I.—Chemical Needs of Life

Animals, unlike plants, require highly organised atoms for nutriment; they can subsist only upon parts of an organism. All parts of the animal body are produced from the fluid circulating within its organism. A destruction of the animal body is constantly proceeding, every motion is the result of a transformation of its structure; every thought, every sensation is accompanied by a change in the composition of the substance of the brain. Food is applied either in the increase of the mass of a structure (nutrition) or in the replacement of a structure wasted (reproduction).

Equally important is the continual absorption of oxygen from the atmosphere. All vital activity results from the mutual action of the oxygen of the atmosphere and the elements of food. According to Lavoisier, an adult man takes into his system every year 827 lb. of oxygen, and yet he does not increase in weight. What, then, becomes of this oxygen?—for no part of it is again expired as oxygen. The carbon and hydrogen of certain parts of the body have entered into combination with the oxygen introduced through the lungs and through the skin, and have been given out in the form of carbonic acid and the vapour of water.

Now, an adult inspires 32-1/2 oz. of oxygen daily; this will convert the carbon of 24 lb. of blood (80 per cent. water) into carbonic acid. He must, therefore, take as much nutriment as will supply the daily loss. And, in fact, it is found that he does so; for the average amount of carbon in the daily food of an adult man is 14 oz., which requires 37 oz. of oxygen for its conversion into carbonic acid. The amount of food necessary for the support of the animal body must be in direct ratio to the quantity of oxygen taken into the system. A bird deprived of food dies on the third day; while a serpent, which inspires a mere trace of oxygen, can live without food for three months. The number of respirations is less in a state of rest than in exercise, and the amount of food necessary in both conditions must vary also.

The capacity of the chest being a constant quantity, we inspire the same volume of air whether at the pole or at the equator; but the weight of air, and consequently of oxygen, varies with the temperature. Thus, an adult man takes into the system daily 46,000 cubic inches of oxygen, which, if the temperature be 77 deg. F., weighs 32-1/2 oz., but when the temperature sinks to freezing-point will weigh 35 oz. It is obvious, also, that in an equal number of respirations we consume more oxygen at the level of the sea than on a mountain. The quantity of oxygen inspired and carbonic acid expired must, therefore, vary with the height of the barometer. In our climate the difference between summer and winter in the carbon expired, and therefore necessary for food, is as much as one-eighth.

II.—The Cause of Animal Heat

Now, the mutual action between the elements of food and the oxygen of the air is the source of animal heat.

This heat is wholly due to the combustion of the carbon and hydrogen in the food consumed. Animal heat exists only in those parts of the body through which arterial blood (and with it oxygen in solution) circulates; hair, wool, or feathers, do not possess an elevated temperature.

As animal heat depends upon respired oxygen, it will vary according to the respiratory apparatus of the animal. Thus the temperature of a child is 102 deg. F., while that of an adult is 99-1/2 deg. F. That of birds is higher than that of quadrupeds or that of fishes or amphibia, whose proper temperature is 3 deg. F higher than the medium in which they live. All animals, strictly speaking, are warm-blooded; but in those only which possess lungs is their temperature quite independent of the surrounding medium. The temperature of the human body is the same in the torrid as in the frigid zone; but the colder the surrounding medium the greater the quantity of fuel necessary to maintain its heat.

The human body may be aptly compared to the furnace of a laboratory destined to effect certain operations. It signifies nothing what intermediate forms the food, or fuel, of the furnace may assume; it is finally converted into carbonic acid and water. But in order to sustain a fixed temperature in the furnace we must vary the quantity of fuel according to the external temperature.

In the animal body the food is the fuel; with a proper supply of oxygen we obtain the heat given out during its oxidation or combustion. In winter, when we take exercise in a cold atmosphere, and when consequently the amount of inspired oxygen increases, the necessity for food containing carbon and hydrogen increases in the same ratio; and by gratifying the appetite thus excited, we obtain the most efficient protection against the most piercing cold. A starving man is soon frozen to death; and everyone knows that the animals of prey in the Arctic regions far exceed in voracity those in the torrid zone. In cold and temperate climates, the air, which incessantly strives to consume the body, urges man to laborious efforts in order to furnish the means of resistance to its action, while in hot climates the necessity of labour to provide food is far less urgent.

Our clothing is merely the equivalent for a certain amount of food.

The more warmly we are clothed the less food we require. If in hunting or fishing we were exposed to the same degree of cold as the Samoyedes we could with ease consume ten pounds of flesh, and perhaps half a dozen tallow candles into the bargain. The macaroni of the Italian, and the train oil of the Greenlander and the Russian, are fitted to administer to their comfort in the climate in which they have been born.

The whole process of respiration appears most clearly developed in the case of a man exposed to starvation. Currie mentions the case of an individual who was unable to swallow, and whose body lost 100 lb. in one month. The more fat an animal contains the longer will it be able to exist without food, for the fat will be consumed before the oxygen of the air acts upon the other parts of the body.

There are various causes by which force or motion may be produced. But in the animal body we recognise as the ultimate cause of all force only one cause, the chemical action which the elements of the food and the oxygen of the air mutually exercise on each other. The only known ultimate cause of vital force, either in animals or in plants, is a chemical process. If this be prevented, the phenomena of life do not manifest themselves, or they cease to be recognisable by our senses. If the chemical action be impeded, the vital phenomena must take new forms.

The heat evolved by the combustion of carbon in the body is sufficient to account for all the phenomena of animal heat. The 14 oz. of carbon which in an adult are daily converted into carbonic acid disengage a quantity of heat which would convert 24 lb. of water, at the temperature of the body, into vapour. And if we assume that the quantity of water vaporised through the skin and lungs amounts to 3 lb., then we have still a large quantity of heat to sustain the temperature of the body.

III.—The Chemistry of Blood-Making

Physiologists conceive that the various organs in the body have originally been formed from blood. If this be admitted, it is obvious that those substances alone can be considered nutritious that are capable of being transformed into blood.

When blood is allowed to stand, it coagulates and separates into a watery fluid called serum, and into the clot, which consists principally of fibrine. These two bodies contain, in all, seven elements, among which sulphur, phosphorus, and nitrogen are found; they contain also the earth of bones. The serum holds in solution common salt and other salts of potash and soda, of which the acids are carbonic, phosphoric, and sulphuric acids. Serum, when heated, coagulates into a white mass called albumen. This substance, along with the fibrine and a red colouring matter in which iron is a constituent, constitute the globules of blood.

Analysis has shown that fibrine and albumen are perfectly identical in chemical composition. They may be mutually converted into each other. In the process of nutrition both may be converted into muscular fibre, and muscular fibre is capable of being reconverted into blood.

All parts of the animal body which form parts of organs contain nitrogen. The principal ingredients of blood contain 17 per cent. of nitrogen, and there is no part of an active organ that contains less than 17 per cent. of this element.

The nutritive process is simplest in the case of the carnivora, for their nutriment is chemically identical in composition with their own tissues. The digestive apparatus of graminivorous animals is less simple, and their food contains very little nitrogen. From what constituents of vegetables is their blood produced?

Chemical researches have shown that all such parts of vegetables as can afford nutriment to animals contain certain constituents which are rich in nitrogen; and experience proves that animals require for their nutrition less of these parts of plants in proportion as they abound in the nitrogenised constituents. These important products are specially abundant in the seeds of the different kinds of grain, and of peas, beans, and lentils. They exist, however, in all plants, without exception, and in every part of plants in larger or smaller quantity. The nitrogenised compounds of vegetables are called vegetable fibrine, vegetable albumen, and vegetable casein. All other nitrogenised compounds occurring in plants are either rejected by animals or else they occur in the food in such very small proportion that they cannot possibly contribute to the increase of mass in the animal body.

The chemical analysis of these three substances has led to the interesting result that they contain the same organic elements, united in the same proportion by weight; and—which is more remarkable—that they are identical in composition with the chief constituents of blood—animal fibrine and animal albumen. By identity, be it remarked, is not here meant merely similarity, but that even in regard to the presence and relative amounts of sulphur, phosphorus, and phosphate of lime no difference can be observed.

How beautifully simple then, by the aid of these discoveries, appears the process of nutrition in animals, the formation of their organs, in which vitality chiefly resides. Those vegetable constituents which are used by animals to form blood contain the essential ingredients of blood ready formed. In point of fact, vegetables produce in their organism the blood of all animals; for the carnivora, in consuming the blood and flesh of the graminivora, consume, strictly speaking, the vegetable principles which have served for the nourishment of the latter. In this sense we may say the animal organism gives to blood only its form; and, further, that it is incapable of forming blood out of other compounds which do not contain the chief ingredients of that fluid.

