Both before and after the appearance of Darwin's work, biolo gists devoted their attention to the study of how useful variations arise. Three views have been held. ( ) Jean Baptiste, chevalier de Lamarck (1744-1829), regarded variation as due to the ac cumulated and inherited effect of use. Thus the giraffe acquires his long neck by the successive efforts of countless generations to browse on leaves just beyond their reach. (2) Darwin, while ac cepting changes in accordance with Lamarck's ideas as exceptional aids to variation, revolutionized biology by showing the primary importance of the struggle for life, when extended over long periods of time, in selecting useful variations which arise acci dentally or in other ways. (3) Darwin also recognized the pos sible occasional effect of discontinuous variations or "sports," when a plant or an animal diverges from its parents in a marked manner. But of late years the rediscovery of the forgotten work of G. J. Mendel, Abbot of Bri_inn (1822-1884), and further study by Hugo de Vries, William Bateson and others, of discon tinuous variations which arise spontaneously, have pointed to the conclusion that in nature such sudden leaps are the normal cause of development. If a "sport" has advantages over the parental type, it tends to survive, while, if it is not as fitted for its life struggle, it is destroyed by natural selection and never establishes itself. Such a theory does something to avoid the difficulty of pure "Darwinism," that organs useful, when fully developed, to an animal or plant are of no advantage in incipient stages.
Closely connected with such problems is the question of inheritance. Lamarck's theory required the inheritance of charac teristics acquired during the life of a parent. But difficulties, such as that of seeing how such a change could affect the simple germ cells, have led some more recent biologists to deny the possibility of any acquired characteristic being transmitted to offspring. This question, like that of the sufficiency of Darwinism and Mendelism to explain the facts of evolution, is still the subject of controversy. (See HEREDITY.) Modern Physics.—Simultaneously with the growth of geology, and the birth of the Darwinian hypothesis, a new development took place in physical science—the development of the concep tion of energy as a quantity invariable in amount throughout a series of physical changes. The genesis of the idea in its modern form may be traced in the work of Newton and C. Huygens (1629-1695), who applied it to the problems of pure dynamics.
But, in the middle of the 19th century, by the work of James Prescott Joule (1818-1889), Lord Kelvin (1824-1907), H. L. F.
von Helmholtz (1821-1894), J. Willard Gibbs R. J. E. Clausius (1822-1888) and others, it was extended to physical processes. The amount of heat produced by friction was found to bear a constant proportion to the work expended, and this experimental result led to the conception of an invariable quantity of something, to which the name of energy was given, manifesting itself in various forms such as heat or mechanical work. Energy thus took its place beside mass as a real quantity, conserved throughout a series of physical changes. Of late years,
as we shall see below, evidence has appeared to show that mass is not constant, but may depend on the velocity when the velocity approaches that of light, while still more recent work suggests that mass and energy are mutually convertible. Since the only essential quality of matter is its mass, these results seem to strike at the root of the metaphysical conception of matter as a real, invariable quantity.
In ordinary physical conditions, the amount of energy in an isolated system remains invariable, but, if changes are going on in the system, the energy tends continually to become less and less available for the performance of useful work. All heat engines require a difference of temperature—a boiler and refrigerator, or their equivalents. We cannot continue to transform heat into mechanical work if all available objects are at a uniform tempera ture. But, if temperature differences exist, they tend to equalize themselves by irreversible processes of thermal conduction, and it becomes increasingly difficult to get useful work out of the supplies of heat. In an isolated system, then, equilibrium will be reached when this process of "dissipation of energy" is complete, and, from this single principle, the whole theory of the equilibrium of physical and chemical systems was worked out by Kelvin, Helmholtz and Willard Gibbs. Such a method avoids altogether the use of atomic and molecular conceptions.
But the other great line of advance in recent physics has been traced by a method which uses atomic and molecular conceptions in an extreme form. The passage of electricity through liquids had been explained by Michael Faraday (1791-1867), Kohlrausch and others as a transference of a succession of electric charges carried by moving particles of matter or ions. At the end of the 19th century these ideas were extended, chiefly by the labours of J. J. Thomson, to elucidate also the conduction of electricity through gases. In 1897 Thomson discovered that, in certain cases, the moving particles which carried the electric current were of much smaller mass than the smallest chemical atom, that of hydrogen, and that these minute particles, to which he gave the name of cor puscles, were identical from whatever substance they were ob tained. They enter into the structure of all matter, and form a common constituent of all chemical atoms. The only known properties of these corpuscles are their mass and their electric charge. Now, a charged body when set in motion spreads electro magnetic energy into the surrounding medium. Thus, more force is needed to produce a given acceleration than if the body were uncharged. The body acts as though its mass were greater than when it is uncharged. Indeed there is reason to believe that the whole apparent mass of the minute corpuscles to which we have referred is an effect of their electric charge. The idea of a mate rial particle thus disappears with that of material mass, and the corpuscle becomes an isolated unit of electricity—an electron, while electricity, mass and energy become different manifestations of the same underlying condition.