, that the Newtonian theory could never be proved to extend through the interstellar spaces, and that the matter of which stars are composed may be entirely different in its properties from the matter with which we are familiar. Within three years the first part of this prophecy was disproved when Bessel measured the distance of the star 61 Cygni; since then the study of the movements of double and multiple stars have shown them conforming to Newton’s law; and as for the matter of which they are composed, we are introduced to a chapter in science which even the boldest speculator of half a century ago would have derided as a baseless dream. The discovery of spectrum analysis and the invention of the spectroscope, completed in 1861 by Kirchhoff and Bunsen, have supplied data for the creation of a stellar chemistry; showing us, for example, hydrogen in Sirius and the nebula of Orion, sodium and potassium, calcium and iron, in the sun; demonstrating the gaseous character of nebulæ; and revealing chemical elements hitherto unknown, such as helium, a mineral first detected in the suns atmosphere, and afterward found in Norway. A still more wonderful result of spectrum analysis is our ability to measure the motion of a star through a slight shifting in the wave-lengths of the light which it emits. In this way we can measure, in the absence of all parallax, the direct approach or recession of a star; and in somewhat similar wise has been discovered the cause of the long-observed variations of brilliancy in Algol. That star, which is about the size of our sun, has a dark companion not much smaller, and the twain are moving around a third body, also dark: the result is an irregular series of eclipses of Algol, and the gravitative forces exerted by the two invisible stars are estimated through their effects upon the spectrum of the bright star. In no department of science has a region of inference been reached more remote than this. From such a flight one may come back gently to more familiar regions while remarking upon the manifold results that have begun to be attained from the application of a sensitive photograph plate to the telescope in place of the human eye. It may suffice to observe that we thus catch the fleeting aspects of sun-spots and preserve them for study; we detect the feeble self-luminosity still left in such a slowly cooling planet as Jupiter; and since the metallic plate does not quickly weary, like the human retina, the cumulative effects of its long exposure reveal the existence of countless stars and nebulæ too remote to be otherwise reached by any visual process. By such photographic methods George Darwin has caught an equatorial ring in the act of detachment from its parent nebula, and the successive phases of the slow process may be watched and recorded by generations of mortals yet to come.
To appreciate the philosophic bearings of this vast enlargement of the mental horizon, let us recall just what happened when Newton first took the leap from earth into the celestial spaces by establishing a law of physics to which moon and apple alike conform. It was the first step, and a very long one, toward proving that the terrestrial and celestial worlds are dynamically akin, that the same kind of order prevails through both alike, that both are parts of one cosmic whole. So late as Kepler’s time, it was possible to argue that the planets are propelled in their elliptic orbits by forces quite unlike any that are disclosed by purely terrestrial experience, and therefore perhaps inaccessible to any rational interpretation. Such imaginary lines of demarcation between earth and heavens were forever swept away by Newton, and the recent work of spectrum analysis simply completes the demonstration that the remotest bodies which the photographic telescope can disclose are truly part and parcel of the dynamic world in which we live.
