II

The molecules and atoms and electrons into which science resolves matter are hypothetical bodies which no human eye has ever seen, or ever can see, but they build up the solid frame of the universe. The air and the rocks are not so far apart in their const.i.tuents as they might seem to our senses. The invisible and indivisible molecules of oxygen which we breathe, and which keep our life-currents going, form about half the crust of the earth. The soft breeze that fans and refreshes us, and the rocks that crush us, are at least half-brothers. And herein we get a glimpse of the magic of chemical combinations. That mysterious property in matter which we call chemical affinity, a property beside which human affinities and pa.s.sions are tame and inconstant affairs, is the architect of the universe. Certain elements attract certain other elements with a fierce and unalterable attraction, and when they unite, the resultant compound is a body totally unlike either of the const.i.tuents. Both substances have disappeared, and a new one has taken their place. This is the magic of chemical change. A physical change, as of water into ice, or into steam, is a simple matter; it is merely a matter of more or less heat; but the change of oxygen and hydrogen into water, or of chlorine gas and the mineral sodium into common salt, is a chemical change. In nature, chlorine and sodium are not found in a free or separate state; they hunted each other up long ago, and united to produce the enormous quant.i.ties of rock salt that the earth holds. One can give his imagination free range in trying to picture what takes place when two or more elements unite chemically, but probably there is no physical image that can afford even a hint of it. A snake trying to swallow himself, or two fishes swallowing each other, or two bullets meeting in the air and each going through the centre of the other, or the fourth dimension, or almost any other impossible thing, from the point of view of tangible bodies, will serve as well as anything. The atoms seem to get inside of one another, to jump down one another's throats, and to suffer a complete transformation. Yet we know that they do not; oxygen is still oxygen, and carbon still carbon, amid all the strange partnerships entered into, and all the disguises a.s.sumed. We can easily evoke hydrogen and oxygen from water, but just how their molecules unite, how they interpenetrate and are lost in one another, it is impossible for us to conceive.

We cannot visualize a chemical combination because we have no experience upon which to found it. It is so fundamentally unlike a mechanical mixture that even our imagination can give us no clew to it. It is thinkable that the particles of two or more substances however fine, mechanically mixed, could be seen and recognized if sufficiently magnified; but in a chemical combination, say like iron sulphide, no amount of magnification could reveal the two elements of iron and sulphur. They no longer exist. A third substance unlike either has taken their place.

We extract aluminum from clay, but no conceivable power of vision could reveal to us that metal in the clay. It is there only potentially. In a chemical combination the different substances interpenetrate and are lost in one another: they are not mechanically separable nor individually distinguishable. The iron in the red corpuscles of the blood is not the metal we know, but one of its many chemical disguises.

Indeed it seems as if what we call the ultimate particles of matter did not belong to the visible order and hence were incapable of magnification.

That mysterious force, chemical affinity, is the true and original magic. That two substances should cleave to each other and absorb each other and produce a third totally unlike either is one of the profound mysteries of science. Of the nature of the change that takes place, I say, we can form no image. Chemical force is selective; it is not promiscuous and indiscriminate like gravity, but specific and individual. Nearly all the elements have their preferences and they will choose no other. Oxygen comes the nearest to being a free lover among the elements, but its power of choice is limited.

Science conceives of all matter as grained or discrete, like a bag of shot, or a pile of sand. Matter does not occupy s.p.a.ce continuously, not even in the hardest substances, such as the diamond; there is s.p.a.ce, molecular s.p.a.ce, between the particles. A rifle bullet whizzing past is no more a continuous body than is a flock of birds wheeling and swooping in the air. Air s.p.a.ces separate the birds, and molecular s.p.a.ces separate the molecules of the bullet. Of course it is unthinkable that indivisible particles of matter can occupy s.p.a.ce and have dimensions.

But science goes upon this hypothesis, and the hypothesis proves itself.

After we have reached the point of the utmost divisibility of matter in the atom, we are called upon to go still further and divide the indivisible. The electrons, of which the atom is composed, are one hundred thousand times smaller, and two thousand times lighter than the smallest particle hitherto recognized, namely, the hydrogen atom. A French physicist conceives of the electrons as rushing about in the interior of the atom like swarms of gnats whirling about in the dome of a cathedral. The smallest particle of dust that we can recognize in the air is millions of times larger than the atom, and millions of millions of times larger than the electron. Yet science avers that the manifestations of energy which we call light, radiant heat, magnetism, and electricity, all come from the activities of the electrons. Sir J.