Animal and vegetable life are, therefore, closely related, for the first substance capable of affording nutriment to animals is the last product of the creative energy of vegetables. The seemingly miraculous in the nutritive power of vegetables disappears in a great degree, for the production of the constituents of blood cannot appear more surprising than the occurrence of the principal ingredient of butter in palm-oil and of horse-fat and train-oil in certain of the oily seeds.

IV.—Food the Fuel of Life

We have still to account for the use in food of substances which are destitute of nitrogen but are known to be necessary to animal life. Such substances are starch, sugar, gum, and pectine. In all of these we find a great excess of carbon, with oxygen and hydrogen in the same proportion as water. They therefore add an excess of carbon to the nitrogenised constituents of food, and they cannot possibly be employed in the production of blood, because the nitrogenised compounds contained in the food already contain exactly the amount of carbon which is required for the production of fibrine and albumen. Now, it can be shown that very little of the excess of this carbon is ever expelled in the form either of solid or liquid compounds; it must be expelled, therefore, in the gaseous state. In short, these compounds are solely expended in the production of animal heat, being converted by the oxygen of the air into carbonic acid and water. The food of carnivorous animals does not contain non-nitrogenised matters, so that the carbon and hydrogen necessary for the production of animal heat are furnished in them from the waste of their tissues.

The transformed matters of the organs are obviously unfit for the further nourishment of the body—that is, for the increase or reproduction of the mass. They pass through the absorbent and lymphatic vessels into the veins, and their accumulation in these would soon put a stop to the nutritive process were it not that the blood has to pass through a filtering apparatus, as it were, before reaching the heart. The venous blood, before returning to the heart, is made to pass through the liver and the kidneys, which separate from it all substances incapable of contributing to nutrition. The new compounds containing the nitrogen of the transformed organs, being utterly incapable of further application in the system, are expelled from the body. Those which contain the carbon of the transformed tissues are collected in the gall-bladder as bile, a compound of soda which, being mixed with water, passes through the duodenum and mixes with chyme. All the soda of the bile, and ninety-nine-hundredths of the carbonaceous matter which it contains, retain the capacity of re-absorption by the absorbents of the small and large intestines—a capacity which has been proved by direct experiment.

The globules of the blood, which in themselves can be shown to take no share in the nutritive process, serve to transport the oxygen which they give up in their passage through the capillary vessels. Here the current of oxygen meets with the carbonaceous substances of the transformed tissues, and converts their carbon into carbonic acid, their hydrogen into water. Every portion of these substances which escapes this process of oxidation is sent back into the circulation in the form of bile, which by degrees completely disappears.

It is obvious that in the system of the graminivora, whose food contains relatively so small a proportion of the constituents of blood, the process of metamorphosis in existing tissues, and consequently their restoration or reproduction, must go on far less rapidly than in the carnivora. Otherwise, a vegetation a thousand times as luxuriant would not suffice for their sustenance. Sugar, gum, and starch, which form so large a proportion of their food, would then be no longer necessary to support life in these animals, because in that case the products of waste, or metamorphosis of organised tissues, would contain enough carbon to support the respiratory process.

When exercise is denied to graminivorous and omnivorous animals this is tantamount to a deficient supply of oxygen. The carbon of the food, not meeting with a sufficient supply of oxygen to consume it, passes into other compounds containing a large excess of carbon—or, in other words, fat is produced. Fat is thus an abnormal production, resulting from a disproportion of carbon in the food to that of the oxygen respired by the lungs or absorbed by the skin. Wild animals in a state of nature do not contain fat. The production of fat is always a consequence of a deficient supply of oxygen, for oxygen is absolutely indispensable for the dissipation of excess of carbon in the food.

V.—Animal Life-Chemistry

The substances of which the food of man is composed may be divided into two classes—into nitrogenised and non-nitrogenised. The former are capable of conversion into blood, the latter incapable of this transformation. Out of those substances which are adapted to the formation of blood are formed all the organised tissues. The other class of substances in the normal state of health serve to support the process of respiration. The former may be called the plastic elements of nutrition; the latter, elements of respiration.

Among the former we may reckon—vegetable fibrine, vegetable albumen, vegetable casein, animal flesh, animal blood.

Among the elements of respiration in our food are—fat, starch, gum, cane sugar, grape-sugar, sugar of milk, pectine, bassorine, wine, beer, spirits.

The nitrogenised constituents of vegetable food have a composition identical with that of the constituents of the blood.

No nitrogenised compound the composition of which differs from that of fibrine, albumen, and casein, is capable of supporting the vital process in animals.

The animal organism undoubtedly possesses the power of forming from the constituents of its blood the substance of its membranes and cellular tissue, of the nerves and brain, of the organic part of cartilages and bones. But the blood must be supplied to it ready in everything but its form—that is, in its chemical composition. If this is not done, a period is put to the formation of blood, and, consequently, to life.

The whole life of animals consists of a conflict between chemical forces and the vital power. In the normal state of the body of an adult these stand in equilibrium: that is, there is equilibrium between the manifestations of the causes of waste and the causes of supply. Every mechanical or chemical agency which disturbs the restoration of this equilibrium is a cause of disease.

Death is that condition in which chemical or mechanical powers gain the ascendancy, and all resistance on the part of the vital force ceases. This resistance never entirely departs from living tissues during life. Such deficiency in resistance is, in fact, a deficiency in resistance to the action of the oxygen of the atmosphere.

Disease occurs when the sum of vital force, which tends to neutralise all causes of disturbance, is weaker than the acting cause of disturbance.

Should there be formed in the diseased parts, in consequence of the change of matter, from the elements of the blood or of the tissue, new products which the neighbouring parts cannot employ for their own vital functions; should the surrounding parts, moreover, be unable to convey these products to other parts where they may undergo transformation, then these new products will suffer, at the place where they have been formed, a process of decomposition analogous to putrefaction.

In certain cases, medicine removes these diseased conditions by exciting in the vicinity of the diseased part, or in any convenient situation, an artificial diseased state (as by blisters), thus diminishing by means of artificial disturbance the resistance offered to the external causes of change in these parts by the vital force. The physician succeeds in putting an end to the original diseased condition when the disturbance artificially excited (or the diminution of resistance in another part) exceeds in amount the diseased state to be overcome.

The accelerated change of matter and the elevated temperature in the diseased part show that the resistance offered by the vital force to the action of oxygen is feebler than in the healthy state. But this resistance only ceases entirely when death takes place. By the artificial diminution of resistance in another part, the resistance in the diseased organ is not, indeed, directly strengthened; but the chemical action, the cause of the change of matter, is diminished in the diseased part, being directed to another part, where the physician has succeeded in producing a still more feeble resistance to the change of matter, to the action of oxygen.



SIR CHARLES LYELL

The Principles of Geology

Sir Charles Lyell, the distinguished geologist, was born at Kinnordy, Forfarshire, Scotland, Nov. 14, 1797. It was at Oxford that his scientific interest was first aroused, and after taking an M.A. degree in 1821 he continued his scientific studies, becoming an active member of the Geological and Linnaean Societies of London. In 1826 he was elected a fellow of the Royal Society, and two years later went with Sir Roderick Murchison on a tour of Europe, and gathered evidence for the theory of geological uniformity which he afterwards promulgated. In 1830 he published his great work, "Principles of Geology: Being an Attempt to Explain the Former Changes of the Earth's Surface by References to Causes now in Action," which converted almost the whole geological world to the doctrine of uniformitarianism, and may be considered the foundation of modern geology. Lyell died in London on February 22, 1875. Besides his great work, he also published "The Elements of Geology," "The Antiquity of Man," "Travels in North America," and "The Student's Elements of Geology."

I.—Uniformity in Geological Development

According to the speculations of some writers, there have been in the past history of the planet alternate periods of tranquillity and convulsion, the former enduring for ages, and resembling the state of things now experienced by man; the other brief, transient, and paroxysmal, giving rise to new mountains, seas, and valleys, annihilating one set of organic beings, and ushering in the creation of another. These theories, however, are not borne out by a fair interpretation of geological monuments; but, on the contrary, nature indicates no such cataclysms, but rather progressive uniformity.

