gas-engine which manufactures its own explosive. During the period of repose which intervenes between periods of activity, its protoplasm is busy in construction, taking from the blood-discs oxygen, and from the blood-fluid carbonaceous and nitrogenous materials, and knitting these together into relatively unstable explosive compounds, which play the part of the mixed air and gas of the gas-engine. A resting muscle may be likened to a complex and well-organized battery of gas-engines. On the stimulus supplied through a nervechannel a series of co-ordinated explosions takes place: the gas-engines are set to work; the muscular fibres contract; the products of the silent explosions are taken up and hurried away by the blood-stream; and the protoplasm prepares a fresh supply of explosive material. Long before the invention of the gas-engine, long before gun-cotton or dynamite were. dreamt of, long before some Chinese or other inventor first mixed the ingredients of gunpowder, organic nature had utilized the principle of controlled explosions in the protoplasmic cell, and thus rendered animal behaviour possible.

Certain cells are, however, more delicately explosive than others. Those, for example, on or near the external surface of the body-those, that is to say, which constitute the endorgans of the special senses-contain explosive material which may be fired by a touch, a sound, an odour, the contact with a sapid fluid or a ray of light. The effects of the explosions in these delicate cells, reinforced in certain neighbouring nervebatteries, are transmitted down the nerves as waves of subtle chemical or electrolytic change, and thus reach that wonderful aggregation of organized and co-ordinated explosive cells, the brain. Here it is again reinforced and directed (who, at present, can say how ?) along fresh nerve-channels to muscles, or glands, or other organized groups of explosives. And in the brain, somehow associated with the explosion of its cells, consciousness, the mind-element, emerges; of which we need only notice here that it belongs to a wholly different order of being from the physical activities and products with which we are at present concerned.

We must not press the explosion analogy too far. The essential thing seems to be that the protoplasm of the cell has the power of building up complex and unstable chemical compounds, which are perhaps stored in its spongy substance; and that these unstable compounds, under the influence of a stimulus (or, possibly, sometimes spontaneously), break down into simpler and more stable compounds. In the case of muscle-cells, this latter change is accompanied by an alteration in length of the fibres, and consequent movements in the animal, the products of the disruptive change being useless or harmful, and being, therefore, removed as soon as possible. But very frequently the products of explosive activity are made use of. In the case of bone-cells, one of the products of disruption is of permanent use to the organism, and constitutes the solid framework of the skeleton. In the case of the secreting cells in the salivary and other digestive glands, some of the disruptive products are of temporary value for the preparation of the food. It is probable that these useful products of disruption, permanent or temporary, took their origin in waste products for which natural selection has found a use, and which have been gradually rendered more and more efficacious in modes of organic behaviour increasingly complex.

In the busy hive of cells which constitutes what we call the animal body, there is thus ceaseless activity. During periods of apparent rest the protoplasm is engaged in constructive work, building up fresh supplies of unstable materials, which, during periods of apparent activity, break up into simpler and more stable substances, some of which are useful to the organism, while others must be got rid of as soon as possible. From another point of view, the cells during apparent rest are storing up energy to be utilized by the animal during its periods of activity. The storing up of available energy may be likened to the winding up of a watch or clock; it is when an organ is at rest that the cells are winding themselves up; and thus we have the apparent paradox that the cell is most active and doing most work when the organ of which it forms a part is at rest. During

the repose of an organ, in fact, the cells are busily working in preparation for the manifestation of energetic action that is to follow. Just as the brilliant display of intellectual activity in a great orator is the result of the silent work of a lifetime, so is the physical manifestation of muscular power the result of the silent preparatory work of the muscle-cells.

It may, perhaps, seem strange that the products of cellular life should be reached by the roundabout process of first producing unstable compounds, from which are then formed. more stable substances, useful for permanent purposes as in bone, or temporary purposes as in the digestive fluids. It seems a waste of power to build up substances unnecessarily complex and stored with an unnecessarily abundant supply of energy. But only thus could the organs be enabled to act under the influence of stimuli, and afford examples of corporate behaviour. They are like charged batteries ready to discharge under the influence of the slightest organic touch. In this way, too, is afforded a means by which the organ is not dependent only upon the products of the immediate activity of the protoplasm at the time of action, but can utilize the store laid up during preceding periods of rest.