J. Thomson conceives of a free electron as dashing about from one atom to another at a speed so great as to change its location forty million times a second. In the electron we have matter dematerialized; the electron is not a material particle. Hence the step to the electric const.i.tution of matter is an easy one. In the last a.n.a.lysis we have pure disembodied energy. "With many of the feelings of an air-man," says Soddy, "who has left behind for the first time the solid ground beneath him," we make this plunge into the demonstrable verities of the newest physics; matter in the old sense--gross matter--fades away. To the three states in which we have always known it, the solid, the liquid, and the gaseous, we must add a fourth, the ethereal--the state of matter which Sir Oliver Lodge thinks borders on, or is identical with, what we call the spiritual, and which affords the key to all the occult phenomena of life and mind.

As we have said, no human eye has ever seen, or will see, an atom; only the mind's eye, or the imagination, sees atoms and molecules, yet the atomic theory of matter rests upon the sure foundation of experimental science. Both the chemist and the physicist are as convinced of the existence of these atoms as they are of the objects we see and touch.

The theory "is a necessity to explain the experimental facts of chemical composition." "Through metaphysics first," says Soddy, "then through alchemy and chemistry, through physical and astronomical spectroscopy, lastly through radio-activity, science has slowly groped its way to the atom." The physicists make definite statements about these hypothetical bodies all based upon definite chemical phenomena. Thus Clerk Maxwell a.s.sumes that they are spherical, that the spheres are hard and elastic like billiard-b.a.l.l.s, that they collide and glance off from one another in the same way, that is, that they collide at their surfaces and not at their centres.

Only two of our senses make us acquainted with matter in a state which may be said to approach the atomic--smell and taste. Odors are material emanations, and represent a division of matter into inconceivably small particles. What are the perfumes we smell but emanations, flying atoms or electrons, radiating in all directions, and continuing for a shorter or longer time without any appreciable diminution in bulk or weight of the substances that give them off? How many millions or trillions of times does the rose divide its heart in the perfume it sheds so freely upon the air? The odor of the musk of certain animals lingers under certain conditions for years. The imagination is baffled in trying to conceive of the number and minuteness of the particles which the fox leaves of itself in the snow where its foot was imprinted--so palpable that the scent of a hound can seize upon them hours after the fox has pa.s.sed! The all but infinite divisibility of matter is proved by every odor that the breeze brings us from field and wood, and by the delicate flavors that the tongue detects in the food we eat and drink. But these emanations and solutions that affect our senses probably do not represent a chemical division of matter; when we smell an apple or a flower, we probably get a real fragment of the apple, or of the flower, and not one or more of its chemical const.i.tuents represented by atoms or electrons. A chemical a.n.a.lysis of odors, if it were possible, would probably show the elements in the same state of combination as the substances from which the odors emanated.

The physicists herd these ultimate particles of matter about; they have a regular circus with them; they make them go through films and screens; they guide them through openings; they count them as their tiny flash is seen on a sensitized plate; they weigh them; they reckon their velocity.

The alpha-rays from radio-active substances are swarms of tiny meteors flying at the incredible speed of twelve thousand miles a second, while the meteors of the midnight sky fly at the speed of only forty miles a second. Those alpha particles are helium atoms. They are much larger than beta particles, and have less penetrative power. Sir J. J. Thomson has devised a method by which he has been able to photograph the atoms.

The photographic plate upon which their flight is recorded suggests a shower of shooting stars. Oxygen is found to be made up of atoms of several different forms.

III

The "free path" of molecules, both in liquids and in gases, is so minute as to be beyond the reach of the most powerful microscope. This free path in liquids is a zigzag course, owing to the perpetual collisions with other molecules. The molecular behavior of liquids differs from that of gases only in what is called surface tension. Liquids have a skin, a peculiar stress of the surface molecules; gases do not, but tend to dissipate and fill all s.p.a.ce. A drop of water remains intact till vaporization sets in; then it too becomes more and more diffused.

When two substances combine chemically, more or less heat is evolved.

When the combination is effected slowly, as in an animal's body, heat is slowly evolved. When the combustion is rapid, as in actual fire, heat is rapidly evolved. The same phenomenon may reach the eye as light, and the hand as heat, though different senses get two different impressions of the same thing. So a mechanical disturbance may reach the ear as sound, and be so interpreted, and reach the hand as motion in matter. In combustion, the oxygen combines rapidly with the carbon, giving out heat and light and carbon dioxide, but why it does so admits of no explanation. Herein again is where life differs from fire; we can describe combustion in terms of chemistry, but after we have described life in the same terms something--and this something is the main thing--remains untouched.

