On the whole, then, we have positive knowledge of the existence, in the sun, stars, and planetary and stellar s.p.a.ces, of such a large proportion of the elements of our globe, and so few indications of any not forming part of it, that we are justified in the statement, that the whole stellar universe is, broadly speaking, constructed of the same series of elementary substances as those we can study upon our earth, and of which the whole realm of nature, animal, vegetable, and mineral, is composed. The evidence of this ident.i.ty of substance is really far more complete than we could expect, considering the very limited means of inquiry that we possess; and we shall, therefore, not be justified in a.s.suming that any important difference exists.

When we pa.s.s from the elements of matter to the laws which govern it, we also find the clearest proofs of ident.i.ty. That the fundamental law of gravitation extends to the whole physical universe is rendered almost certain by the fact that double stars move round their common centre of gravity in elliptical orbits which correspond well with both observation and calculation. That the laws of light are the same both here and in inter-planetary s.p.a.ce is indicated by the fact that the actual measurement of the velocity of light on the earth's surface gives a result so completely identical with that prevailing to the limits of the solar system, that the measurement of the sun's distance, by means of the eclipses of Jupiter's satellites combined with the measured velocity of light, agrees almost exactly with that obtained by means of the transits of Venus, or through our nearest approach to the planets Mars or Eros.

Again, the more recondite laws of light are found to be identical in sun and stars with those observed within the narrow bounds of laboratory experiments. The minute change of position of spectral lines caused by the source of light moving towards or away from us enables us to determine this kind of motion in the most distant stars, in the planets, or in the moon, and these results can be tested by the motion of the earth either in its...o...b..t or in its rotation; and these latter tests agree with the theoretical determination of what must occur, dependent on the wave-lengths of the different dark lines of the solar spectrum determined by measurements in the laboratory.

In like manner, minute changes in the widening or narrowing of spectral lines, their splitting up, their increase or decrease in number, and their arrangement so as to form flutings, can all be interpreted by experiments in the laboratory, showing that such phenomena are due to alterations of temperature, of pressure, or of the magnetic field, thus proving that the very same physical and chemical laws act in the same way here and in the remotest depths of s.p.a.ce.

These various discoveries give us the certain conviction that the whole material universe is essentially one, both as regards the action of physical and chemical laws, and also in its mechanical relations of form and structure. It consists throughout of the very same elements with which we are so familiar on our earth; the same ether whose vibrations bring us light and heat, electricity and magnetism, and a whole host of other mysterious and as yet imperfectly known forces; gravitation acts throughout its vast extent; and in whatever direction and by whatever means we obtain a knowledge of the stellar universe, we find the same mechanical, physical, and chemical laws prevailing as upon our earth, so that we have in some cases been actually enabled to reproduce in our laboratories phenomena with which we had first become acquainted in the sun or among the stars.

We may therefore feel it to be an almost certain conclusion that--the elements being the same, the laws which act upon, and combine, and modify those elements being the same--organised living beings wherever they may exist in this universe must be, fundamentally, and in essential nature, the same also. The outward forms of life, if they exist elsewhere, may vary almost infinitely, as they do vary on the earth; but, throughout all this variety of form--from fungus or moss to rose-bush, palm or oak; from mollusc, worm, or b.u.t.terfly to humming-bird, elephant, or man--the biologist recognises a fundamental unity of substance and of structure, dependent on the absolute requirements of the growing, moving, developing, living organism, built up of the same elements, combined in the same proportions, and subject to the same laws. We do not say that organic life _could_ not exist under altogether diverse conditions from those which we know or can conceive, conditions which may prevail in other universes constructed quite differently from ours, where other substances replace the matter and ether of our universe, and where other laws prevail. But, _within_ the universe we know, there is not the slightest reason to suppose organic life to be possible, except under the same general conditions and laws which prevail here. We will, therefore, now proceed to describe, very generally, what are the conditions essential to the existence and the continuous development of vegetable and animal life.