All this enlargement of the mental horizon, from Newton to Kirchhoff, had reference to space. The nineteenth century has witnessed an equally notable enlargement with reference to time. The beginnings of scientific geology were much later than those of astronomy. The phenomena were less striking and far more complicated; it took longer, therefore, to bring men’s minds to bear upon them. Antagonism on the part of theologians was also slower in dying out. The complaint against Newton, that he substituted Blind Gravitation for an Intelligent Deity, was nothing compared to the abuse that was afterwards lavished upon geologists for disturbing the accepted Biblical chronology. At the time when Priestley discovered oxygen, educated men were still to be found who could maintain with a sober face that fossils had been created already dead and petrified, just for the fun of the thing. The writings of Buffon were preparing men’s minds for the belief that the earth’s crust has witnessed many and important changes, but there could be no scientific geology until further progress was made in physics and chemistry. It was only in 1763 that Joseph Black discovered latent heat, and thus gave us a clue to what happens when water freezes and melts, or when it is turned into steam. It was in 1786 that the publication of James Hutton’s Theory of the Earth ushered in the great battle between Neptunians and Plutonists which prepared the way for scientific geology. When the new science won its first great triumph with Lyell in 1830, the philosophic purport of the event was the same that was being proclaimed by the progress of astronomy. Newton proved that the forces which keep the planets in their orbits are not strange or supernatural forces, but just such as we see in operation upon this earth every moment of our lives. Geologists before Lyell had been led to the conclusion that the general aspect of the earth’s surface with which we are familiar is by no means its primitive or its permanent aspect, but that there has been a succession of ages, in which the relations of land and water, of mountain and plain, have varied to a very considerable extent; in which soils and climates have undergone most complicated vicissitudes; and in which the earth’s vegetable products and its animal populations have again and again assumed new forms, while the old forms have passed away. In order to account for such wholesale changes, geologists were at first disposed to imagine violent catastrophes brought about by strange agencies, — agencies which were perhaps not exactly supernatural, but were in some vague, unspecified way different from those which are now at work in the visible and familiar order of nature. But Lyell proved that the very same kind of physical processes which are now going on about us would suffice, during a long period of time, to produce the changes in the inorganic world which distinguish one geological period from another. Here, in Lyell’s geological investigations, there was for the first time due attention paid to the immense importance of the prolonged and cumulative action of slight and unobtrusive causes. The continual dropping that wears away stones might have served as a text for the whole series of beautiful researches of which he first summed up the results in 1830. As astronomy was steadily advancing toward the proof that in the abysses of space the physical forces at work are the same as our terrestrial forces, so geology, in carrying us back to enormously remote periods of time, began to teach that the forces at work have all along been the same forces that are operative now. Of course, in that early stage when the earth’s crust was in process of formation, when the temperature was excessively high, there were phenomena here such as can no longer be witnessed, but for which we must look to big planets like Jupiter; in that intensely hot atmosphere violent disturbances occur, and chemical elements are dissociated which we are accustomed to find in close combination here. But ever since our earth cooled to a point at which its solid crust acquired stability, since the earliest mollusks and vertebrates began to swim in the seas and worms to crawl in the damp ground, if at almost any time we could have come here on a visit, we should doubtless have found things going on at measured pace very much as at present, — here and there earthquake and avalanche, fire and flood, but generally rain falling, sunshine quickening, herbage sprouting, creatures of some sort browsing, all as quiet and peaceful as a daisied field in June, without the slightest visible presage of the continuous series of minute secular changes that were gradually to transform a Carboniferous world into what was by and by to be a Jurassic world, and that again into what was after a while to be an Eocene world, and so on, until the aspect of the world that we know to-day should noiselessly steal upon us.
When once the truth of Lyell’s conclusions began to be distinctly realized, their influence upon men’s habits of thought and upon the drift of philosophic speculation was profound. The conception of Evolution was irresistibly forced upon men’s attention. It was proved beyond question that the world was not created in the form in which we find it to-day, but has gone through many phases, of which the later are very different from the earlier; and it was shown that, so far as the inorganic world is concerned, the changes can be much more satisfactorily explained by a reference to the ceaseless, all-pervading activity of gentle, unobtrusive causes such as we know than by an appeal to imaginary catastrophes such as we have no means of verifying. It began to appear, also, that the facts which form the subject-matter of different departments of science are not detached and independent groups of facts, but that all are intimately related one with another, and that all may be brought under contribution in illustrating the history of cosmic events. It was a sense of this interdependence of different departments that led Auguste Comte to write his Philosophie Positive, the first volume of which appeared in 1830, in which he sought to point out the methods which each science has at command for discovering truth, and the manner in which each might be made to contribute toward a sound body of philosophic doctrine. The attempt had a charm and a stimulus for many minds, but failed by being enlisted in the service of sundry sociological vagaries upon which the author’s mind was completely wrecked. “Positivism,” from being the name of a potent scientific method, became the name of one more among the myriad ways of having a church and regulating the details of life.
