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Printed in Great Britain by Blackie & Son, Limited , Glasg07v


Frederick O. Bower,

Sc.D., D.Sc., LL.D., F.R.S.

James H. Jeans,

M.A., D.Sc., LL.D., F.R.S. , Secretary of the Royal Society.

Harold Jeffreys,

M.A., D.Sc., Fellow of St. John’s College, Cambridge.

Ernest W. MacBride,

M.A., D.Sc., LL.D., F.R.S., Professor of Zoology, Imperial College of Science.

William M‘Dougall,

M.B., F.R.S., Professor of Psychology, Harvard University.

Conwy Lloyd Morgan,

D.Sc., LL.D., F.R.S., Professor Emeritus, Bristol University.

Marcus Seymour Pembrey,

M.A., M.D., F.R.S., Professor of Physiology, University of London.

Alfred Arthur Robb,

Sc.D., D.Sc., Ph.D., F.R.S.

G. Elliot Smith,

M.A., Litt.D., M.D., F.R.C.P., F.R.S., Professor of Anatomy, University of London.

Frederick Soddy,

M.A , F.R.S., Professor of Physical Chemistry, Oxford.

Alfred Edward Taylor,

M.A., D.Litt., Litt.D., F.B.A., Professor of Moral Philosophy, University of Edinburgh.

William W. Watts,

Sc.D., LL.D., M.Sc., F.R.S., Professor of Geology, Imperial College of Science and Technology.

The Rev. James Maurice Wilson,

D.D., F.G.S., Canon of Worcester.

Digitized by the Internet Archive in 2020 with funding from University of Toronto



The present volume is the outcome of a suggestion made to the Publishers by Mr. Allan Ferguson, D.Sc., of the East London College. Asked by a young friend how the doctrine of evolution now stood after the general upheaval of fundamental theories in the last twenty years, Dr. Ferguson was unable to refer him to any recent authoritative statement on the subject by British scientists.

The Publishers decided to make an attempt to have this want supplied. The question whether such a project was capable of execution depended upon whether the co-operation of representative British scientists could be secured. The list of the contributors, as printed on page v, gives answer to this question.

The duties of the editor have been to collate the various sections, and to prevent overlapping where it might have occurred.





By James H. Jeans, M.A., D.Sc., F.R.S.

Primitive Cosmogonies Greek Cosmogony The Cosmogony of Kant The Nebular Hypothesis of Laplace Modern Cos¬ mogony Giant and Dwarf Stars The Source of Stellar Energy The Course of Stellar Evolution The Ages of the Stars The Birth of Stars The Structure of the Universe The Origin of the Solar System --------


The Evolution of the Earth as a Planet

By Harold Jeffreys, M.A., D.Sc.

Early Changes in the System Birth of the Moon How the Orbits became nearly Circular The Solidification of the Earth Moun¬ tain Building Geological Time The Tides The Age of the Earth Continent Formation The Earth’s Ellipticity The Moon’s Rotation and Figure Origin of the Atmosphere -


Geology v

By William W. Watts, Sc.D., F.R.S.

Geological History Theories of Darwin Bearing of Geology on Evolution Imperfection of the Geological Record Recent Geo-




logical Work The General Progression of Life Special Adap¬ tations Lineages Changing Environment Some Biological Peculiarities Geographical Distribution Retreat and Swarming Blind Leads Correlated Evolution Brain Evolution Difficulties Evolution of the Earth Itself - - 59



By Conwy Lloyd Morgan, D.Sc., F.R.S.

Introduction Emergent Evolution In Search of Abiogenesis Biology and Psychology Psycho-biology The Hormic Schema Two Stories Distinguished A Psychological Schema The Twofold Story in Anthropology Concomitants of Emotional Enjoyment An Approach to Heredity A Plain Tale and an Interpretative Story Some General Considerations Towards a Bio-chemical Schema Concepts of Evolution - 107



By Frederick O. Bower, Sc.D., D.Sc., F.R.S.

Methods of Inquiry Lines of Inquiry Various Uses of these Lines of Inquiry Study of Ferns as an Example of Phyletic Method Criteria of Comparison for Ferns Application of the Pale¬ ontological Check Ferns represent a Skein of Advancing Lines Results of Comparison in Respect of the Several Criteria General Conclusion from the above Comparisons Impulses and Limitations Operative in Evolution Importance of the Long History of Ferns Mnemic Theory of Semon and Sir F. Darwin 163



By Ernest W. MacBride, M.A., D.Sc., F.R.S.