FOOTNOTE:

[10] _The Astrophysical Journal_, vol. xiv., July 1901, p. 17.

CHAPTER X

THE ESSENTIAL CHARACTERS OF THE LIVING ORGANISM

Before trying to comprehend the physical conditions on any planet which are essential for the development and maintenance of a varied and complex system of organic life comparable to that of our earth, we must obtain some knowledge of what life is, and of the fundamental nature and properties of the living organism.

Physiologists and philosophers have made many attempts to define 'life,'

but in most cases in aiming at absolute generality they have been vague and uninstructive. Thus De Blainville defined it as 'The twofold internal movement of composition and decomposition, at once general and continuous'; while Herbert Spencer's latest definition was 'Life is the continuous adjustment of internal relations to external relations.' But neither of these is sufficiently precise, explanatory, or distinctive, and they might almost be applied to the changes occurring in a sun or planet, or to the elevation and gradual formation of a continent. One of the oldest definitions, that of Aristotle, seems to come nearer the mark: 'Life is the a.s.semblage of the operations of nutrition, growth, and destruction.' But these definitions of 'life' are unsatisfactory, because they apply to an abstract idea rather than to the actual living organism. The marvel and mystery of life, as we know it, resides in the body which manifests it, and this living body the definitions ignore.

The essential points in the living body, as seen in its higher developments, are, first, that it consists throughout of highly complex but very unstable forms of matter, every particle of which is in a continual state of growth or decay; that it absorbs or appropriates dead matter from without; takes this matter into the interior of its body; acts upon it mechanically and chemically, rejecting what is useless or hurtful; and so transforming the remainder as to renew every atom of its own structure internal and external, at the same time throwing off, particle by particle, all the worn-out or dead portions of its own substance. Secondly, in order to be able to do all this, its whole body is permeated throughout by branching vessels or porous tissues, by which liquids and gases can reach every part and carry on the various processes of nutrition and excretion above referred to. As Professor Burdon Sanderson well puts it: 'The most distinctive peculiarity of living matter as compared with non-living is, that it is ever changing while ever the same.' And these changes are the more remarkable because they are accompanied, and even produced, by a very large amount of mechanical work--in animals by means of their normal activities in search of food, in a.s.similating that food, in continually renewing and building up their whole organism, and in many other ways; in plants by building up their structure, which often involves raising tons of material high into the air, as in forest trees. As a recent writer puts it: 'The most prominent, and perhaps the most fundamental, phenomenon of life is what may be described as the _Energy Traffic_ or the function of _trading in energy_. The chief physical function of living matter seems to consist in absorbing energy, storing it in a higher potential state, and afterwards partially expending it in the kinetic or active form.'[11]

Thirdly--and perhaps most marvellous of all--all living organisms have the power of reproduction or increase, in the lowest forms by a process of self-division or 'fission,' as it is termed, in the higher by means of reproductive cells, which, though in their earliest stage quite indistinguishable physically or chemically in very different species, yet possess the mysterious power of developing a perfect organism, identical with its parents in all its parts, shapes, and organs, and so wonderfully resembling them, that the minutest distinctive details of size, form, and colour, in hair or feathers, in teeth or claws, in scales, spines, or crests, are reproduced with very close accuracy, though often involving metamorphic changes during growth of so strange a nature that, if they were not familiar to us but were narrated as occurring only in some distant and almost inaccessible region, would be treated as travellers' tales, incredible and impossible as those of Sindbad the Sailor.

In order that the substance of living bodies should be able to undergo these constant changes while preserving the same form and structure in minute details--that they should be, as it were, in a constant state of flux while remaining sensibly unchanged, it is necessary that the molecules of which they are built up should be so combined as to be easily separated and as easily united--be, as it is termed, _labile_ or flowing; and this is brought about by their chemical composition, which, while consisting of few elements, is yet highly complex in structure, a large number of chemical atoms being combined in an endless variety of ways.