Igneous rocks have been supposed to afford evidence of ancient paroxysms of nature, but we cannot consider igneous rocks proof of any exceptional paroxysms. Rather, we find ourselves compelled to regard igneous rocks as an aggregate effect of innumerable eruptions, of various degrees of violence, at various times, and to consider mountain chains as the accumulative results of these eruptions. The incumbent crust of the earth is never allowed to attain that strength and coherence which would be necessary in order to allow the volcanic force to accumulate and form an explosive charge capable of producing a grand paroxysmal eruption. The subterranean power, on the contrary, displays, even in its most energetic efforts, an intermittent and mitigated intensity. There are no proofs that the igneous rocks were produced more abundantly at remote periods.

Nor can we find proof of catastrophic discontinuity when we examine fossil plants and fossil animals. On the contrary, we find a progressive development of organic life at successive geological periods.

In Palaeozoic strata the entire want of plants of the most complex organisation is very striking, for not a single dicotyledonous angiosperm has yet been found, and only one undoubted monocotyledon. In Secondary, or Mesozoic, times, palms and some other monocotyledons appeared; but not till the Upper Cretaceous era do we meet with the principal classes and orders of the vegetable kingdom as now known. Through the Tertiary ages the forms were perpetually changing, but always becoming more and more like, generically and specifically, to those now in being. On the whole, therefore, we find progressive development of plant life in the course of the ages.

In the case of animal life, progression is equally evident. Palaeontological research leads to the conclusion that the invertebrate animals flourished before the vertebrate, and that in the latter class fish, reptiles, birds, and mammalia made their appearance in a chronological order analogous to that in which they would be arranged zoologically according to an advancing scale of perfection in their organisation. In regard to the mammalia themselves, they have been divided by Professor Owen into four sub-classes by reference to modifications of their brain. The two lowest are met with in the Secondary strata. The next in grade is found in Tertiary strata. And the highest of all, of which man is the sole representative, has not yet been detected in deposits older than the Post-Tertiary.

It is true that in passing from the older to the newer members of the Tertiary system we meet with many chasms, but none which separate entirely, by a broad line of demarcation, one state of the organic world from another. There are no signs of an abrupt termination of one fauna and flora, and the starting into life of new and wholly distinct forms. Although we are far from being able to demonstrate geologically an insensible transition from the Eocene to the Miocene, or even from the latter to the recent fauna, yet the more we enlarge and perfect our general survey the more nearly do we approximate to such a continuous series, and the more gradually are we conducted from times when many of the genera and nearly all the species were extinct to those in which scarcely a single species flourished which we do not know to exist at present. We must remember, too, that many gaps in animal and floral life were due to ordinary climatic and geological factors. We could, under no circumstances, expect to meet with a complete ascending series.

The great vicissitudes in climate which the earth undoubtedly experienced, as shown by geological records, have been held to be themselves proof of sudden violent revolutions in the life-history of the world. But all the great climatic vicissitudes can be accounted for by the action of factors still, in operation—subsidences and elevations of land, alterations in the relative proportions and position of land and water, variations in the relative position of our planet to the sun and other heavenly bodies.

Altogether, the conclusion is inevitable that from the remotest period there has been one uniform and continuous system of change in the animate and inanimate world, and accordingly every fact collected respecting the factors at present at work in forming and changing the world, affords a key to the interpretation of its part. And thus, although we are mere sojourners on the surface of the planet, chained to a mere point in space, enduring but for a moment of time, the human mind is enabled not only to number worlds beyond the unassisted ken of mortal eye, but to trace the events of indefinite ages before the creation of our race, and to penetrate into the dark secrets of the ocean and the heart of the solid globe.

II.—Changes in the Inorganic World now in Progress

The great agents of change in the inorganic world may be divided into two principal classes—the aqueous and the igneous. To the aqueous belong rain, rivers, springs, currents, and tides, and the action of frost and snow; to the igneous, volcanoes and earthquakes. Both these classes are instruments of degradation as well as of reproduction. But they may also be regarded as antagonist forces, since the aqueous agents are incessantly labouring to reduce the inequalities of the earth's surface to a level; while the igneous are equally active in restoring the unevenness of the external crust, partly by heaping up new matter in certain localities, and partly by depressing one portion of the earth's envelope and forcing out another.

We will treat in the first place of the aqueous agents.

RAIN AND RIVERS. When one considers that in some parts of the world as much as 500 or 600 inches of rain may fall annually, it is easy to believe that rain qua rain may be a denuding and plastic agent, and in some parts of the world we find evidence of its action in earth pillars or pyramids. The best example of earth pillars is seen near Botzen, in the Tyrol, where there are hundreds of columns of indurated mud, varying in height from 20 feet to 100 feet. These columns are usually capped by a single stone, and have been separated by rain from the terrace of which they once formed a part.

As a rule, however, rain acts through rivers. The power of rivers to denude and transport is exemplified daily. Even a comparatively small stream when swollen by rain may move rocks tons in weight, and may transport thousands of tons of gravel. The greatest damage is done when rivers are dammed by landslips or by ice. In 1818 the River Dranse was blocked by ice, and its upper part became a lake. In the hot season the barrier of ice gave way, and the torrent swept before it rocks, forests, houses, bridges, and cultivated land. For the greater part of its course the flood resembled a moving mass of rock and mud rather than of water. Some fragments of granite rock of enormous size, which might be compared to houses, were torn out and borne down for a quarter of a mile.

The rivers of unmelted ice called the glaciers act more slowly, but they also have the power of transporting gravel, sand, and boulders to great distances, and of polishing and scoring their rocky channels. Icebergs, too, are potent geological agents. Many of them are loaded with 50,000 to 100,000 tons of rock and earth, which they may carry great distances. Also in their course they must break, and polish, and scratch the peaks and points of submarine mountains.

Coast ice, likewise, may transport rocks and earth. Springs also must be considered as geological agents affecting the face of the globe.

But running water not only denudes it, but also creates land, for lakes, seas, rivers are seen to form deltas. That Egypt was the gift of the Nile was the opinion of the Egyptian priests, and there can be no doubt that the fertility of the alluvial plain above Cairo, and the very existence of the delta below that city, are due to the action of that great river, and to its power of transporting mud from the interior of Africa and depositing it on its inundated plains as well as on that space which has been reclaimed from the Mediterranean and converted into land. The delta of the Ganges and Brahmapootra is more than double that of the Nile. Even larger is the delta of the Mississippi, which has been calculated to be 12,300 square miles in area.

TIDES AND CURRENTS. The transporting and destroying and constructive power of tides and currents is, in many respects, analogous to that of rivers, but extends to wider areas, and is, therefore, of more geological importance. The chief influence of the ocean is exerted at moderate depths below the surface on all areas which are slowly rising, or attempting, as it were, to rise above the sea; but its influence is also seen round the coast of every continent and island.

* * * * *

We shall now consider the igneous agents that act on the earth's surface. These agents are chiefly volcanoes and earthquakes, and we find that both usually occur in particular parts of the world. At various times and at various places within historical times volcanic eruptions and earthquakes have both proved their potency to alter the face of the earth.

The principal geological facts and theories with regard to volcanoes and earthquakes are as follows.

The primary causes of the volcano and the earthquake are to a great extent the same, and connected with the development of heat and chemical action at various depths in the interior of the globe.

Volcanic heat has been supposed to be the result of the original high temperature of the molten planet, and the planet has been supposed to lose heat by radiation. Recent inquiries, however, suggest that the apparent loss of heat may arise from the excessive local development of volcanic action.

Whatever the original shape of our planet, it must in time have become spheroidal by the gradual operation of centrifugal force acting on yielding materials brought successively within its action by aqueous and igneous causes.

The heat in mines and artesian wells increases as we descend, but not in uniform ratio in different regions. Increase at a uniform ratio would imply such heat in the central nucleus as must instantly fuse the crust.

Assuming that there are good astronomical grounds for inferring the original fluidity of the planet, yet such pristine fluidity need not affect the question of volcanic heat, for the volcanic action of successive periods belongs to a much more modern state of the globe, and implies the melting of different parts of the solid crust one after the other.

The supposed great energy of the volcanic forces in the remoter periods is by no means borne out by geological observations on the quantity of lava produced by single eruptions in those several periods.

