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-honoured 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 Oersted, 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 remodelled 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 sketch 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 travelling; 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