Sufficient has now been said to illustrate the nature of some of the physical processes which accompany organic behaviour in its corporate aspect. The fact that should stand out clearly is that the animal body is stored with large quantities of available energy resident in highly complex and unstable chemical compounds, elaborated by the constituent cells. These unstable compounds, eminently explosive according to our analogy, are built up of materials derived from two different sources-from the nutritive matter (containing carbon, hydrogen, and nitrogen) absorbed during digestion, and from oxygen taken up from the air during respiration. The cells thus become charged with energy that can be set free on the application of the appropriate stimulus, which may be likened to the spark that fires the explosive.

Let us note, in conclusion, that it is through the bloodsystem, ramifying to all parts of the body, and the nerve

system, the ramifications of which are not less perfect, that one of the larger and higher animals is knit together into an organic whole. The former carries to the cell the raw materials for the elaboration of its explosive products, and, after the explosions, carries off the waste products which result therefrom. The nerve-fibres carry the stimuli by which the explosive is fired, while the central nervous system organizes, co-ordinates, and controls the explosions, and initiates the elaboration of the explosive compounds. Blood and nerves co-operate to render corporate behaviour possible.

IV. THE BEHAVIOUR OF PLANTS

A short parenthetic section on the behaviour of plants may serve further to illustrate the nature of organic behaviour. We have seen that Paramecium is apparently attracted by faintly acid solutions, and have briefly considered Dr. Jennings's interpretation of the facts disclosed by careful observation. In the ferns the female element, or ovum, is contained in a minute flask-shaped structure (archegonium), in the neck and mouth of which mucilaginous matter, with a slightly acid reaction, is developed; and this is said to exercise an attractive influence on the freely swimming ciliated male elements, or spermatozoids, which are necessary for fertilization. "Now, it has been shown by experiment that the spermatozoids of ferns are attracted by certain chemical substances, and especially by malic acid. If artificial archegonia are prepared (consisting of tiny capillary glass-tubes) and filled with mucilage to which a small quantity of this acid has been added, they are found, when placed in water containing fern-spermatozoids, to exercise the same attraction upon them which the real archegonia exercise in nature. The malic acid gradually diffuses out into the water, and the spermatozoids are influenced by it, so that they move in the direction in which the substance is more concentrated, i.e. towards the tube. Although it cannot be proved that the archegonia themselves contain malic acid, as they are too small for a recognizable quantity to be obtained

from them, yet there can be little doubt that the natural archegonia owe their attractive influence to the same chemical agent which has proved efficacious in experiment." In the light of Dr. Jennings's observations, it is perhaps not improbable that this so-called attractive influence is similar to that seen in Paramecium; and that the spermatozoids enter the organic acid in the course of their random movements, and there remain. Be that as it may, the male elements collect in the mucilaginous mass, and pass down the neck of the flask until one reaches and coalesces with the female element, or ovum, and effects its fertilization. Here we have organic behaviour unmistakably directed to a biological end-behaviour which may indeed be accompanied by some dim form of consciousness, but which is due to a purely organic reaction. It is scarcely satisfactory to say that the spermatozoids "possess a certain power of perception, by which their movements are guided." If consciousness be present, it is probably merely an accompaniment of the response, and has no directive influence on its nature and character.

In the higher plants, as in the higher animals, the differentiation and the orderly marshalling of the cell-progeny arising from the coalescent male and female elements, afford, during development, examples of corporate organic behaviour which can be more readily described than explained, but which not less clearly subserve definite biological ends, and in many cases, such as the direction of growth in radicles and roots, the curling of tendrils, and the reaction to the influence of light and warmth, are related to and evoked by the environing conditions. More closely resembling familiar modes of behaviour in animals are such movements as are seen in the "tentacles" which project from the upper surface and margin of the Sundew leaf. Their knobbed ends secrete a sticky matter, which glistens in the sun, and to which small foreign bodies readily adhere. If particles of limestone, sand, or clay, such as may

* D. H. Scott, "An Introduction to Structural Botany," part ii., "Flowerless Plants," pp. 70, 71.

† Ibid., p. 71.