The facts of radio-activity alone demonstrate the truth of the atomic theory. The beta rays, or emanations from radium, penetrating one foot of solid iron are very convincing. And this may go on for hundreds of years without any appreciable diminution of size or weight of the radio-active substance. "A gram of such substance," says Sir Oliver Lodge, "might lose a few thousand of atoms a second, and yet we could not detect the loss if we continued to weigh it for a century." The volatile essences of organic bodies which we detect in odors and flavors, are not potent like the radium emanations. We can confine them and control them, but we cannot control the rays of radio-active matter any more than we can confine a spirit. We can separate the three different kinds of rays--the alpha, the beta, and the gamma--by magnetic devices, but we cannot cork them up and isolate them, as we can musk and the attar of roses.

And these emanations are taking place more or less continuously all about us and we know it not. In fact, we are at all times subjected to a molecular bombardment of which we never dream; minute projectiles, indivisible points of matter, are shot out at us in the form of electrons from glowing metals, from lighted candles, and from other noiseless and unsuspected batteries at a speed of tens of thousands of miles a second, and we are none the wiser for it. Indeed, if we could see or feel or be made aware of it, in what a different world we should find ourselves! How many million-or billion-fold our sense of sight and touch would have to be increased to bring this about! We live in a world of collisions, disruptions, and hurtling missiles of which our senses give us not the slightest evidence, and it is well that they do not.

There is a tremendous activity in the air we breathe, in the water we drink, in the food we eat, and in the soil we walk upon, which, if magnified till our senses could take it in, would probably drive us mad.

It is in this interior world of molecular activity, this world of electric vibrations and oscillations, that the many transformations of energy take place. This is the hiding-place of the lightning, of the electrons which moulded together make the thunderbolt. What an underworld of mystery and power it is! In it slumbers all the might and menace of the storm, the power that rends the earth and shakes the heavens. With the mind's eye one can see the indivisible atoms giving up their electrons, see the invisible hosts, in numbers beyond the power of mathematics to compute, being summoned and marshalled by some mysterious commander and hurled in terrible fiery phalanxes across the battlefield of the storm.

The physicist describes the atom and talks about it as if it were "a tangible body which one could hold in his hand like a baseball." "An atom," Sir Oliver Lodge says, "consists of a globular ma.s.s of positive electricity with minute negative electrons embedded in it." He speaks of the spherical form of the atom, and of its outer surface, of its centre, and of its pa.s.sing through other atoms, and of the electrons that revolve around its centre as planets around a sun. The electron, one hundred thousand times smaller than an atom, yet has surface, and that surface is a dimpled and corrugated sheet--like the cover of a mattress.

What a flight of the scientific imagination is that!

The disproportion between the size of an atom and the size of an electron is vastly greater than that between the sun and the earth.

Represent an atom, says Sir Oliver Lodge, by a church one hundred and sixty feet long, eighty feet broad, and forty feet high; the electrons are like gnats inside it. Yet on the electric theory of matter, electrons are all of the atom there is; there is no church, but only the gnats rushing about. We know of nothing so empty and hollow, so near a vacuum, as matter in this conception of it. Indeed, in the new physics, matter is only a hole in the ether. Hence the newspaper joke about the bank sliding down and leaving the woodchuck-hole sticking out, looks like pretty good physics. The electrons give matter its inertia, and give it the force we call cohesion, give it its toughness, its strength, and all its other properties. They make water wet, and the diamond hard.

They are the fountain-head of the immense stores of the inter-atomic energy, which, if it could be tapped and controlled, would so easily do all the work of the world. But this we cannot do. "We are no more competent," says Professor Soddy, "to make use of these supplies of atomic energy than a savage, ignorant of how to kindle a fire, could make use of a steam-engine." The natural rate of flow of this energy from its atomic sources we get as heat, and it suffices to keep life going upon this planet. It is the source of all the activity we see upon the globe. Its results, in the geologic ages, are stored up for us in coal and oil and natural gas, and, in our day, are available in the winds, the tides, and the waterfalls, and in electricity.