While the ponderous mechanical intellect of Comte was striving to elicit the truth from themes beyond its grasp, one of the world’s supreme poets had already discerned some of the deeper aspects of science presently to be set forth. By temperament and by training Goethe was one of the first among evolutionists. The belief in an evolution of higher from lower organisms could not fail to be strongly suggested to a mind like his as soon as the classification of plants and animals had begun to be conducted upon scientific principles. It is not for nothing that a table of classes, orders, families, genera, and species, when graphically laid out, resembles a family tree. It was not long after Linnæus that believers in some sort of a development theory, often fantastic enough, began to be met with. The facts of morphology gave further suggestions in the same direction. Such facts were first generalized on a grand scale by Goethe in his beautiful little essay on The Metamorphoses of Plants, written in 1790, and his Introduction to Morphology, written in 1795, but not published until 1807. In these profound treatises, which were too far in advance of their age to exert much influence at first, Goethe laid the philosophic foundations of comparative anatomy in both vegetal and animal worlds. The conceptions of metamorphosis and of homology, which were thus brought forward, tended powerfully toward a recognition of the process of evolution. It was shown that what under some circumstances grows into a stem with a whorl of leaves, under other circumstances grows into a flower; it was shown that in the general scheme of the vertebrate skeleton a pectoral fin, a fore leg, and a wing occupy the same positions: thus was strongly suggested the idea that what under some circumstances developed into a fin might under other circumstances develop into a leg or a wing. The revelations of palæontology, showing various extinct adult forms, with corresponding organs in various degrees of development, went far to strengthen this suggestion, until an unanswerable argument was reached with the study of rudimentary organs, which have no meaning except as remnants of a vanished past during which the organism has been changing. The study of comparative embryology pointed in the same direction, for it was soon observed that the embryos and larvæ of the higher forms of each group of animals pass, “in the course of their development, through a series of stages in which they more or less completely resemble the lower forms of the group” (Balfour, Embryology, i. 2).
Before the full significance of such facts of embryology and morphology could be felt, it was necessary that the work of classification should be carried far beyond the point at which it had been left by Linnæus. In mapping out the relationships in the animal kingdom, the great Swedish naturalist had relied less than his predecessors upon external or superficial characteristics; the time was arriving when classification should be based upon a thorough study of internal structure, and this was done by a noble company of French anatomists, among whom Cuvier was chief. It was about 1817 that Cuvier’s gigantic work reached its climax in bringing palæontology into alliance with systematic zoölogy, and effecting that grand classification of animals in space and time which at once cast into the shade all that had gone before it. During the past fifty years there have been great changes made in Cuvier’s classification, especially in the case of the lower forms of animal life. His class of Radiata has been broken up, other divisions in his invertebrate world have been modified beyond recognition, his vertebrate scheme has been overhauled in many quarters, his attempt to erect a distinct order for Man has been overthrown. Among the great anatomists concerned in this work the greatest name is that of Huxley. The classification most generally adopted to-day is Huxley’s, but it is rather a modification of Cavier’s than a new development. So enduring has been the work of the great Frenchman.
With Cuvier the analysis of the animal organism made some progress in such wise that anatomists began to concentrate their attention upon the study of the development and characteristic functions of organs. Philosophically this was a long step in advance, but a still longer one was taken at about the same time by that astonishing youth whose career has no parallel in the history of science. When Xavier Bichat died in 1802, in his thirty-first year, he left behind him a treatise on comparative anatomy in which the subject was worked up from the study of the tissues and their properties. The path thus broken by Bichat led to the cell doctrine of Schleiden and Schwann, matured about 1840, which remains, with some modifications, the basis of modern biology. The advance along these lines contributed signally to the advancement of embryology, which reached a startling height in 1829 with the publication of Baer’s memorable treatise, in which the development of an ovum is shown to consist in a change from homogeneity to heterogeneity through successive differentiations. But while Baer thus arrived at the very threshold of the law of evolution, he was not in the true sense an evolutionist; he had nothing to say to phylogenetic evolution, or the derivation of the higher forms of life from lower forms through physical descent with modifications. Just so with Cuvier. When he effected his grand classification, he prepared the way most thoroughly for a general theory of evolution, but he always resisted any such inference from his work. He was building better than he knew.
