Science

A position similar to that won for physics by Galileo and New ton was only reached in chemistry a century later. Then Lavoisier, by the test of the balance, showed that the total mass involved remained unchanged throughout a chemical reaction, and, by his re-discovery of oxygen, explained combustion as the combination of ordinary substances possessing mass and weight.

Newton brought the existing state of the solar system within the cognizance of known dynamical principles, and the logical ex tension of such principles to explain the origin of that system was made by the speculations of Pierre Simon, marquis de Laplace (1749-1827), and developed by those who followed him down to Sir James Jeans in our own day. They have imagined a primitive state of nebulosity from which, by the action of known dynamical processes, the stellar universe and our sun and planets would be evolved.

The discovery by Sir George G. Stokes (1819-1903), J. B. L. Foucault (1819-1868), R. W. Bunsen (1811-1898) and G. R. Kirchhoff (1824-1887), that the spectroscope gave a means of investigating the chemical composition of the sun and the stars, brought another set of phenomena within the range of experi ment, while the differences observed in stellar spectra suggested once more the idea of cosmical development, familiar from the nebular hypothesis of Laplace.

Geology.

Similar principles were during the 19th century applied to the history of the earth. The earlier conceptions of the origin of the rocks imagined catastrophes of fire or water, proc esses alien to those of everyday experience. But the "uniformi tarian" school, founded by James Hutton (1726-1797) and ex pounded by Sir Charles Lyell (1797-1875), produced evidence to show that much, at any rate, of the structure of the surface of the globe was produced by processes still going on under our eyes. The deposition of material by the action of seas, rivers and volcanoes, was seen to need only time enough to produce beds of rock like those which make up our mountains. Comparison of the fossil remains of plants and animals found in different strata then enabled geologists to classify the rocks, and place them in a chronological sequence. Moreover, it became evident that a series of animal and plant types was associated with the gradual forma tion of the rocks, and that the age both of the earth itself and of the organic life f ound on it was much greater than had been suspected. The few thousand years of received cosmogonies stretched out into untold millions, during which the same familiar laws described the phenomena of development. The remains and

traces of man, found, it is true, only in the later sedimentary de posits of the earth, were enough to prove his existence through ages beside which the dawn of history was but as yesterday. As Newton had extended known principles throughout the gigantic spaces of the heavens, so the later astronomers and geologists pushed them back over enormous epochs of time.

Physiology and Biology.

Turning back to the 17th century, we find William Harvey starting physiology on its true course by his discovery of the circulation of the blood. By experiments on living animals, Harvey (1578-1657), von Haller (1708-1777), Bernard (1813-1878) and many others, investigated the mecha nism of circulation, respiration, digestion and the other functions of the body, and thus made modern medicine and surgery possible. Moreover, they showed that physical and chemical principles are applicable in physiology, and gave new support to the idea of "man as a machine," which, whether or no it be a complete ex planation, is an almost necessary assumption for physiological research.

The part played by micro-organisms in disease, made clear by Pasteur (1822-1895) and his disciples, the multitudinous life of tropic lands observed during voyages of scientific exploration, the revelations of geology—all combined with the new knowledge of physiology to prepare the way for the biological work of Charles Darwin (1809-1882). The origin of living beings from a few an cestral types was an old conception, but Darwin first found an adequate intelligible cause in the slow action of sexual selection, in conjunction with the pressure of the struggle for life, which allowed only those individuals most suited to the environment by favourable variation to survive and rear their offspring. The advantage thus given to beings with useful variations may develop into permanent modifications in the course of ages, and, when the parent types have disappeared, their common posterity may ex hibit the marked differences characteristic of the separate and dis tinct species now existent. From the point of view of the history of science, the significance of Darwin's theory lies in the new and vast extension it gives to the field in which causes intelligible to the human mind can be sought as explanations of phenomena. Thus evolution is co-ordinated in the history of thought with the Newtonian theory of gravitation, with physical and chemical theories of physiology and with the uniformitarian theory of geology.