History of the Term Evolution The Darwinian Theory: Origin of Species; Natural Selection, Sexual Selection; Origin of Inter¬ specific Sterility; Origin of Variations, The Inheritance of the




Effects of Use and Disuse Pure Line Experiments Mutations or Sports: Mendel’s Laws Tornier on the Origin of Mutations Weismann’s Dogma of the Impossibility of the Inheritability of the Effects of Use and Disuse Kammerer’s Experiments on the Inheritability of the Effects of Habit Durkhen’s Experi¬ ments on the Effects of Habit Neuter Insects Mimicry In¬ direct Proofs of the Inheritability of the Effects of Habit: The Evolution of the Camel; The Evidence of Systematic Zoology;

The Life-history of the Cat-fish Evolution of Annelida and Mollusca deduced from Embryology - - - - -21




By Marcus S. Pembrey, M.A., M.D., F.R.S.

The Need of Biological Conceptions in Physiology Differentiation of Structure and Division of Labour Physiology of the Human Embryo and Foetus Evidence from the Nursery in favour of Evolution Variability as a Factor in Evolution Use and Disuse as Factors in Evolution Are any Characters acquired? The Germ Cells do not live a Life apart from the Common Life of the Organism Physiological Processes of Animals and Plants Evolution of the Warm-blooded Animal Adaptation and Struggle for Existence The Theory of Evolution as a Guide in Physiology The Development of the Mammary Gland Physiology as a Guide in Everyday Life ----- 263



By G. Elliot Smith, M.A., M.D., F.R.S.

Charles Darwin and the Evolution of Man The Evidence of Evolu¬ tion Fossil Remains of Man Fossil Apes The Discoveries in 19 1 1 The Affinities of Apes and Men The Anthropoid Apes The Early Primates Vision and Man’s Evolution Evolution and Language Evolution and Culture The Glamour of a Fashionable Phrase The Fall of Man and the Degradation of Culture The Lost Ten Tribes and Atlantis Dr. William Robertson’s History of America The Doctrine of Psychic Unity American Civilization inspired by Asia The Phase




of Instability in Ethnological Opinion The Early Believers in the Theory of Diffusion The Use of the Term Evolution in Ethnology The Psychology of Invention Early Coloniza¬ tion by the Egyptians False Analogies The Psychological Factor Psychology as the Unifying Factor Egypt the Cradle of Civilization - -- -- -- -- 287


Mental Evolution

By William M‘Dougall, M.B., F.R.S.

Darwin: Spencer: Wallace The Dualistic Theory: Body and Soul Mental Powers of Animals Lamarckian Theory of Lapsed Intelligence Attack on the Lamarckian Theory by Physiologists Attack on the Lamarckian Theory by Weismann Neo- Darwinism - - -- -- -- _ 221

The Evidence for Mental Evolution Evidence from Comparative Psychology Man and the Lower Animals: Ideas: Reason:

Will: Instinct Evidence from Comparative Anatomy Evidence from Mental Life of the Child Search for Mind dowm the Scale - 329

Two Descriptions of the Course of Mental Evolution The Attempt to describe the Evolution of Mind Lloyd Morgan: Emergent Evolution The Description of the Evolution of Higher from Lower Forms of Mind The Seven Marks of Pur¬ posive Striving Amoeba Consciousness Continuity Pur¬ posiveness and Awareness Development of Rudimentary Mind Point of Divergence: Vertebrates and Insects The Apes Language Judgment: Traditional Knowledge: Character - 336

The Problem of the Agency at Work in Mental Evolution - 352


Physics and Chemistry

By Frederick Soddy, M.A., F.R.S.

The Idea of Evolution as it applies to Matter Substances and Qualities The Chemical Elements and their Relationships Prout’s Hypothesis The Spectra of the Sun and Stars The Atomic Theory of Electricity The Electrical Theory of Matter Radioactivity The a, (3, and 7 Rays The a-Ray and £-Ray Changes Isotopes The Atomic Weight of Lead Aston’s Work The Einstein Relation between Mass and Energy




Chemical Consequences of the Electron Theory The Modern Picture of Atomic Structure Artificial Transmutation Is the Idea of Evolution applicable to Matter? - - - - -355


Time and Space

By Alfred A. Robb, Sc.D., D.Sc., Ph.D., F.R.S.