IV

The electric const.i.tution of matter is quite beyond anything we can imagine. The atoms are little worlds by themselves, and the whole mystery of life and death is in their keeping. The whole difference in the types of mind and character among men is supposed to be in their keeping. The different qualities and properties of bodies are in their keeping. Whether an object is hot or cold to our senses, depends upon the character of their vibrations; whether it be sweet or sour, poisonous or innocuous to us, depends upon how the atoms select their partners in the whirl and dance of their activities. The hardness and brilliancy of the diamond is supposed to depend upon how the atoms of carbon unite and join hands.

I have heard the view expressed that all matter, as such, is dead matter, that the molecules of hydrogen, oxygen, carbon, nitrogen, iron, phosphorus, calcium, and so on, in a living body, are themselves no more alive than the same molecules in inorganic matter. Nearly nine tenths of a living body is water; is not this water the same as the water we get at the spring or the brook? is it any more alive? does water undergo any chemical change in the body? is it anything more than a solvent, than a current that carries the other elements to all parts of the body? There are any number of chemical changes or reactions in a living body, but are the atoms and molecules that are involved in such changes radically changed? Can oxygen be anything but oxygen, or carbon anything but carbon? Is what we call life the result of their various new combinations? Many modern biologists hold to this view. In this conception merely a change in the order of arrangement of the molecules of a substance--which follows which or which is joined to which--is fraught with consequences as great as the order in which the letters of the alphabet are arranged in words, or the words themselves are arranged in sentences. The change of one letter in a word often utterly changes the meaning of that word, and the changing of a word in the sentence may give expression to an entirely different idea. Reverse the letters in the word "G.o.d," and you get the name of our faithful friend the dog.

Huxley and Tyndall both taught that it was the way that the ultimate particles of matter are compounded that makes the whole difference between a cabbage and an oak, or between a frog and a man. It is a hard proposition. We know with scientific certainty that the difference between a diamond and a piece of charcoal, or between a pearl and an oyster-sh.e.l.l, is the way that the particles of carbon in the one case, and of calcium carbide in the other, are arranged. We know with equal certainty that the difference between certain chemical bodies, like alcohol and ether, is the arrangement of their ultimate particles, since both have the same chemical formula. We do not spell acetic acid, alcohol, sugar, starch, animal fat, vegetable oils, glycerine, and the like, with the same letters; yet nature compounds them all of the same atoms of carbon, hydrogen, and oxygen, but in different proportions and in different orders.

Chemistry is all-potent. A mechanical mixture of two or more elements is a simple affair, but a chemical mixture introduces an element of magic. No conjurer's trick can approach such a transformation as that of oxygen and hydrogen gases into water. The miracle of turning water into wine is tame by comparison. Dip plain cotton into a mixture of nitric and sulphuric acids and let it dry, and we have that terrible explosive, guncotton. Or, take the cellulose of which cotton is composed, and add two atoms of hydrogen and one of oxygen, and we have sugar. But we are to remember that the difference here indicated is not a quant.i.tative, but a qualitative one, not one affecting bulk, but affecting structure.

Truly chemistry works wonders. Take ethyl alcohol, or ordinary spirits of wine, and add four more atoms of carbon to the carbon molecule, and we have the poison carbolic acid. Pure alcohol can be turned into a deadly poison, not by adding to, but simply by taking from it; take out one atom of carbon and two of hydrogen from the alcohol molecule, and we have the poison methyl alcohol. But we are to remember that the difference here indicated is not a quant.i.tative, but a qualitative one, not one affecting bulk, but affecting structure.

In our atmosphere we have a mechanical mixture of nitrogen and oxygen, four parts of nitrogen to one of oxygen. By uniting the nitrogen and oxygen chemically (N_{2}O) we have nitrous oxide, laughing-gas. Ordinary starch is made up of three different elements--six parts of carbon, ten parts of hydrogen, and five parts of oxygen (C_{6}H_{10}O_{5}). Now if we add water to this compound, we have a simple mixture of starch and water, but if we bring about a chemical union with the elements of water (hydrogen and oxygen), we have grape sugar. This sugar is formed in green leaves by the agency of sunlight, and is the basis of all plant and animal food, and hence one of the most important things in nature.