The hesitancy of such men as Cuvier and Baer was no doubt due partly to the apparent absence of any true cause for physical modifications in species, partly to the completeness with which their own great work absorbed their minds. Often in the history of science we witness the spectacle of a brilliant discoverer traveling in triumph along some new path, but stopping just short of the goal which subsequent exploration has revealed. There it stands looming up before his face, but he is blind to its presence through the excess of light which he has already taken in. The intellectual effort already put forth has left no surplus for any further sweep of comprehension, so that further advance requires a fresher mind and a new start with faculties unjaded and unwarped. To discover a great truth usually requires a succession of thinkers. Among the eminent anatomists who in the earlier part of our century were occupied with the classification of animals, there were some who found themselves compelled to believe in phylogenetic evolution, although they could frame no satisfactory theory to account for it. The weight of evidence was already in favor of such evolution, and these men could not fail to see it. Foremost among them was Jean Baptiste Lamarck, whose work was of supreme importance. His views were stated in 1809, in his Philosophie Zoölogique, and further illustrated in 1815, in his voluminous treatise on invertebrate animals. Lamarck entirely rejected the notion of special creations, and he pointed out some of the important factors in evolution, especially the law that organs and faculties tend to increase with exercise, and to diminish with disuse. His weakest point was the disposition to imagine some inherent and ubiquitous tendency toward evolution, whereas a closer study of nature has taught us that evolution occurs only where there is a concurrence of favorable conditions. Among others who maintained some theory of evolution were the two Geoffroy Saint-Hilaire, father and son, and the two great botanists, Naudin in France and Hooker in England. In 1852 the case of evolution as against special creations was argued by Herbert Spencer with convincing force, and in 1855 appeared The Principles of Psychology, by the same author, a book which is from beginning to end an elaborate illustration of the process of evolution, and is divided from everything that came before it by a gulf as wide as that which divides the Copernican astronomy from the Ptolemaic.
The followers of Cuvier regarded the methods and results of these evolutionists with strong disapproval. In the excess of such a feeling they even went so far as to condemn all philosophic thinking on subjects within the scope of natural history as visionary and unscientific. Why seek for any especial significance in the fact that every spider and every lobster is made up of just twenty segments? Is it not enough to know the fact? Children must not ask too many questions. It is the business of science to gather facts, not to seek for hidden implications. Such was the mental attitude into which men of science were quite commonly driven, between 1830 and 1860, by their desire to blink the question of evolution. A feeling grew up that the true glory of a scientific career was to detect for the two hundredth time an asteroid, or to stick a pin through a beetle with a label attached bearing your own latinized name, Browni, or Jonesii, or Robinsoniense. This feeling was especially strong in France, and was not confined to physical science. It was exhibited a few years later in the election of some Swedish or Norwegian naturalist (whose name I forget) to the French Academy of Sciences instead of Charles Darwin: the former had described some new kind of fly, the latter was only a theorizer! The study of origins in particular was to be frowned upon. In 1863 the Linguistic Society of Paris passed a by-law that no communications bearing upon the origin of language would be received. In the same mood Sir Henry Maine’s treatise on Ancient Law was condemned at a leading American university: it was enough for us to know our own laws; those of India might interest British students who might have occasion to go there, but not Americans. Such crude notions, utterly hostile to the spirit of science, were unduly favored fifty years ago by the persistent unwillingness to submit the phenomena of organic nature to the kind of scientific explanation which facts from all quarters were urging upon us.