The old notion that the crystalline rocks, whether stratified or unstratified, such as granite and gneiss, were produced in the lower parts of the earth's crust at the expense of a central nucleus slowly cooling from a state of fusion by heat has now had to be given up, now that granite is found to be of all ages, and now that we know the metamorphic rocks to be altered sedimentary strata, implying the denudation of a previously solidified crust.

The powerful agency of steam or aqueous vapour in volcanic eruptions leads us to compare its power of propelling lava to the surface with that which it exerts in driving water up the pipe of an Icelandic geyser. Various gases also, rendered liquid by pressure at great depths, may aid in causing volcanic outbursts, and in fissuring and convulsing the rocks during earthquakes.

The chemical character of the products of recent eruptions suggests that large bodies of salt water gain access to the volcanic foci. Although this may not be the primary cause of volcanic eruptions, which are probably due to the aqueous vapour intimately mixed with molten rock, yet once the crust is shattered through, the force and frequency of eruptions may depend in some measure on the proximity of large bodies of water.

The permanent elevation and subsidence of land now observed, and which may have been going on through past ages, may be connected with the expansion and contraction of parts of the solid crust, some of which have been cooling from time to time, while others have been gaining heat.

In the preservation of the average proportion of land and sea, the igneous agents exert a conservative power, restoring the unevenness of the surface which the levelling power of water in motion would tend to destroy. If the diameter of the planet remains always the same, the downward movements of the crust must be somewhat in excess, to counterbalance the effects of volcanoes and mineral springs, which are always ejecting material so as to raise the level of the surface of the earth. Subterranean movements, therefore, however destructive they may be during great earthquakes, are essential to the well-being of the habitable surface, and even to the very existence of terrestrial and aquatic species.

III.—Changes of the Organic World now in Progress

In 1809 Lamarck introduced the idea of transmutation of species, suggesting that by changes in habitat, climate, and manner of living one species may, in the course of generations, be transformed into a new and distinct species.

In England, however, the idea remained dormant till in 1844 a work entitled the "Vestiges of Creation" reinforced it with many new facts. In this work the unity of plan exhibited by the whole organic creation, fossil and recent, and the mutual affinities of all the different classes of the animal and vegetable kingdoms, were declared to be in harmony with the idea of new forms having proceeded from older ones by the gradually modifying influence of environment. In 1858 the theory was put on a new and sound basis by Wallace and Darwin, who added the conception of natural selection, suggesting that variations in species are naturally produced, and that the variety fittest to survive in the severe struggle for existence must survive, and transmit the advantageous variation, implying the gradual evolution of new species. Further, Darwin showed that other varieties may be perpetuated by sexual selection.

On investigating the geographical distribution of animals and plants we find that the extent to which the species of mammalia, birds, insects, landshells, and plants (whether flowering or cryptogamous) agree with continental species; or the degree in which those of different islands of the same group agree with each other has an unmistakable relation to the known facilities enjoyed by each class of crossing the ocean. Such a relationship accords well with the theory of variation and natural selection, but with no other hypothesis yet suggested for explaining the origin of species.

From what has been said of the changes which are always going on in the habitable surface of the world, and the manner in which some species are constantly extending their range at the expense of others, it is evident that the species existing at any particular period may, in the course of ages, become extinct one after the other.

If such, then, be the law of the organic world, if every species is continually losing some of its varieties, and every genus some of its species, it follows that the transitional links which once, according to the doctrine of transmutation, must have existed, will, in the great majority of cases, be missing. We learn from geological investigations that throughout an indefinite lapse of ages the whole animate creation has been decimated again and again. Sometimes a single representative alone remains of a type once dominant, or of which the fossil species may be reckoned by hundreds. We rarely find that whole orders have disappeared, yet this is notably the case in the class of reptiles, which has lost some orders characterised by a higher organisation than any now surviving in that class. Certain genera of plants and animals which seem to have been wholly wanting, and others which were feebly represented in the Tertiary period, are now rich in species, and appear to be in such perfect harmony with the present conditions of existence that they present us with countless varieties, confounding the zoologist or botanist who undertakes to describe or classify them.

We have only to reflect on the causes of extinction, and we at once foresee the time when even in these genera so many gaps will occur, so many transitional forms will be lost, that there will no longer be any difficulty in assigning definite limits to each surviving species. The blending, therefore, of one generic or specific form into another must be an exception to the general rule, whether in our own time or in any period of the past, because the forms surviving at any given moment will have been exposed for a long succession of antecedent periods to those powerful causes of extinction which are slowly but incessantly at work in the organic and inorganic worlds.

They who imagine that, if the theory of transmutation be true, we ought to discover in a fossil state all the intermediate links by which the most dissimilar types have been formerly connected together, expect a permanence and completeness of records such as is never found. We do not find even that all recently extinct plants have left memorials of their existence in the crust of the earth; and ancient archives are certainly extremely defective. To one who is aware of the extreme imperfection of the geological record, the discovery of one or two missing links is a fact of small significance; but each new form rescued from oblivion is an earnest of the former existence of hundreds of species, the greater part of which are irrevocably lost.

A somewhat serious cause of disquiet and alarm arises out of the supposed bearing of this doctrine of the origin of species by transmutation on the origin of man, and his place in nature. It is clearly seen that there is such a close affinity, such an identity in all essential points, in our corporeal structure, and in many of our instincts and passions with those of the lower animals—that man is so completely subjected to the same general laws of reproduction, increase, growth, disease, and death—that if progressive development, spontaneous variation, and natural selection have for millions of years directed the changes of the rest of the organic world, we cannot expect to find that the human race has been exempted from the same continuous process of evolution.

Such a near bond of connection between man and the rest of the animate creation is regarded by many as derogatory to our dignity. But we have already had to exchange the pleasing conceptions indulged in by poets and theologians as to the high position in the scale of being held by our early progenitors for humble and more lowly beginnings, the joint labours of the geologist and archaeologist having left us in no doubt of the ignorance and barbarism of Palaeolithic man.

It is well, too, to remember that the high place we have reached in the scale of being has been gained step by step, by a conscientious study of natural phenomena, and by fearlessly teaching the doctrines to which they point. It is by faithfully weighing evidence without regard to preconceived notions, by earnestly and patiently searching for what is true, not what we wish to be true, that we have attained to that dignity, which we may in vain hope to claim through the rank of an ideal parentage.



JAMES CLERK MAXWELL

A Treatise on Electricity and Magnetism

James Clerk Maxwell, the first professor of experimental physics at Cambridge, was born at Edinburgh on November 13, 1831, and before he was fifteen was already famous as a writer of scientific papers. In 1854 he graduated at Cambridge as second wrangler. Two years later he became professor of natural philosophy at Marischal College, Aberdeen. Vacating his chair in 1860 for one at King's College, London, Maxwell contributed largely to scientific literature. His great lifework, however, is his famous "Treatise on Electricity and Magnetism," which was published in 1873, and is, in the words of a critic, "one of the most splendid monuments ever raised by the genius of a single individual." It was in this work that he constructed his famous theory if electricity in which "action at a distance" should be replaced by "action through a medium," and first enunciated the principles of an electro-magnetic theory of light which has formed the basis of nearly all modern physical science. He died on November 5, 1879.

I.—The Nature of Electricity

Let a piece of glass and a piece of resin be rubbed together. They will be found to attract each other. If a second piece of glass be rubbed with a second piece of resin, it will be found that the two pieces of glass repel each other and that the two pieces of resin are also repelled from one another, while each piece of glass attracts each piece of resin. These phenomena of attraction and repulsion are called electrical phenomena, and the bodies which exhibit them are said to be "electrified," or to be "charged with electricity."

Bodies may be electrified in many other ways, as well as by friction. When bodies not previously electrified are observed to be acted on by an electrified body, it is because they have become "electrified by induction." If a metal vessel be electrified by induction, and a second metallic body be suspended by silk threads near it, and a metal wire be brought to touch simultaneously the electrified body and the second body, this latter body will be found to be electrified. Electricity has been transferred from one body to the other by means of the wire.

There are many other manifestations of electricity, all of which have been more or less studied, and they lead to the formation of theories of its nature, theories which fit in, to a greater or less extent, with the observed facts. The electrification of a body is a physical quantity capable of measurement, and two or more electrifications can be combined experimentally with a result of the same kind as when two quantities are added algebraically. We, therefore, are entitled to use language fitted to deal with electrification as a quantity as well as a quality, and to speak of any electrified body as "charged with a certain quantity of positive or negative electricity."