The physical basis of life, as Huxley termed it, is protoplasm, a substance which consists essentially of only four common elements, the three gases, nitrogen, hydrogen, and oxygen, with the non-metallic solid, carbon; hence all the special products of plants and animals are termed carbon-compounds, and their study const.i.tutes one of the most extensive and intricate branches of modern chemistry. Their complexity is indicated by the fact that the molecule of sugar contains 45, and that of stearine no less than 173, const.i.tuent atoms. The chemical compounds of carbon are far more numerous than those of all the other chemical elements combined; and it is this wonderful variety and the complexity of its possible combinations which explain the fact, that all the various animal tissues--skin, horn, hair, nails, teeth, muscle, nerve, etc., consist of the same four elements (with occasionally minute quant.i.ties of sulphur, phosphorus, lime, or silica, in some of them), as proved by the marvellous fact that these tissues are all produced as well by the gra.s.s-eating sheep or ox as by the fish or flesh-eating seal or tiger. And the marvel is still further increased when we consider that the innumerable diverse substances produced by plants and animals are all formed out of the same three or four elements. Such are the endless variety of organic acids, from prussic acid to those of the various fruits; the many kinds of sugars, gums, and starches; the number of different kinds of oil, wax, etc.; the variety of essential oils which are mostly forms of turpentines, with such substances as camphor, resins, caoutchouc, and gutta-percha; and the extensive series of vegetable alkaloids, such as nicotine from tobacco, morphine from opium, strychnine, curarine, and other poisons; quinine, belladonna, and similar medicinal alkaloids; together with the essential principles of our refres.h.i.+ng drinks, tea, coffee, and cocoa, and others too numerous to be named here--all alike consisting solely of the four common elements from which almost our whole organism is built up. If this were not indisputably proved, it would scarcely be credited.

Professor F.J. Allen considers that the most important element in protoplasm, and that which confers upon it its most essential properties in the living organism--its extreme mobility and transposibility--is nitrogen.

This element, though inert in itself, readily enters into compounds when energy is supplied to it, the most striking ill.u.s.tration of which is the formation of ammonia, a compound of nitrogen and hydrogen, produced by electric discharges through the atmosphere. Ammonia, and certain oxides of nitrogen produced in the atmosphere in the same way, are the chief sources of the nitrogen a.s.similated by plants, and through them by animals; for although plants are continually in contact with the free nitrogen of the atmosphere, they are unable to absorb it. By their leaves they absorb oxygen and carbon-dioxide to build up their woody tissues, while by their roots they absorb water in which ammonia and oxides of nitrogen are dissolved, and from these they produce the protoplasm which builds up the whole substance of the animal world. The energy required to produce these nitrogen-compounds is given up by them when undergoing further changes, and thus the production of ammonia by electricity in the atmosphere, and its being carried by rain into the soil, const.i.tute the first steps in that long series of operations which culminates in the production of the higher forms of life.

But the remarkable transformations and combinations continually going on in every living body, which are, in fact, the essential conditions of its life, are themselves dependent on certain physical conditions which must be always present. Professor Allen remarks: 'The sensitiveness of nitrogen, its p.r.o.neness to change its state of combination and energy, appear to depend on certain conditions of temperature, pressure, etc., which exist at the surface of this earth. Most vital phenomena occur between the temperature of freezing water and 104 F. If the general temperature of the earth's surface rose or fell 72 F. (a small amount relatively), the whole course of life would be changed, even perchance to extinction.'

Another important, and even more essential fact, in connection with life, is the existence in the atmosphere of a small but nearly constant proportion of carbonic acid gas, this being the source from which the whole of the carbon in the vegetable and animal kingdoms is primarily derived.