Order in Time The Michelson-Morley Experiment Theoretical Inadequacy of Usual Treatment Conical Order Theory of a Block Universe A Logical Difficulty Considered - - 405



By A. E. Taylor, M.A., D.Litt.

Science and Philosophy The Principle of Analogy General Character of Evolutionary Processes Analogy not Identity' Emergence of the Moral Darwin’s Hypothesis Scientific, not Philosophical Darwinism a Specifically Biological Theory Spencer’s Problem Philosophical, not Scientific Evolutionary Speculations of the Greeks partly Scientific, partly Philosophical Anaximander Empedocles Non-evolutionary Character of Aristotelianism Hegel and Evolution Reasons for this Attitude Provisional Character of Science Evolution cannot be the Last Word of Philosophy Implications of Evolu¬ tion Evolution (a) Presupposes Environment and En¬ vironed ”, and their Interaction ( b ) Presupposes the Eternal (c) Is always of some Part of the Real Modification not wholly determined by Environment Spencer’s Evolution Formula Empty Variety not fully explicable Explaining and Explaining Away Origin and Value Reality of the Genuinely New An Application to Ethics Importance of the Background Total Cause and Part Cause Evolution and Creation Compatible Mental Characters, in what sense Heritable Complexity and Stability Secondary Evolution by Degeneration Stability of Environment Relative The best Adapted Type not necessarily the highest Moral Value independent of Origin Ambiguity of the term Progress The Evolutionary Moralist and Moral Tradition Independence of the Moral Standard Indirect bearing of Evidence as to the Origin on question of Value Origins need to be studied without Prepossessions





The Religious Effect of the Idea of Evolution

By The Rev. James Maurice Wilson, D.D., F.G.S.


Section I. Preliminary Considerations. Some Personal Con¬ siderations On whom the Idea of Evolution may have a Reli¬ gious Effect Postscript ------- 477

Section II. The Purging and Dissolving Effect of Evolution on Theology. What was the Theology that was Affected? Pre- Darwinian Difficulties Darwin’s Origin of Species The Need of Second Thoughts of God ------ 484

Section III. The Illuminating and Constructive Effect of the Idea of Evolution. Evolution suggests or confirms Second Thoughts about God Contrast of Past and Future Effects of Evolution Some Special Effects on Theology Evolution and Soteriology Evolution and Christology Presuppositions on which Earlier and Later Thoughts of God are Based This a Subject for Theologians rather than Evolutionists Some further Remarks - -- -- -- -- - 491

Section IV. The Direct Religious Effect of the Idea of Evolution on the Popular Standards and Motives of Morality. Direct Effect of Evolution on Society Altered Relations of Science and Religion Evolution and Continued Life after Death Summary

of Leading Thoughts in this Chapter Note to Section IV - 507

INDEX . 517




NEBUL/E . 22

Circular Nebula Lenticular Nebula Nebula in Virgo Nebula in Coma Berenices.

Nebulae . 24

Spiral Nebula in Canes Venatici Spiral Nebula in Ursa Major.

Map of the World to illustrate the history of civilization before

the world- wide European Diffusion - - - - - 312

Tracks of a Particles . 378



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Primitive Cosmogonies

As the present book bears witness, the concept of evolution has permeated almost every branch of natural science. Its writers trace evolutionary processes in man and beast, in trees and flowers, even in the inanimate chemical elements. Our remote ancestors had no conception of such detailed evolution, but often had vague ideas of evolutionary development having occurred in the universe as a whole. While they thought that men and birds had been men and birds from their creation, they speculated that the earth, sun, moon, and stars had per¬ chance in some remote past been quite other than they are now. Thus it was through the door of cosmogony that Evolution entered the temple of Science.

Naturally enough the most primitive cosmogonies of all display no conception of evolution or gradual change. Generally they predicate only a simple creative act; a supernatural being, who may stand anywhere in the scale of life, from a crow or a raven to a magnified old man, takes raw material and fashions the earth and the heavens much as a potter takes clay and fashions his vessels. The process may be even more rudi¬ mentary: the creator-hero of the Thlinkit Indians merely steals sun, moon, and stars out of a box and hangs them up where they illuminate the earth. In the cosmogonies of more intel¬ lectual races, the conception of aeons or ages of development begins to appear. For instance the Mexicans recognized five aeons which they described as suns the suns of earth,

(I) 479 ) 1 2



of fire, of air, of water, and of the present. Often there was no clear idea of evolutionary change occurring during the progress of an aeon; each aeon would end with a terrific physical catastrophe or cataclysm of the general nature of the Deluge which occurs in so many cosmogonies, and it was through these catastrophes that development occurred. Only when we reach the age of Greek Culture do we find unmistakable traces of the true evolutionary idea.