During the period from 1830 to 1860 the factor in evolution which had hitherto escaped detection was gradually laid hold of and elaborately studied by Charles Darwin. In the nature of his speculations and the occasion that called them forth, he was a true disciple of Lyell. The work of that great geologist led directly up to Darwinism. As long as it was supposed that each geologic period was separated from the periods before and after it by Titanic convulsions which revolutionized the face of the globe, it was possible for men to acquiesce in the supposition that these convulsions wrought an abrupt and a wholesale destruction of organic life, and that the lost forms were replaced by an equally abrupt and wholesale supernatural creation of new forms at the beginning of each new period. But as people ceased to believe in the convulsions such an explanation began to seem improbable, and it was completely discredited by the fact that many kinds of plants and animals have persisted with little or no change during several successive periods, side by side with other kinds in which there have been extensive variation and extinction.
In connection with this a fact of great significance was elicited. Between the fauna and flora of successive periods in the same geographical region there is apt to be a manifest family likeness, indicating that the later are connected with the earlier through the bonds of physical descent. It was a case of this sort that attracted Darwin’s attention in 1835. The plants and animals of the Galapagos Islands are either descended, with specific modifications, from those of the mainland of Ecuador, or else there must have been an enormous number of special creations. The case is one which at a glance presents the notion of special creations in an absurd light. But what could have caused the modifications? What was wanted was to be able to point to some agency, similar to agencies now in operation, and therefore intelligible, which could be proved to be capable of making specific changes in plants and animals. Darwin’s solution of the problem was so beautiful, it seems now so natural and inevitable, that we may be in danger of forgetting how complicated and abstruse the problem really was. Starting from the known experiences of breeders of domestic animals and cultivated plants, and duly considering the remarkable and sometimes astonishing changes that are wrought by simple selection, the problem was to detect among the multifarious phenomena of organic nature any agency capable of accomplishing what man thus accomplishes by selection. In detecting the agency of natural selection, working perpetually through the preservation of favored individuals and races in the struggle for existence, Darwin found the true cause for which men were waiting. With infinite patience and caution he applied his method of explanation to one group of organic phenomena after another, meeting in every quarter with fresh and often unexpected verification. After more than twenty years a singular circumstance led him to publish an account of his researches. The same group of facts had set a younger naturalist to work upon the same problem, and a similar process of thought had led to the same solution. Without knowing what Darwin had done, Alfred Russel Wallace made the same discovery, and sent from the East Indies, in 1858, his statement of it to Darwin as to the man whose judgment upon it he should most highly prize. This made publication necessary for Darwin. The vast treasures of theory and example which he had accumulated were given to the world, the notion of special creations was exploded, and the facts of phylogenetic evolution won general acceptance.
Under the influence of this great achievement men in every department of science began to work in a more philosophical spirit. Naturalists, abandoning the mood of the stamp-collector, saw in every nook and corner some fresh illustration of Darwin’s views. One serious obstacle to any general statement of the doctrine of evolution was removed. It was in 1861 that Herbert Spencer began to publish such a general systematic statement. His point of departure was the point reached by Baer in 1829, the change from homogeneity to heterogeneity. The theory of evolution had already received in Spencer’s hands a far more complete and philosophical treatment than ever before, when the discovery of natural selection came to supply the one feature which it lacked. Spencer’s thought is often more profound than Darwin’s, but he would be the first to admit the indispensableness of natural selection to the successful working out of his own theory.