The leaves of plants absorb carbonic acid gas from the atmosphere, and the peculiar substance, chlorophyll, from which they derive their green colour, has the power, under the influence of sunlight, to decompose it, using the carbon to build up its own structure and giving out the oxygen. In the laboratory the carbon can only be separated from the oxygen by the application of heat, under which certain metals burn by combining with the oxygen, thus setting free the carbon. Chlorophyll has a highly complex chemical structure very imperfectly known, but it is said to be only produced when there is iron in the soil.

The leaves of plants, so often looked upon as mere ornamental appendages, are among the most marvellous structures in living organisms, since in decomposing carbonic acid at ordinary temperatures they do what no other agency in nature can perform. In doing this they utilise a special group of ether-waves which alone appear to have this power. The complexity of the processes going on in leaves is well indicated in the following quotation:--

'We have seen how green leaves are supplied with gases, water, and dissolved salts, and how they can trap special ether-waves. The active energy of these waves is used to trans.m.u.te the simple inorganic compounds into complex organic ones, which in the process of respiration are reduced to simpler substances again, and the potential energy transformed into kinetic. These metabolic changes take place in living cells full of intense activities. Currents course through the protoplasm and cell-sap in every direction, and between the cells which are also united by strands of protoplasm. The gases used and given off in respiration and a.s.similation are floated in and out, and each protoplasm particle burned or unburned is the centre of an area of disturbance. Pure protoplasm is influenced equally by all rays: that a.s.sociated with chlorophyll is affected by certain red and violet rays in particular. These, especially the red ones, bring about the dissociation of the elements of the carbonic acid, the a.s.similation of the carbon, and the excretion of the oxygen.'[12]

It is this vigorous life-activity ever at work in the leaves, the roots, and the sap-cells, that builds up the plant, in all its wondrous beauty of bud and foliage, flower and fruit; and at the same time produces, either as useful or waste-products, all that wealth of odours and flavours, of colours and textures, of fibres and varied woods, of roots and tubers, of gums and oils and resins innumerable, that, taken altogether, render the world of vegetable life perhaps more varied, more beautiful, more enjoyable, more indispensable to our higher nature than even that of animals. But there is really no comparison between them. We _could_ have plants without animals; we could _not_ have animals without plants. And all this marvel and mystery of vegetable life, a mystery which we rarely ponder over because its effects are so familiar, is usually held to be sufficiently explained by the statement that it is all due to the special properties of protoplasm. Well might Huxley say, that protoplasm is not only a substance but a structure or mechanism, a mechanism kept at work by solar heat and light, and capable of producing a thousand times more varied and marvellous results than all the human mechanism ever invented.

But besides absorbing carbonic acid from the atmosphere, separating and utilising the carbon and giving out the oxygen, plants as well as animals continually absorb oxygen from the atmosphere, and this is so universally the case that oxygen is said to be the food of protoplasm, without which it cannot continue to live; and it is the peculiar but quite invisible structure of the protoplasm which enables it to do this, and also in plants to absorb an enormous amount of water as well.

But although protoplasm is so complex chemically as to defy exact a.n.a.lysis, being an elaborate structure of atoms built up into a molecule in which each atom must occupy its true place (like every carved stone in a Gothic cathedral), yet it is, as it were, only the starting-point or material out of which the infinitely varied structures of living bodies are formed. The extreme mobility and changeability of the structure of these molecules enables the protoplasm to be continually modified both in const.i.tution and form, and, by the subst.i.tution or addition of other elements, to serve special purposes. Thus when sulphur in small quant.i.ties is absorbed and built into the molecular structure, proteids are formed. These are most abundant in animal structures, and give the nouris.h.i.+ng properties to meat, cheese, eggs, and other animal foods; but they are also found in the vegetable kingdom, especially in nuts and seeds such as grain, peas, etc.