While admitting electricity to the rank of a physical quantity, we must not too hastily assume that it is, or is not, a substance, or that it is, or is not, a form of energy, or that it belongs to any known category of physical quantities. All that we have proved is that it cannot be created or annihilated, so that if the total quantity of electricity within a closed surface is increased or diminished, the increase or diminution must have passed in or out through the closed surface.

This is true of matter, but it is not true of heat, for heat may be increased or diminished within a closed surface, without passing in or out through the surface, by the transformation of some form of energy into heat, or of heat into some other form of energy. It is not true even of energy in general if we admit the immediate action of bodies at a distance.

There is, however, another reason which warrants us in asserting that electricity, as a physical quantity, synonymous with the total electrification of a body, is not, like heat, a form of energy. An electrified system has a certain amount of energy, and this energy can be calculated. The physical qualities, "electricity" and "potential," when multiplied together, produce the quantity, "energy." It is impossible, therefore, that electricity and energy should be quantities of the same category, for electricity is only one of the factors of energy, the other factor being "potential."

Electricity is treated as a substance in most theories of the subject, but as there are two kinds of electrification, which, being combined, annul each other, a distinction has to be drawn between free electricity and combined electricity, for we cannot conceive of two substances annulling each other. In the two-fluid theory, all bodies, in their unelectrified state, are supposed to be charged with equal quantities of positive and negative electricity. These quantities are supposed to be so great than no process of electrification has ever yet deprived a body of all the electricity of either kind. The two electricities are called "fluids" because they are capable of being transferred from one body to another, and are, within conducting bodies, extremely mobile.

In the one-fluid theory everything is the same as in the theory of two fluids, except that, instead of supposing the two substances equal and opposite in all respects, one of them, generally the negative one, has been endowed with the properties and name of ordinary matter, while the other retains the name of the electric fluid. The particles of the fluid are supposed to repel each other according to the law of the inverse square of the distance, and to attract those of matter according to the same law. Those of matter are supposed to repel each other and attract those of electricity. This theory requires us, however, to suppose the mass of the electric fluid so small that no attainable positive or negative electrification has yet perceptibly increased or diminished the mass or the weight of a body, and it has not yet been able to assign sufficient reasons why the positive rather than the negative electrification should be supposed due to an excess quantity of electricity.

For my own part, I look for additional light on the nature of electricity from a study of what takes place in the space intervening between the electrified bodies. Some of the phenomena are explained equally by all the theories, while others merely indicate the peculiar difficulties of each theory. We may conceive the relation into which the electrified bodies are thrown, either as the result of the state of the intervening medium, or as the result of a direct action between the electrified bodies at a distance. If we adopt the latter conception, we may determine the law of the action, but we can go no further in speculating on its cause.

If, on the other hand, we adopt the conception of action through a medium, we are led to inquire into the nature of that action in each part of the medium. If we calculate on this hypothesis the total energy residing in the medium, we shall find it equal to the energy due to the electrification of the conductors on the hypothesis of direct action at a distance. Hence, the two hypotheses are mathematically equivalent.

On the hypothesis that the mechanical action observed between electrified bodies is exerted through and by means of the medium, as the action of one body on another by means of the tension of a rope or the pressure of a rod, we find that the medium must be in a state of mechanical stress. The nature of the stress is, as Faraday pointed out, a tension along the lines of force combined with an equal pressure in all directions at right angles to these lines. This distribution of stress is the only one consistent with the observed mechanical action on the electrified bodies, and also with the observed equilibrium of the fluid dielectric which surrounds them. I have, therefore, assumed the actual existence of this state of stress.

Every case of electrification or discharge may be considered as a motion in a closed circuit, such that at every section of the circuit the same quantity of electricity crosses in the same time; and this is the case, not only in the voltaic current, where it has always been recognised, but in those cases in which electricity has been generally supposed to be accumulated in certain places. We are thus led to a very remarkable consequence of the theory which we are examining, namely, that the motions of electricity are like those of an incompressible fluid, so that the total quantity within an imaginary fixed closed surface remains always the same.

The peculiar features of the theory as developed in this book are as follows.

That the energy of electrification resides in the dielectric medium, whether that medium be solid or gaseous, dense or rare, or even deprived of ordinary gross matter, provided that it be still capable of transmitting electrical action.

That the energy in any part of the medium is stored up in the form of a constraint called polarisation, dependent on the resultant electromotive force (the difference of potentials between two conductors) at the place.

That electromotive force acting on a dielectric produces what we call electric displacement.

That in fluid dielectrics the electric polarisation is accompanied by a tension in the direction of the lines of force combined with an equal pressure in all directions at right angles to the lines of force.

That the surfaces of any elementary portion into which we may conceive the volume of the dielectric divided must be conceived to be electrified, so that the surface density at any point of the surface is equal in magnitude to the displacement through that point of the surface reckoned inwards.

That, whatever electricity may be, the phenomena which we have called electric displacement is a movement of electricity in the same sense as the transference of a definite quantity of electricity through a wire.

II.—Theories of Magnetism

Certain bodies—as, for instance, the iron ore called loadstone, the earth itself, and pieces of steel which have been subjected to certain treatment—are found to possess the following properties, and are called magnets.

If a magnet be suspended so as to turn freely about a vertical axis, it will in general tend to set itself in a certain azimuth, and, if disturbed from this position, it will oscillate about it.

It is found that the force which acts on the body tends to cause a certain line in the body—called the axis of the magnet—to become parallel to a certain line in space, called the "direction of the magnetic force."

The ends of a long thin magnet are commonly called its poles, and like poles repel each other; while unlike poles attract each other. The repulsion between the two magnetic poles is in the straight line joining them, and is numerically equal to the products of the strength of the poles divided by the square of the distance between them; that is, it varies as the inverse square of the distance. Since the form of the law of magnetic action is identical with that of electric action, the same reasons which can be given for attributing electric phenomena to the action of one "fluid," or two "fluids" can also be used in favour of the existence of a magnetic matter, fluid or otherwise, provided new laws are introduced to account for the actual facts.

At all parts of the earth's surface, except some parts of the polar regions, one end of a magnet points in a northerly direction and the other in a southerly one. Now a bar of iron held parallel to the direction of the earth's magnetic force is found to become magnetic. Any piece of soft iron placed in a magnetic field is found to exhibit magnetic properties. These are phenomena of induced magnetism. Poisson supposes the magnetism of iron to consist in a separation of the magnetic fluids within each magnetic molecule. Weber's theory differs from this in assuming that the molecules of the iron are always magnets, even before the application of the magnetising force, but that in ordinary iron the magnetic axes of the molecules are turned indifferently in every direction, so that the iron as a whole exhibits no magnetic properties; and this theory agrees very well with what is observed.

The theories establish the fact that magnetisation is a phenomenon, not of large masses of iron, but of molecules; that is to say, of portions of the substance so small that we cannot by any mechanical method cut them in two, so as to obtain a north pole separate from the south pole. We have arrived at no explanation, however, of the nature of a magnetic molecule, and we have therefore to consider the hypothesis of Ampere—that the magnetism of the molecule is due to an electric current constantly circulating in some closed path within it.

Ampere concluded that if magnetism is to be explained by means of electric currents, these currents must circulate within the molecules of the magnet, and cannot flow from one molecule to another. As we cannot experimentally measure the magnetic action at a point within the molecule, this hypothesis cannot be disproved in the same way that we can disprove the hypothesis of sensible currents within the magnet. In spite of its apparent complexity, Ampere's theory greatly extends our mathematical vision into the interior of the molecules.

III.—The Electro-Magnetic Theory of Light

We explain electro-magnetic phenomena by means of mechanical action transmitted from one body to another by means of a medium occupying the space between them. The undulatory theory of light also assumes the existence of a medium. We have to show that the properties of the electro-magnetic medium are identical with those of the luminiferous medium.

To fill all space with a new medium whenever any new phenomena are to be explained is by no means philosophical, but if the study of two different branches of science has independently suggested the idea of a medium; and if the properties which must be attributed to the medium in order to account for electro-magnetic phenomena are of the same kind as those which we attribute to the luminiferous medium in order to account for the phenomena of light, the evidence for the physical existence of the medium is considerably strengthened.