The work of Spencer is beyond precedent for comprehensiveness and depth. He began by showing that as a generalization of embryology Baer’s law needs important emendations, and he went on to prove that, as thus rectified, the law of the development of an ovum is the law which covers the evolution of our planetary system, and of life upon the earth’s surface in all its myriad manifestations. In Spencer’s hands, the time-honored Nebular Theory propounded by Immanuel Kant in 1755, the earliest of all scientific theories of evolution, took on fresh life and meaning; and at the same time the theories of Lamarck and Darwin as to organic evolution were worked up along with his own profound generalization of the evolution of mind into one coherent and majestic whole. Mankind have reason to be grateful that the promise of that daring prospectus which so charmed and dazzled us in 1860 is at last fulfilled; that after six-and-thirty years, despite all obstacles and discouragements, the Master’s work is virtually done.
Such a synthesis could not have been achieved, nor even attempted, without the extraordinary expansion of molecular physics that marked the first half of the nineteenth century. When Priestley discovered oxygen, the undulatory theory of light, the basis of all modern physics, had not been established. It had indeed been propounded as long ago as 1678 by the illustrious Christian Huyghens, whom we should also remember as the discoverer of Saturn’s rings and the inventor of the pendulum clock. But Huyghens was in advance of his age, and the overshadowing authority of Newton, who maintained a rival hypothesis, prevented due attention being paid to the undulatory theory until the beginning of the present century, when it was again taken up and demonstrated by Fresnel and Thomas Young. About the same time, our fellow countryman, Count Rumford, was taking the lead in that series of researches which culminated in the discovery of the mechanical equivalent of heat by Dr. Joule, in 1843. One of Priestley’s earliest books, the one which made him a doctor of laws and a fellow of the Royal Society, was a treatise on electricity, published in 1767. It was a long step from that book to the one in which the Danish physicist Gersted, in 1820, demonstrated the intimate correlation between electricity and magnetism, thus preparing the way for Faraday’s great discovery of magneto-electric induction in 1831. By the middle of our century the work in these various departments of physics had led to the detection of the deepest truth in science, the law of correlation and conservation, which we owe chiefly to Helmholtz, Mayer, and Grove. It was proved that light and heat and the manifestations of force which we group together under the name of electricity are various modes of undulatory motion transformable one into another; and that in the operations of nature energy is never annihilated, but only changed from one form into another. This generalization includes the indestructibility of matter, and thus lies at the bottom of all chemistry and physics and of all science.
Returning to that chemistry with which we started, we may recall two laws that were propounded early in the century, one of which was instantly adopted, while the other had to wait for its day. Dalton’s law of definite and multiple proportions has been ever since 1808 the corner-stone of chemical science, and the atomic theory by which he sought to explain the law has exercised a profound influence upon all modern speculation. The other law, announced by Avogadro in 1811, that, “under the same conditions of pressure and temperature, equal volumes of all gaseous substances, whether elementary or compound, contain the same number of molecules,” was neglected for nearly fifty years, and then, when it was taken up and applied, it remodeled the whole science of chemistry and threw a flood of light upon the internal constitution of matter. In this direction a new world of speculation is opening up before us, full of wondrous charm. The amazing progress made since Priestley’s day may be summed up in a single contrast. In 1781 Cavendish ascertained the bare fact that water is made up of oxygen and hydrogen; within ninety years from that time Sir William Thomson was able to tell us that “if the drop of water were magnified to the size of the earth, the constituent atoms would be larger than peas, but not so large as billiard-balls.” Such a statement is confessedly provisional, but, allowing for this, the contrast is no less striking.
Concerning the various and complicated applications of physical science to the arts, by which human life has been so profoundly affected in the present century, a mere catalogue of them would tax our attention to little purpose. As my object in the present paper is simply to trace the broad outlines of advance in pure science, I pass over these applications, merely observing that the perpetual interaction between theory and practice is such that each new invention is liable to modify the science in which it originated, either by encountering fresh questions or by suggesting new methods, or in both these ways. The work of men like Pasteur and Koch cannot fail to influence biological theory as much as medical practice. The practical uses of electricity are introducing new features into the whole subject of molecular physics, and in this region, I suspect, we are to look for some of the most striking disclosures of the immediate future.