These are generally known as nitrogenous foods, and are very nutritious, but not so easily digestible as meat. Proteids exist in very varied forms and often contain phosphorus as well as sulphur, but their main characteristic is the large proportion of nitrogen they contain, while many other animal and vegetable products, as most roots, tubers, and grains, and even fats and oils, are mainly composed of starch and sugar. In its chemical and physiological aspects protein is thus described by Professor W.D. Haliburton:--'Proteids are produced only in the living laboratory of animals and plants; proteid matter is the all-important material present in protoplasm. This molecule is the most complex that is known; it always contains five and often six or even seven elements. The task of thoroughly understanding its composition is necessarily vast, and advance slow. But, little by little, the puzzle is being solved, and this final conquest of organic chemistry, when it does arrive, will furnish physiologists with new light on many of the dark places of physiological science.'[13]

What makes protoplasm and its modifications still more marvellous is the power it possesses of absorbing and moulding a number of other elements in various parts of living organisms for special uses. Such are silica in the stems of the gra.s.s family, lime and magnesia in the bones of animals, iron in blood, and many others. Besides the four elements const.i.tuting protoplasm, most animals and plants contain also in some parts of their structure sulphur, phosphorus, chlorine, silicon, sodium, pota.s.sium, calcium, magnesium, and iron; while, less frequently, fluorine, iodine, bromine, lithium, copper, manganese, and aluminium are also found in special organs or structures; and the molecules of all these are carried by the protoplasmic fluids to the places where they are required and built into the living structure, with the same precision and for similar ends as brick and stone, iron, slate, wood, and gla.s.s are each utilised in their proper places in any large building.[14] The organism, however, is not built, but grows. Every organ, every fibre, cell, or tissue is formed from diverse materials, which are first decomposed into their elementary molecules, transformed by the protoplasm or by special solvents formed from it, carried to the places where they are needed by the vital fluids, and there built up atom by atom or molecule by molecule into the special structures of which they are to form a part.

But even this marvel of growth and repair of every individual organism is far surpa.s.sed by the greater marvel of reproduction. Every living thing of the higher orders arises from a single microscopic cell, when fertilised, as it is termed, by the absorption of another microscopic cell derived from a different individual. These cells are often, even under the highest powers of the microscope, hardly distinguishable from other cells which occur in all animals and plants and of which their structure is built up; yet these special cells begin to grow in a totally different manner, and instead of forming one particular part of the organism, develop inevitably into a complete living thing with all the organs, powers, and peculiarities of its parents, so as to be recognisably of the same species. If the simple growth of the fully formed organism is a mystery, what of this growth of thousands of complex organisms each with all its special peculiarities, yet all arising from minute germs or cells the diverse natures of which are wholly indistinguishable by the highest powers of the microscope? This, too, is said to be the work of protoplasm under the influence of heat and moisture, and modern physiologists hope some day to learn 'how it is done.' It may be well here to give the views of a modern writer on this point. Referring to a difficulty which had been stated by Clerk-Maxwell twenty-five years ago, that there was not room in the reproductive cell for the millions of molecules needed to serve as the units of growth for all the different structures in the body of the higher animals, Professor M'Kendrick says:--'But to-day, it is reasonable from existing data to suppose that the germinal vesicle might contain a million of millions of organic molecules. Complex arrangements of these molecules suited for the development of all the parts of a highly complicated organism, might satisfy all the demands of the theory of heredity.