According to the theory of emission, the transmission of light energy is effected by the actual transference of light-corpuscles from the luminous to the illuminated body. According to the theory of undulation there is a material medium which fills the space between the two bodies, and it is by the action of contiguous parts of this medium that the energy is passed on, from one portion to the next, till it reaches the illuminated body. The luminiferous medium is therefore, during the passage of light through it, a receptacle of energy. This energy is supposed to be partly potential and partly kinetic, and our theory agrees with the undulatory theory in assuming the existence of a medium capable of becoming a receptacle for two forms of energy.

Now, the properties of bodies are capable of quantitative measurement. We therefore obtain the numerical value of some property of the medium—such as the velocity with which a disturbance is propagated in it, which can be calculated from experiments, and also observed directly in the case of light. If it be found that the velocity of propagation of electro-magnetic disturbance is the same as the velocity of light, we have strong reasons for believing that light is an electro-magnetic phenomenon.

It is, in fact, found that the velocity of light and the velocity of propagation of electro-magnetic disturbance are quantities of the same order of magnitude. Neither of them can be said to have been determined accurately enough to say that one is greater than the other. In the meantime, our theory asserts that the quantities are equal, and assigns a physical reason for this equality, and it is not contradicted by the comparison of the results, such as they are.

Lorenz has deduced from Kirchoff's equations of electric currents a new set of equations, indicating that the distribution of force in the electro-magnetic field may be considered as arising from the mutual action of contiguous elements, and that waves, consisting of transverse electric currents, may be propagated, with a velocity comparable with that of light, in non-conducting media. These conclusions are similar to my own, though obtained by an entirely different method.

The most important step in establishing a relation between electric and magnetic phenomena and those of light must be the discovery of some instance in which one set of phenomena is affected by the other. Faraday succeeded in establishing such a relation, and the experiments by which he did so are described in the nineteen series of his "Experimental Researches." Suffice it to state here that he showed that in the case of aray of plane-polarised light the effect of the magnetic force is to turn the plane of polarisation round the direction of the ray as an axis, through a certain angle.

The action of magnetism on polarised light leads to the conclusion that in a medium under the action of a magnetic force, something belonging to the same mathematical class as an angular velocity, whose axis is in the direction of the magnetic force, forms part of the phenomenon. This angular velocity cannot be any portion of the medium of sensible dimensions rotating as a whole. We must, therefore, conceive the rotation to be that of very small portions of the medium, each rotating on its own axis.

This is the hypothesis of molecular vortices. The displacements of the medium during the propagation of light will produce a disturbance of the vortices, and the vortices, when so disturbed, may react on the medium so as to affect the propagation of the ray. The theory proposed is of a provisional kind, resting as it does on unproved hypotheses relating to the nature of molecular vortices, and the mode in which they are affected by the displacement of the medium.

IV.—Action at a Distance

There appears to be some prejudice, or a priori objection, against the hypothesis of a medium in which the phenomena of radiation of light and heat, and the electric actions at a distance, take place. It is true that at one time those who speculated as to the cause of physical phenomena were in the habit of accounting for each kind of action at a distance by means of a special aethereal fluid, whose function and property it was to produce these actions. They filled all space three and four times over with aethers of different kinds, the properties of which consisted merely to "save appearances," so that more rational inquirers were willing to accept not only Newton's definite law of attraction at a distance, but even the dogma of Cotes that action at a distance is one of the primary properties of matter, and that no explanation can be more intelligible than this fact. Hence the undulatory theory of light has met with much opposition, directed not against its failure to explain the phenomena, but against its assumption of the existence of a medium in which light is propagated.

The mathematical expression for electro-dynamic action led, in the mind of Gauss, to the conviction that a theory of the propagation of electric action would in time be found to be the very keystone of electro-dynamics. Now, we are unable to conceive of propagation in time, except either as the flight of a material substance through space or as the propagation of a condition of motion or stress in a medium already existing in space.

In the theory of Neumann, the mathematical conception called potential, which we are unable to conceive as a material substance, is supposed to be projected from one particle to another, in a manner which is quite independent of a medium, and which, as Neumann has himself pointed out, is extremely different from that of the propagation of light. In other theories it would appear that the action is supposed to be propagated in a manner somewhat more similar to that of light.

But in all these theories the question naturally occurs: "If something is transmitted from one particle to another at a distance, what is its condition after it had left the one particle, and before it reached the other?" If this something is the potential energy of the two particles, as in Neumann's theory, how are we to conceive this energy as existing in a point of space coinciding neither with the one particle nor with the other? In fact, whenever energy is transmitted from one body to another in time, there must be a medium or substance in which the energy exists after it leaves one body, and before it reaches the other, for energy, as Torricelli remarked, "is a quintessence of so subtile a nature that it cannot be contained in any vessel except the inmost substance of material things."

Hence all these theories lead to the conception of a medium in which the propagation takes place, and if we admit this medium as an hypothesis, I think we ought to endeavour to construct a mental representation of all the details of its action, and this has been my constant aim in this treatise.



ELIE METCHNIKOFF

The Nature of Man

Elie Metchnikoff, Sub-Director of the Pasteur Institute in Paris, was born May 15, 1845, in the province of Kharkov, Russia, and has worked at the Pasteur Institute since 1888. The greater part of Metchnikoff's work is concerned with the most intimate processes of the body, and notably the means by which it defends itself from the living agents of disease. He is, indeed, the author of a standard treatise entitled "Immunity in Infective Diseases." His early work in zoology led him to study the water-flea, and thence to discover that the white cells of the human blood oppose, consume, and destroy invading microbes. Latterly, Metchnikoff has devoted himself in some measure to more general and especially philosophical studies, the outcome of which is best represented by the notable volume on "The Nature of Man," which was published at Paris in 1903.

I.—Disharmonies in Nature

Notwithstanding the real advance made by science, it cannot be disputed that a general uneasiness disturbs the whole world to-day, and the frequency of suicide is increased greatly among civilised peoples. Yet if science turns to study human nature, there may be grounds for hope. The Greeks held human nature and the human body in high esteem, and among the Romans such a philosopher as Seneca said, "Take nature as your guide, for so reason bids you and advises you; to live happily is to live naturally." In our own day Herbert Spencer has expressed again the Greek ideal, seeking the foundation of morality in human nature itself.

But it has often been taught that human nature is composed of two hostile elements, a body and a soul. The soul alone was to be honoured, while the body was regarded as the vile source of evils. This doctrine has had many disastrous consequences, and it is not surprising that in consequence of it celibacy should have been regarded as the ideal state. Art fell from the Greek ideal until the Renaissance, with its return to that ideal, brought new vigour. When the ancient spirit was born again its influence reached science and even religion, and the Reformation was a defence of human nature. The Lutheran doctrines resumed the principle of a "development as complete as possible of all the natural powers" of man, and compulsory celibacy was abolished.

The historical diversity of opinion regarding human nature is what has led me to the attempt to give an exposition of human nature in its strength and in its weakness. But, before dealing with the man himself, we must survey the lower forms of life.

The facts of the organised world, before the appearnace of man, teach us that though we find change and development, development does not always take a progressive march. We are bound to believe, for instance, that the latest products of evolution are not human beings, but certain parasites which live only upon, or in, the human body. The law in nature is not of constant progress, but of constant tendency towards adaptation. Exquisite adaptations, or harmonies, in nature are constantly met with in the world of living beings. But, on the other hand, any close investigation of organisation and life reveals that beside many most perfect harmonies, there are facts which prove the existence of incomplete harmony, or even absolute disharmony. Rudimentary and useless organs are widely distributed. Many insects are exquisitely adapted for sucking the nectar of flowers; many others would wish to do the same, but their want of adaptation baffles them.

It is plain that an instinct, or any other form of disharmony, leading to destruction, cannot increase or even endure very long. The perversion of the maternal instinct, tending to abandonment of the young, is destructive to the stock. In consequence, individuals affected by it do not have the opportunity of transmitting the perversion. If all rabbits, or a majority of them, left their young to die through neglect, it is evident that the species would soon die out. On the contrary, mothers guided by their instinct to nourish and foster their offspring will produce a vigorous generation capable of transmitting the healthy maternal instinct so essential for the preservation of the species. For such a reason harmonious characters are more abundant in nature than injurious peculiarities. The latter, because they are injurious to the individual and to the species, cannot perpetuate themselves indefinitely.

In this way there comes about a constant selection of characters. The useful qualities are handed down and preserved, while noxious characters perish and so disappear. Although disharmonies tend to the destruction of a species, they may themselves disappear without having destroyed the race in which they occur.

