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During the decade, many of the familiar figures of the philo- sophical world were removed by death. This was only to be ex- pected in the case of octogenarians like Wundt and Windelband and J. F. Brentano, still more in that of a nonagenarian like the Italian positivist, R. Ardigo (1828-1020). In Great Britain there were few losses among the elder philosophers since the death of the nonagenarian, A. Campbell Fraser. Of the elderly, but not old, the United States lost their leading philosopher, Josiah Royce; France, Gaston Milhaud and Emile Boutroux (1845-1921); and Austria, Alexis Meinong. Still more regrettable was the death of men who were still young or in their intellectual prime, as Louis Couturat in France and Oswald Kulpe in Germany.

(A. E. T.)

PHYSIOLOGY (see 21.554;. Since 1910 increased attention has been paid, in physiological research, to phenomena common to living beings in general, and recent investigation has added considerably to our knowledge and corrected earlier theories.

As distinguished from Morphology, the science of the form and structure of living organisms, Physiology may be said to be concerned with their activities, chemical and physical. But there can be no hard and fast line between the two bodies of doctrine. A function depends on the way the machine is made. And many provinces of modern research, such as those depend- ing on changes of general form in response to external agents, combine morphology and physiology. For convenience of re- search, physiology is often divided into biochemistry and bio- physics. But this distinction can scarcely be regarded as a scientific one, since in all vital processes both chemical and physical factors intervene. It is true that some of the problems of the biochemist consist in the elucidation of the chemical nature of vital products, and might be looked upon as the chemical side of morphology; but the methods of investiga- tion distinguish them from those of the organic chemist.

Animal and Plant. There is no real or fundamental difference beCween the animal and plant organism. Great as may appear to be the external differences between a dog and a tree, when we proceed to examine the physiological factors of which their life is made up, we find that the elementary processes are essentially alike. The most striking contrast, that of move- ment, does not exist in the simplest members of the two king- doms. While certain plants, such as algae and bacteria, are motile in some stages, certain polyps and ascidians become fixed in the later periods of their life. Other instances might be given. The difference between the net result of the chemical changes occurring in the green plants and those occurring in animals is due to the presence in the former of the green pig- ment, chlorophyll, and does not show itself in fungi. By the aid of chlorophyll, the energy of the sun's light is used to build up the carbon dioxide, formed in the combustion of food by all cells, into sugar and oxygen. These again become available as sources of energy to living matter. In this connexion, it may be noted that the work of Willstatter and Stoll has made it practically certain that carbon dioxide and water become at- tached in some way to the chlorophyll particles, a molecular rearrangement takes place with addition of energy when light is absorbed, a peroxide of formaldehyde is produced, and this is then decomposed into gaseous oxygen and formaldehyde by the agency of an enzyme (catalase). From formaldehyde the higher sugars are readily produced by polymerization. The precise chemistry of the reactions is 'not yet clear it may be that formic acid and hydrogen peroxide result from the action of water on the formaldehyde peroxide. In this case, catalase splits up hydrogen peroxide into water and oxygen, while the formic acid is reduced by light (absorbed by chlorophyll) to formaldehyde. However this may be, the final result of the process is that, in the light, green plants take up carbon diox- ide and give off oxygen. In the dark, they behave like animals, giving off carbon dioxide by combustion. Such combustion, of course, is actually proceeding in the light also, although ob- scured by the opposite phenomenon.

Vitalism. Some discussion, not very profitable as far as physiology is concerned, has taken place as to the existence of

a form of energy, or, as held by some, of a kind of directing agency peculiar to living beings. The aim of all physiological experi- mentation is to express vital processes in terms of physical and chemical laws. We call this " explaining " them. The methods used, moreover, are those of physics and chemistry. Owing to the complexity of the phenomena, we have to be content in many cases with expression in terms of simpler physiological laws, leaving the further analysis of these laws to the future. As Claude Bernard pointed out, the laws of physics and chemistry manifest their action in systems of a special kind, differing from those familiar to workers in the former sciences, so that the separation of a biological group is quite justified. At the same time, there is intimate connexion and overlapping, a process subject to continual increase.

As to the existence of a peculiar form of " vital " energy, it is to be admitted that we have no direct method of measuring the energy contained in a given mass of carbon and oxygen, although we know that a definite quantity of work can be obtained by allowing them to combine. Thus, the energy of living things cannot be looked upon as altogether peculiar. Indeed, measure- ments of any form of energy are usually made by converting them into other forms. The first law of energetics tells us that this is permissible. The striking fact that any organism behaves as a unified and coordinated set of activities has led to the assumption of a directing agency of some kind. We shall see later that notable progress has been made in the explanation of the factors responsible for many aspects of this integration.

The position of consciousness in physiological processes is by no means clear. While it may be. held that the province of physiology is to investigate what can be made out experiment- ally in the nervous centres, it does not seem impossible that con- sciousness may ultimately fall into place as one of the functions of nervous tissue.

Relation to Physical Chemistry. One of the most striking aspects of the development of modern physiology is the im- portant part played by considerations derived from physical chemistry. The osmotic pressure of solutions, the action of the various inorganic ions arising from the electrolytic dissociation of salts, acids and bases, especially the hydrogen ion, and the maintenance of the neutrality necessary to the normal function of the cell, together with the properties of matter in the colloidal state and of heterogeneous systems in general may be mentioned. The absorption of substances at the surfaces of contact of phases that do not mix has been found to explain many otherwise puz- zling facts, and is doubtless a prominent factor in the formation of the membrane covering the surface of cells in general and regulating the passage of dissolved substances into and out of the cell protoplasm.

This is perhaps the most appropriate place to refer to the rela- tion of vital processes to the second law of energetics. In one of its aspects, this law points out how the transformation of heat energy into other forms is limited, owing to our conditions of existence being at a temperature so far above absolute zero. The properties which we recognize as " vital " are especially manifested during the change of one form of energy into another. In this process, it is of much importance from the point of view of economy that the free chemical energy of our food should not be " degraded " to heat before utilization in muscular work, and so on. Similarly, the energy of the sun's light must be con- verted to chemical energy by the green plant as directly as pos- sible, without passing through the stage of heat. Accordingly we find that arrangements are made to ensure that loss from such causes shall be as small as possible.

Nature of Protoplasm. As usually seen in such organisms as the amoeba or the leucocytes of the blood, protoplasm exhibits the properties of a liquid. It contains numerous particles in Brownian movement, a fact which shows the absence of obstacles holding them in place. This liquid, however, is itself a colloidal solution and can be seen under special modes of illumination to be closely packed with extremely minute particles. Under some states of activity, as in dividing cells or when electrically stimulated, this " hydrosol," to use Graham's word, becomes a