Carbon is a solid, and is seen in its pure state in the diamond, the hardest body in nature and the most valued of all precious stones, but it enters largely into all living bodies and is an important const.i.tuent of all the food we eat. As a gas, united with the oxygen of the air, forming carbon dioxide, it was present at the beginning of life, and probably helped kindle the first vital spark. In the shape of wood and coal, it now warms us and makes the wheels of our material civilization go round. Diamond stuff, through the magic of chemistry, plays one of the principle roles in our physical life; we eat it, and are warmed and propelled by it, and cheered by it. Taken as carbonic acid gas into our lungs, it poisons us; taken into our stomachs, it stimulates us; dissolved in water, it disintegrates the rocks, eating out the carbonate of lime which they contain. It is one of the princ.i.p.al actors in the drama of organized matter.

V

We have a good ill.u.s.tration of the power of chemistry, and how closely it is d.o.g.g.i.ng the footsteps of life, in the many organic compounds it has built up out of the elements, such as sugar, starch, indigo, camphor, rubber, and so forth, all of which used to be looked upon as impossible aside from life-processes. It is such progress as this that leads some men of science to believe that the creation of life itself is within the reach of chemistry. I do not believe that any occult or transcendental principle bars the way, but that some unknown and perhaps unknowable condition does, as mysterious and unrepeatable as that which separates our mental life from our physical. The trans.m.u.tation of the physical into the psychical takes place, but the secret of it we do not know. It does not seem to fall within the law of the correlation and the conservation of energy.

Free or single atoms are very rare; they all quickly find their mates or partners. This eagerness of the elements to combine is one of the mysteries. If the world of visible matter were at one stroke resolved into its const.i.tuent atoms, it would practically disappear; we might smell it, or taste it, if we were left, but we could not see it, or feel it; the water would vanish, the solid ground would vanish--more than half of it into oxygen atoms, and the rest mainly into silicon atoms.

The atoms of different bodies are all alike, and presumably each holds the same amount of electric energy. One wonders, then, how the order in which they are arranged can affect them so widely as to produce bodies so unlike as, say, alcohol and ether. This brings before us again the mystery of chemical arrangement or combination, so different from anything we know among tangible bodies. It seems to imply that each atom has its own individuality. Mix up a lot of pebbles together, and the result would be hardly affected by the order of the arrangement, but mix up a lot of people, and the result would be greatly affected by the fact of who is elbowing who. It seems the same among the mysterious atoms, as if some complemented or stimulated those next them, or had an opposite effect. But can we think of the atoms in a chemical compound as being next one another, or merely in juxtaposition? Do we not rather have to think of them as identified with one another to an extent that has no parallel in the world of ponderable bodies? A kind of sympathy or affinity makes them one in a sense that we only see realized among living beings.

Chemical activity is the first step from physical activity to vital activity, but the last step is taken rarely--the other two are universal. Chemical changes involve the atom. What do vital changes involve? We do not know. We can easily bring about the chemical changes, but not so the vital changes. A chemical change destroys one or more substances and produces others totally unlike them; a vital change breaks up substances and builds up other bodies out of them; it results in new compounds that finally cover the earth with myriads of new and strange forms.

X

THE VITAL ORDER

I

The mechanistic theory of life--the theory that all living things can be explained and fully accounted for on purely physico-chemical principles--has many defenders in our day. The main aim of the foregoing chapters is to point out the inadequacy of this view. At the risk of wearying my reader I am going to collect under the above heading a few more considerations bearing on this point.

A thing that grows, that develops, cannot, except by very free use of language, be called a machine. We speak of the body as a machine, but we have to qualify it by prefixing the adjective living--the living machine, which takes it out of the mechanical order of things fabricated, contrived, built up from without, and puts it in the order we call vital, the order of things self-developed from within, the order of things autonomous, as contrasted with things automatic. All the mechanical principles are operative in the life processes, but they have been vitalized, not changed in any way but in the service of a new order of reality. The heart with its chambers and valves is a pump that forces the blood through the system, but a pump that works itself and does not depend upon pneumatic pressure--a pump in which vital energy takes the place of gravitational energy. The peristaltic movement in the intestines involves a mechanical principle, but it is set up by an inward stimulus, and not by outward force. It is these inward stimuli, which of course involve chemical reactions, that afford the motive power for all living bodies and that put the living in another order from the mechanical. The eye is an optical instrument,--a rather crude one, it is said,--but it cannot be separated from its function, as can a mere instrument--the eye sees as literally as the brain thinks. In breathing we unconsciously apply the principle of the bellows; it is a bellows again which works itself, but the function of which, in a very limited sense, we can inhibit and control. An artificial, or man-made, machine always implies an artificer, but the living machine is not made in any such sense; it grows, it arises out of the organizing principle that becomes active in matter under conditions that we only dimly understand, and that we cannot reproduce.