A word must be said of the historical sciences, which have witnessed as great changes as any others, mainly through the introduction of the comparative method of inquiry. The first two great triumphs of the comparative method were achieved contemporaneously in two fields of inquiry very remote from one another: the one was the work of Cuvier, above mentioned; the other was the founding of the comparative philology of the Aryan languages by Franz Bopp, in 1816. The work of Bopp exerted as powerful an influence throughout all the historical fields of study as Cuvier exerted in biology. The young men whose minds were receiving their formative impulses between 1825 and 1840, under the various influences of Cuvier and Saint-Hilaire, Lyell, Goethe, Bopp, and other such great leaders, began themselves to come to the foreground as leaders of thought about 1860: on the one hand, such men as Darwin, Gray, Huxley, and Wallace; on the other hand, such as Kuhn and Schleicher, Maine, Maurer, Mommsen, Freeman, and Tylor. The point of the comparative method, in whatever field it may be applied, is that it brings before us a great number of objects so nearly alike that we are bound to assume for them an origin and general history in common, while at the same time they present such differences in detail as to suggest that some have advanced further than others in the direction in which all are traveling; some, again, have been abruptly arrested, others perhaps even turned aside from the path. In the attempt to classify such phenomena, whether in the historical or in the physical sciences, the conception of development is presented to the student with irresistible force. In the case of the Aryan languages, no one would think of doubting their descent from a common original: just side by side is the parallel case of one sub-group of the Aryan languages, namely, the seven Romance languages which we know to have been developed out of Latin since the Christian era. In these cases we can study the process of change resulting in forms that are more or less divergent from their originals. In one quarter a form is retained with little modification; in another it is completely blurred, as the Latin metipsissimus becomes medesimo in Italian, but mismo in Spanish, while in modern French there is nothing left of it but même. So in Sanskrit and in Lithuanian we find a most ingenious and elaborate system of conjugation and declension, which in such languages as Greek and Latin is more or less curtailed and altered, and which in English is almost completely lost. Yet in Old English there are quite enough vestiges of the system to enable us to identify it with the Lithuanian and Sanskrit.
So the student who applies the comparative method to the study of human customs and institutions is continually finding usages, beliefs, or laws existing in one part of the world that have long since ceased to exist in another part; yet where they have ceased to exist they have often left unmistakable traces of their former existence. In Australasia we find types of savagery ignorant of the bow and arrow; in aboriginal North America, a type of barbarism familiar with the art of pottery, but ignorant of domestic animals or of the use of metals; among the earliest Romans, a higher type of barbarism, familiar with iron and cattle, but ignorant of the alphabet. Along with such gradations in material culture we find associated gradations in ideas, in social structure, and in deep-seated customs. Thus, some kind of fetichism is apt to prevail in the lower stages of barbarism, and some form of polytheism in the higher stages. The units of composition in savage and barbarous societies are always the clan, the phratry, and the tribe. In the lower stages of barbarism we see such confederacies as those of the Iroquois; in the highest stage, at the dawn of civilization, we begin to find nations imperfectly formed by conquest without incorporation, like aboriginal Peru or ancient Assyria. In the lower stages we see captives tortured to death, then at a later stage sacrificed to the tutelar deities, then later on enslaved and compelled to till the soil. Through the earlier stages of culture, as in Australasia and aboriginal America, we find the marriage tie so loose and paternity so uncertain that kinship is reckoned only through the mother; but in the highest stage of barbarism, as among the earliest Greeks, Romans, and Jews, the more definite patriarchal family is developed, and kinship begins to be reckoned through the father. It is only after that stage is reached that inheritance of property becomes fully developed, with the substitution of individual ownership for clan ownership, and so on to the development of testamentary succession, individual responsibility for delict and crime, and the substitution of contract for status. In all such instances—and countless others might be cited—we see the marks of an intelligible progression, a line of development which human ideas and institutions have followed. But in the most advanced societies we find numerous traces of such states of things as now exist only among savage or barbarous societies. Our own ancestors were once polytheists, with plenty of traces of fetichism. They were organized in clans, phratries, and tribes. There was a time when they used none but stone tools and weapons; when there was no private property in land, and no political structure higher than the tribe. Among the forefathers of the present civilized inhabitants of Europe are unmistakable traces of human sacrifices and of the reckoning of kinship through the mother only. When we have come to survey large groups of facts of this sort, the conclusion is irresistibly driven home to us that the more advanced societies have gone through various stages now represented here and there by less advanced societies; that there is a general path of social development, along which, owing to special circumstances, some peoples have advanced a great way, some a less way, some but a very little way; and that by studying existing savages and barbarians we get a valuable clue to the interpretation of prehistoric times. All these things are to-day commonplaces among students of history and archæology; sixty years ago they would have been scouted as idle vagaries. It is the introduction of such methods of study that is making history scientific. It is enabling us to digest the huge masses of facts that are daily poured in upon us by decipherers of the past, — monuments, inscriptions, pottery, weapons, ethnological reports, and all that sort of thing, — and to make all contribute toward a coherent theory of the career of mankind upon the earth.