This continuous process of natural selection, which offers so good an explanation of the transmutation and origin of species by means of preservation of useful and destruction of harmful characters, was discovered by Darwin and Wallace, and was established by the splendid researches of the former of these.

Long before the appearance of man on the face of the earth, there were some happy beings well adapted to their environment, and some unhappy creatures that followed disharmonious instincts so as to imperil or to destroy their lives. Were such creatures capable of reflection and communication, plainly the fortunate among them, such as orchids and certain wasps, would be on the side of the optimists; they would declare this the best of all possible worlds, and insist that to secure happiness it is necessary only to follow natural instincts. On the other hand, the disharmonious creatures, those ill adapted to the conditions of life, would be pessimistic philosophers. Consider the case of the ladybird, driven by hunger and with a preference for honey, which searches for it on flowers and meets only with failure, or of insects driven by their instincts into the flames, only to lose their wings and their lives; such creatures, plainly, would express as their idea of the world that it was fashioned abominably, and that existence was a mistake.

II.—Disharmonies in Man

As for man, the creature most interesting to us, in what category does he fall? Is he a being whose nature is in harmony with the conditions in which he has to live, or is he out of harmony with his environment? A critical examination is needed to answer these questions, and to such an examination the pages to follow are devoted.

Science has proved that man is closely akin to the higher monkeys or anthropoid apes—a fact which we must reckon with if we are to understand human nature. The details of anatomy which show the kinship between man and the apes are numerous and astonishing. All the facts brought to light during the last forty years have supported this truth, and no single fact has been brought against it. Quite lately it has been shown that there are remarkable characters in the blood, such that, though by certain tests the fluid part of human blood can be readily distinguished from that of any other creature, the anthropoid apes, and they alone, furnish an exception to this rule. There is thus verily a close blood-relationship between the human species and the anthropoid apes.

But how man arose we do not know. It is probable that he owes his origin to a mutation—a sudden change comparable with that which De Vries observed in the case of the evening primrose. The new creature possessed a brain of abnormal size placed in a spacious cranium which allowed a rapid development of intellectual faculties. This peculiarity would be transmitted to the descendants, and as it was a very considerable advantage in the struggle for existence, the new race would hold its own, propagate, and prevail.

Although he is a recent arrival on the earth, man has already made great progress, as compared with his ancestors the anthropoid apes, and we learn the same if we compare the higher and lower races of mankind. Yet there remain many disharmonies in the organisation of man, as, for instance, in his digestive system. A simple instance of this kind is furnished by the wisdom teeth. The complete absence of all four wisdom teeth has no influence on mastication, and their presence is very frequently the source of illness and danger. In man they are indeed rudimentary organs, providing another proof of our simian origin. The vermiform appendix, so frequently the cause of illness and death, is another rudimentary organ in the human body, together with the part of the digestive canal to which it is attached. The organ is a very old part of the constitution of mammals, and it is because it has been preserved long after its function has disappeared that we find it occurring in the body of man.

I believe that not only the appendix, but a very large part of the alimentary canal is superfluous, and worse than superfluous. It is, of course, of great importance to the horse, the rabbit, and some other mammals that live exclusively on grain and herbage. The latter part of the alimentary canal, however, must be regarded as one of the organs possessed by man and yet harmful to his health and life. It is the cause of a series of misfortunes. The human stomach also is of little value, and can easily be dispensed with, as surgery has proved. It is because we inherit our alimentary canal from creatures of different dietetic habits that it is impossible for us to take our nutriment in the most perfect form. If we were only to eat substances that could be almost completely absorbed, serious complications would be produced. A satisfactory system of diet has to make allowance for this, and in consequence of the structure of the alimentary canal has to include in the food bulky and indigestible materials, such as vegetables. Lastly, it may be noted that the instinct of appetite in man is largely aberrant. The widespread results of alcoholism show plainly the prevalent existence in man of a want of harmony between the instinct for choosing food and the instinct of preservation.

Far stronger than the social instinct, and far older, is the love of life and the instinct of self-preservation. Devices for the protection of life were developed long before the evolution of mankind, and it is quite certain that animals, even those highest in the scale of life, are unconscious of the inevitability of death and the ultimate fate of all living things. This knowledge is a human acquisition. It has long been recognised that the old attach a higher value to life than do the young. The instinctive love of life and fear of death are of importance in the study of human nature, impossible to over-estimate.

The instinctive love of life is preserved in the aged in its strongest form. I have carefully studied the aged to make certain on this point. It is a terrible disharmony that the instinctive love of life should manifest itself so strongly when death is felt to be so near at hand. Hence the religions of all times have been concerned with the problem of death.

III.—Science the Only Remedy for Human Disharmonies

In religion and in philosophy throughout their whole history we find attempts to combat the ills arising from the disharmonies of the human constitution.

Ancient and modern philosophies, like ancient and modern religions, have concerned themselves with the attempt to remedy the ills of human existence, and instinctive fear of death has always ensured that great attention has been paid to the doctrine of immortality.

Science, the youngest daughter of knowledge, has begun to investigate the great problems affecting humanity. Her first steps, taken along the lines first clearly laid down by Bacon, were slow and halting. But medical science has lately made great progress, and has gone very far to control disease, especially in consequence of the work of Pasteur. It is said that science has failed because, for instance, tuberculosis persists, but tuberculosis is propagated not because of the failure of science, but because of the ignorance and stupidity of the population. To diminish the spread of tuberculosis, of typhoid fever, of dysentery, and of many other diseases, it is necessary only to follow the rules of scientific hygiene without waiting for specific remedies.

Science offers us much hope also when it is directed to the study of old age and the phenomena which lead to death.

Man, who is the descendant of some anthropoid ape, has inherited a constitution adapted to an environment very different from that which now surrounds him. He is possessed of a brain very much more highly developed than that of his ancestors, and has entered on a new path in the evolution of the higher organisms. The sudden change in his natural conditions has brought about a large series of organic disharmonies, which become more and more acutely felt as he becomes more intelligent and more sensitive; and thus there has arisen a number of sorrows which poor humanity has tried to relieve by all the means in its power. Humanity in its misery has put question after question to science, and has lost patience at the slowness of the advance of knowledge. It has declared that the answers already found by science are futile and of little interest. But science, confident of its methods, has quietly continued to work. Little by little the answers to some of the questions that have been set have begun to appear.

Man, because of the fundamental disharmonies in his constitution, does not develop normally. The earlier phases of his development are passed through with little trouble; but after maturity greater or lesser abnormality begins, and ends in old age and death that are premature and pathological. Is not the goal of existence the accomplishment of a complete and physiological cycle in which occurs a normal old age, ending in the loss of the instinct of life and the appearance of the instinct of death? But before attaining the normal end, coming after the appearance of the instinct of death, a normal life must be lived; a life filled all through with the feeling that comes from the accomplishment of function. Science has been able to tell us that man, the descendant of animals, has good and evil qualities in his nature, and that his life is made unhappy by the evil qualities.

But the constitution of man is not immutable, and perhaps it may be changed for the better. Morality should be based not on human nature in its existing condition, but on ideal human nature, as it may be in the future. Before all things, it is necessary to try to amend the evolution of human life, that is to say, to transform its disharmonies into harmonies. This task can be undertaken only by science, and to science the opportunity of accomplishing it must be given. Before it is possible to reach the goal mankind must be persuaded that science is all-powerful and that the deeply-rooted existing superstitions are pernicious. It will be necessary to reform many customs and many institutions that now seem to rest on enduring foundations. The abandonment of much that is habitual, and a revolution in the mode of education, will require long and painful effort. But the conviction that science alone is able to redress the disharmonies of the human constitution will lead directly to the improvement of education and to the solidarity of mankind.



The Prolongation of Life

Professor Metchnikoff's volume, on "The Prolongation of Life: Studies in Optimistic Philosophy," was published in 1907, and is in some respects the most original of his works. In it he carries much further the arguments and the studies to which he made brief allusion in "The Nature of Man," and he lays down certain principles for the prolongation of life which have been put into practice by a large number of people during the last two or three years, and are steadily gaining more attention. Sour milk as an article of diet appears to have a peculiar value in arresting the supposed senile changes which are largely due to auto-intoxication or self-poisoning.