The vital and the mechanical cooperate in all our bodily functions.

Swallowing our food is a mechanical process, the digestion of it is a chemical process and the a.s.similation and elimination of it a vital process. Inhaling and exhaling the air is a mechanical process, the oxidation of the blood is a chemical process, and the renewal of the corpuscles is a vital process. Growth, a.s.similation, elimination, reproduction, metabolism, and secretion, are all vital processes which cannot be described in terms of physics and chemistry. All our bodily movements--lifting, striking, walking, running--are mechanical, but seeing, hearing, and tasting, are of another order. And that which controls, directs, coordinates, and inhibits our activities belongs to a still higher order, the psychic. The world of thoughts and emotions within us, while dependent upon and interacting with the physical world without us, cannot be accounted for in terms of the physical world. A living thing is more than a machine, more than a chemical laboratory.

We can a.n.a.lyze the processes of a tree into their mechanical and chemical elements, but there is besides a kind of force there which we must call vital. The whole growth and development of the tree, its manner of branching and gripping the soil, its fixity of species, its individuality--all imply something that does not belong to the order of the inorganic, automatic forces. In the living animal how the psychic stands related to the physical or physiological and arises out of it, science cannot tell us, but the relation must be real; only philosophy can grapple with that question. To resolve the psychic and the vital into the mechanical and chemical and refuse to see any other factors at work is the essence of materialism.

II

Any contrivance which shows an interdependence of parts, that results in unity of action, is super-mechanical. The solar system may be regarded as a unit, but it has not the purposive unity of a living body. It is one only in the sense that its separate bodies are all made of one stuff, and obey the same laws and move together in the same direction, but a living body is a unit because all its parts are in the service of one purposive end. An army is a unit, a flock of gregarious birds, a colony of ants or bees, is a unit because the spirit and purpose of one is the spirit and purpose of all; the unity is psychological.

Only living bodies are adaptive. Adaptation, of course, has its physics or its chemistry, because it is a physical phenomenon; but there is no adaptation of a rock or a clay-bank to its environment; there is only mechanical and chemical adjustment. The influence of the environment may bring about chemical and physical changes in a non-living body, but they are not purposive as in a living body. The fat in the seeds of plants in northern countries is liquid and solid at a lower temperature than in tropical climates. Living organisms alone react in a formative or deformative way to external stimuli. In warm climates the fur of animals and the wool of sheep become thin and light. The colder the climate, the thicker these coverings. Such facts only show that in the matter of adaptation among living organisms, there is a factor at work other than chemistry and physics--not independent of them, but making a purposive use of them. Cut off the central shoot that leads the young spruce tree upwards, and one of the shoots from the whirl of lateral branches below it slowly rises up and takes the place of the lost leader. Here is an action not prompted by the environment, but by the morphological needs of the tree, and it ill.u.s.trates how different is its unity from the unity of a mere machine. I am only aiming to point out that in all living things the material forces behave in a purposive way to a degree that cannot be affirmed of them in non-living, and that, therefore, they imply intelligence.

Evidently the cells in the body do not all have the same degree of life,--that is, the same degree of irritability. The bone cells and the hair cells, for instance, can hardly be so much alive--or so irritable--as the muscle cells; nor these as intensely alive as the nerve and brain cells. Does not a bird possess a higher degree of life than a mollusk, or a turtle? Is not a brook trout more alive than a mud-sucker? You can freeze the latter as stiff as an icicle and resuscitate it, but not the former. There is a scale of degrees in life as clearly as there is a scale of degrees in temperature. There is an endless gradation of sensibilities of the living cells, dependent probably upon the degree of differentiation of function. Anaesthetics dull or suspend this irritability. The more highly developed and complex the nervous system, the higher the degree of life, till we pa.s.s from mere physical life to psychic life. Science might trace this difference to cell structure, but what brings about the change in the character of the cell, or starts the cells to building a complex nervous system, is a question unanswerable to science. The biologist imagines this and that about the invisible or hypothetical molecular structure; he a.s.signs different functions to the atoms; some are for endosmosis, others for contraction, others for conduction of stimuli. Intramolecular oxygen plays a part. Other names are given to the mystery--the micellar strings of Naegeli, the bioph.o.r.es of Weismann, the plastidules of Haeckel; they all presuppose millions of molecules peculiarly arranged in the protoplasm.