In the course of the foregoing survey one fact stands out with especial prominence: it appears that about half a century ago the foremost minds of the world, with whatever group of phenomena they were occupied, had fallen, and were more and more falling, into a habit of regarding things, not as having originated in the shape in which we now find them, but as having been slowly metamorphosed from some other shape through the agency of forces similar in nature to forces now at work. Whether planets, or mountains, or mollusks, or subjunctive moods, or tribal confederacies were the things studied, the scholars who studied them most deeply and most fruitfully were those who studied them as phases in a process of development. The work of such scholars has formed the strong current of thought in our time, while the work of those who did not catch these new methods has been dropped by the way and forgotten; and as we look back to Newton’s time we can see that ever since then the drift of scientific thought has been setting in this direction, and with increasing steadiness and force.
Now, what does all this drift of scientific opinion during more than two centuries mean? It can, of course, have but one meaning. It means that the world is in a process of development, and that gradually, as advancing knowledge has enabled us to take a sufficiently wide view of the world, we have come to see that it is so. The old statical conception of a world created all at once in its present shape was the result of very narrow experience; it was entertained when we knew only an extremely small segment of the world. Now that our experience has widened, it is outgrown and set aside forever; it is replaced by the dynamical conception of a world in a perpetual process of evolution from one state into another state. This dynamical conception has come to stay with us. Our theories as to what the process of evolution is may be more or less wrong and are confessedly tentative, as scientific theories should be. But the dynamical conception, which is not the work of any one man, be he Darwin or Spencer or any one else, but the result of the cumulative experience of the last two centuries, this is a permanent acquisition. We can no more revert to the statical conception than we can turn back the sun in his course. Whatever else the philosophy of future generations may be, it must be some kind of a philosophy of evolution.
Such is the scientific conquest achieved by the nineteenth century, a marvelous story without any parallel in the history of human achievement. The swiftness of the advance has been due partly to the removal of the ancient legal and social trammels that beset free thinking in every conceivable direction. It is largely due also to the use of correct methods of research. The waste of intellectual effort has been less than in former ages. The substitution of Lavoisier’s balance for Stahl’s a priori reasoning is one among countless instances of this. Sound scientific method is a slow acquisition of the human mind, and for its more rapid introduction, in Priestley’s time and since, we have largely to thank the example set by those giants of a former age, Galileo and Kepler, Descartes and Newton.
The lessons that might be derived from our story are many. But one that we may especially emphasize is the dignity of Man whose persistent seeking for truth is rewarded by such fruits. We may be sure that the creature whose intelligence measures the pulsations of molecules and unravels the secret of the whirling nebula is no creature of a day, but the child of the universe, the heir of all the ages, in whose making and perfecting is to be found the consummation of God’s creative work.