I.—Senile Debility

When we study old age in man and the lower animals, we observe certain features common to both. But often among vertebrates there are found animals whose bodies withstand the ravages of time much better than that of man. I think it a fair inference that senility, that precocious senescence which is one of the greatest sorrows of humanity, is not so profoundly seated in the constitution of the higher animals as has generally been supposed. The first facts which we must accept are that human beings who reach extreme old age may preserve their mental qualities, notwithstanding serious physical decay, and that certain of the higher animals can resist the influence of time much longer than is the case with man under present conditions.

Many theories have been advanced regarding the cause of senility. It is certain that many parts of the body continue to thrive and grow even in old age, as, for instance, the nails and hair. But I believe that I have proved that in many parts of the body, especially the higher elements, such as nervous and muscular cells, there is a destruction due to the activity of the white cells of the blood. I have shown also that the blanching of the hair in old age is due to the activity of these white cells, which destroy the hair pigment. Progressive muscular debility is an accompaniment of old age; physical work is seldom given to men over sixty years of age, as it is notorious that they are less capable of it. Their muscular movements are feebler, and soon bring on fatigue; their actions are slow and painful. Even old men whose mental vigour is unimpaired admit their muscular weakness. The physical correlate of this condition is an actual atrophy of the muscles, and has for long been known to observers. I have found that the cause of this atrophy is the consumption of the muscle fibres by what I call phagocytes, or eating cells, a certain kind of white blood cells.

In the case of certain diseases we find symptoms, which look like precocious senility, due to the poison of the disease. It is no mere analogy to suppose that human senescence is the result of a slow but chronic poisoning of the organism. Such poisons, if not completely destroyed or got rid of, weaken the tissues, the functions of which become altered or enfeebled in which the latter have the advantage. But we must make further studies before we can answer the question whether our senescence can be ameliorated.

The duration of the life of animals varies within very wide limits. As a general rule, small animals do not live so long as large ones, but there is no absolute relation between size and longevity, since parrots, ravens, and geese live much longer than many mammals, and than some much larger birds. Buffon long ago argued that the total duration of life bore some definite relation to the length of the period of growth, but further inquiry shows that such a relation cannot be established. Nevertheless, there is something intrinsic in each kind of animal which sets a definite limit to the length of years it can attain. The purely physiological conditions which determine this limit leave room for a considerable amount of variation in longevity. Duration of life, therefore, is a character which can be influenced by the environment.

The duration of life in mammals is relatively shorter than in birds, and in the so-called cold-blooded vertebrates. No indication as to the cause of this difference can be found elsewhere than in the organs of digestion. Mammals are the only group of vertebrate animals in which the large intestine is much developed. This part of the alimentary canal is not important, for it fulfils no notable digestive function. On the other hand, it accommodates among the intestinal flora many microbes which damage health by poisoning the body with their products. Among the intestinal flora there are many microbes which are inoffensive, but others are known to have pernicious properties, and auto-intoxication, or self-poisoning, is the cause of the ill-health which may be traced to their activity. It is indubitable that the intestinal microbes or their poisons may reach the system generally, and bring harm to it. I infer from the facts that the more the digestive tract is charged with microbes, the more it is a source of harm capable of shortening life. As the large intestine not only is that part of the digestive tube most richly charged with microbes, but is relatively more capacious in mammals than in any other vertebrates, it is a just inference that the duration of life of mammals has been notably shortened as the result of chronic poisoning from an abundant intestinal flora.

When we come to study the duration of human life, it is impossible to accept the view that the high mortality between the ages of seventy and seventy-five indicates a natural limit to human life. The fact that many men from seventy to seventy-five years old are well preserved, both physically and intellectually, makes it impossible to regard that age as the natural limit of human life. Philosophers such as Plato, poets such as Goethe and Victor Hugo, artists such as Michael Angelo, Titian, and Franz Hals, produced some of their most important works when they had passed what some regard as the limit of life. Moreover, deaths of people at that age are rarely due to senile debility. Centenarians are really not rare. In France, for instance, nearly 150 centenarians die every year, and extreme longevity is not limited to the white races. Women more frequently become centenarians than men—a fact which supports the general proposition that male mortality is always greater than that of the other sex.

It has been noticed that most centenarians have been people who were poor or in humble circumstances, and whose life has been extremely simple. It may well be said that great riches do not bring a very long life. Poverty generally brings with it sobriety, especially in old age, and sobriety is certainly favourable to long life.

II.—The Study of Natural Death

It is surprising to find how little science really knows about death. By natural death I mean to denote death due to the nature of the organism, and not to disease. We may ask whether natural death really occurs, since death so frequently comes by accident or by disease; and certainly the longevity of many plants is amazing. Such ages as three, four, and five thousand years are attributed to the baobab at Cape Verd, certain cypresses, and the sequoias of California. It is plain that among the lower and higher plants there are cases where natural death does not exist; and, further, so far as I can ascertain, it looks as if poisons produced by their own bodies were the cause of natural death among the higher plants where it does occur.

In the human race cases of what may be called natural death are extremely rare; the death of old people is usually due to infectious disease, particularly pneumonia, or to apoplexy. The close analogy between natural death and sleep supports my view that it is due to an auto-intoxication of the organism, since it is very probable that sleep is due to "poisoning" by the products of organic activity.

Although the duration of the life of man is one of the longest amongst mammals, men find it too short. Ought we to listen to the cry of humanity that life is too short, and that it will be well to prolong it? If the question were merely one of prolonging the life of old people, without modifying old age itself, the answer would be doubtful. It must be understood, however, that the prolongation of life will be associated with the preservation of intelligence and of the power to work. When we have reduced or abolished such causes of precocious senility as intemperance and disease, it will no longer be necessary to give pensions at the age of sixty or seventy years. The cost of supporting the old, instead of increasing, will diminish progressively. We must use all our endeavors to allow men to complete their normal course of life, and to make it possible for old men to play their parts as advisers and judges, endowed with their long experience of life.

From time immemorial suggestions have been made for the prolongation of life. Many elixirs have been sought and supposed to have been found, but general hygienic measures have been the most successful in prolonging life and in lessening the ills of old age. That is the teaching of Sir Herman Weber, himself of very great age, who advises general hygienic principles, and especially moderation in all respects. He advises us to avoid alcohol and other stimulants, as well as narcotics and soothing drugs. Certainly the prolongation of life which has come to pass in recent centuries must be attributed to the advance of hygiene; and if hygiene was able to prolong life when little developed, as was the case until recently, we may well believe that with our greater knowledge a much better result will be obtained.

III.—The Use of Lactic Acid

The general measures of hygiene directed against infectious diseases play a part in prolonging the lives of old people; but, in addition to the microbes which invade the body from outside, there is a rich source of harm in microbes which inhabit the body. The most important of these belong to the intestinal flora which is abundant and varied. Now the attempt to destroy the intestinal microbes by the use of chemical agents has little chance of success, and the intestine itself may be harmed more than the microbes. If, however, we observe the new-born child we find that, when suckled by its mother, its intestinal microbes are very different and much fewer than if it be fed with cows' milk. I am strongly convinced that it is advantageous to protect ourselves by cooking all kinds of food which, like cows' milk, are exposed to the air. It is well-known that other means—as, for instance, the use of lactic acid—will prevent food outside the body from going bad. Now as lactic fermentation serves so well to arrest putrefaction in general, why should it not be used for the same purpose within the digestive tube? It has been clearly proved that the microbes which produce lactic acid can, and do, control the growth of other microbes within the body, and that the lactic microbe is so much at home in the human body that it is to be found there several weeks after it has been swallowed.

From time immemorial human beings have absorbed quantities of lactic microbes by consuming in the uncooked condition substances such as soured milk, kephir, sauerkraut, or salted cucumbers, which have undergone lactic fermentation. By these means they have unknowingly lessened the evil consequences of intestinal putrefaction. The fact that so many races make soured milk and use it copiously is an excellent testimony to its usefulness, and critical inquiry shows that longevity, with few traces of senility, is conspicuous amongst peoples who use sour milk extensively.

A reader who has little knowledge of such matters may be surprised by my recommendation to absorb large quantities of microbes, as the general belief is that microbes are all harmful. This belief, however, is erroneous. There are many useful microbes, amongst which the lactic bacilli have an honourable place. If it be true that our precocious and unhappy old age is due to poisoning of the tissues, the greater part of the poison coming from the large intestine, inhabited by numberless microbes, it is clear that agents which arrest intestinal putrefaction must at the same time postpone and ameliorate old age. This theoretical view is confirmed by the collection of facts regarding races which live chiefly on soured milk, and amongst which great ages are common.