Doubtless the germ was a material system through and through. The conception of the physicist was, that molecules were in various states of movement; and the thinkers were striving toward a kinetic theory of molecules and of atoms of solid matter, which might be as fruitful as the kinetic theory of gases. There were motions atomic and molecular. It was conceivable that the peculiarities of vital action might be determined by the kind of motion that took place in the molecules of what we call living matter. It might be different in kind from some of the motions dealt with by physicists. Life is continually being created from non-living material--such, at least, is the existing view of growth by the a.s.similation of food. The creation of living matter out of non-living may be the transmission to the dead matter of molecular motions which are _sui generis_ in form.' This is the modern physiological view of 'how it may be done,' and it seems hardly more intelligible than the very old theory of the origin of stone axes, given by Adria.n.u.s Tollius in 1649, and quoted by Mr. E.B. Tylor, who says:--'He gives drawings of some ordinary stone axes and hammers and tells how naturalists say that they are generated in the sky by a fulgureous exhalation conglobed in a cloud by the circ.u.mfixed humour, and are, as it were, baked hard by intense heat, and the weapon becomes pointed by the damp mixed with it flying from the dry part, and leaving the other end denser, but the exhalations press it so hard that it breaks through the cloud and makes thunder and lightning. But--he says--if this is really the way in which they are generated, it is odd they are not round, and that they have holes through them. It is hardly to be believed, he thinks.'[15] And so, when the physiologists, determined to avoid the a.s.sumption of anything beyond matter and motion in the germ, impute the whole development and growth of the elephant or of man from minute cells internally alike, by means of 'kinds of motion' and the 'transmission of motions which are _sui generis_ in form,' many of us will be inclined to say with the old author--'It is hardly to be believed, I think.'

This brief statement of the conclusions arrived at by chemists and physiologists as to the composition and structure of organised living things has been thought advisable, because the non-scientific reader has often no conception of the incomparable marvel and mystery of the life-processes he has always seen going on, silently and almost unnoticed, in the world around him. And this is still more the case now that two-thirds of our population are crowded into cities where, removed from all the occupations, the charms, and the interests of country life, they are driven to seek occupation and excitement in the theatre, the music-hall, or the tavern. How little do these know what they lose by being thus shut out from all quiet intercourse with nature; its soothing sights and sounds; its exquisite beauties of form and colour; its endless mysteries of birth, and life, and death. Most people give scientific men credit for much greater knowledge than they possess in these matters; and many educated readers will, I feel sure, be surprised to find that even such apparently simple phenomena as the rise of the sap in trees are not yet completely explained. As to the deeper problems of life, and growth, and reproduction, though our physiologists have learned an infinite amount of curious or instructive facts, they can give us no intelligible explanation of them.

The endless complexities and confusing amount of detail in all treatises on the physiology of animals and plants are such, that the average reader is overwhelmed with the ma.s.s of knowledge presented to him, and concludes that after such elaborate researches everything must be known, and that the almost universal protests against the need of any causes but the mechanical, physical, and chemical laws and forces are well founded. I have, therefore, thought it advisable to present a kind of bird's-eye view of the subject, and to show, in the words of the greatest living authorities on these matters, both how complex are the phenomena and how far our teachers are from being able to give us any adequate explanation of them.

I venture to hope that the very brief sketch of the subject I have been able to give will enable my readers to form some faint general conception of the infinite complexity of life and the various problems connected with it; and that they will thus be the better enabled to appreciate the extreme delicacy of those adjustments, those forces, and those complex conditions of the environment, that alone render life, and above all the grand age-long panorama of the development of life, in any way possible. It is to these conditions, as they prevail in the world around us, that we will now direct our attention.

FOOTNOTES:

[11] Professor F.J. Allen: _What is Life?_

[12] Art. 'Vegetable Physiology' in _Chambers's Encyclopaedia_.

[13] Address to the British a.s.sociation, 1902, Section Physiology.

[14] This enumeration of the elements that enter into the structure of plants and animals is taken from Professor F.J. Allen's paper already referred to.

[15] _Early History of Mankind_, 2nd ed. p. 227.

CHAPTER XI

THE PHYSICAL CONDITIONS ESSENTIAL FOR ORGANIC LIFE

The physical conditions on the surface of our earth which appear to be necessary for the development and maintenance of living organisms may be dealt with under the following headings:--

1. Regularity of heat-supply, resulting in a limited range of temperature.

2. A sufficient amount of solar light and heat.

3. Water in great abundance, and universally distributed.

4. An atmosphere of sufficient density, and consisting of the gases which are essential for vegetable and animal life. These are Oxygen, Carbonic-acid gas, Aqueous vapour, Nitrogen, and Ammonia. These must all be present in suitable proportions.

5. Alternations of day and night.