No early cosmogony contemplated the creation either of heavens or earth except out of some already existent raw material; the creation of something out of nothing was far too abstruse a concept for the primitive thinker. Thus every cosmogony encounters two problems, the first being concerned with the nature of the primeval world-stuff, and the second with the process of forming it into worlds. But it was not until Greek intellect came into play that the first problem was seen to exist at all.

Greek Cosmogony

Six hundred years before Christ we find Thales of Miletus (640-546 B.c.) speculating that water was the original raw material of the universe; the earth, which he regarded as a flat disc, was supposed still to be floating in an ocean of the elemental fluid. To him all things, although made of water (7 ravra vScop ccttl), were full of gods, and he conceived that the attractive powers of magnets showed them to be specially endowed with souls. During the succeeding century the claims of water to be the basic substance of the universe were chal¬ lenged first by Anaximander, who saw the elemental principle as something intermediate between fire and air on the one hand and water and earth on the other ”, and then in turn by Anaximenes and Heraclitus who urged the claims of air and fire respectively. Finally Empedocles (c. 490-430 b.c.) put forth the doctrine of the co-existence of the four elements, fire, air, water, and earth, each eternal, indestructible, and unchangeable, from whose union in different proportions all things were made. In its general philosophic outlook this doctrine, at least in the



form in which Plato gives it in his Laws ”, is not far removed from modern evolutionary cosmogony. Plato’s Athenian introduces the thesis that all things come, have come and will come, into existence either by nature, by art, or by chance ”, and amplifies his doctrine as follows:

The philosophers say that fire and water and earth and air all exist by nature and chance and none of them by art, and that the bodies which come next in order the earth, sun, moon, and stars have been created by means of these ab¬ solutely inanimate existences. The various elements are moved by chance and also by inherent forces according to certain affinities amongst them of hot with cold, dry with moist, soft with hard, and according to all the other accidental mix¬ tures of opposites which have of necessity happened. After this fashion has been created the whole of heaven and all that is therein, as well as all animals and plants and all the seasons. These come from these elements, not by any action of mind or of any God, or from art, but by nature and chance only.”

Largely as a consequence of the approval of this doctrine by Plato and Aristotle, the learned world was content with cosmogonies of this vague type for the next two thousand years until, in fact, Bacon had impressed on men the im¬ portance of testing all scientific conjectures by a direct appeal to nature, and Newton had made them familiar with the existence of natural laws of universal applicability. The time had now come to make a bonfire of all speculations which were unsupported either by comparison with observation or by reasoning based on natural knowledge, and to clear the road for a scientific cosmogony.

The Cosmogony of Kant

The earliest cosmogonies to appear were those of Des¬ cartes (1644), Swedenborg (1734), and Kant (17 55). The first of these could hardly be called scientific, and the authors of the last two were in no sense scientists. The cosmogony of Kant is, nevertheless, of interest, in that it anticipated in many respects the more famous nebular hypothesis of Laplace.



Kant appears to have been stirred to scientific speculation by reading a summary of an English book, A New Theory of the Universe , by Thomas Wright of Durham. The new theory was a theory only of the present arrangement, not of the evolutionary development, of the universe, and its only cos¬ mogonic interest is that of having aroused Kant.

Kant took as his raw material a limitless waste of primordial matter, which he supposed to consist of hard atoms or mole¬ cules of the type described by Lucretius. As a result of their mutual gravitational attractions, these atoms fell in upon one another, and as they continually collided with, and rebounded from, one another, became ever hotter and hotter, just as the bullet becomes hot on striking the target. In brief Kant’s raw material was initially an enormously vast cold nebula; in the first stage of its evolution it was a less vast hot nebula. This was, and still is, in accordance with scientific knowledge, but at the next step Kant was called on to pay the price of his ignor¬ ance of fundamental mechanical principles.

A well-known scientific principle, called the Conservation of Mass ”, asserts that mass can neither be created nor de¬ stroyed. It may appear to be destroyed when, for instance, we drop a bank-note into the fire, but if we collect all the ashes and other products of combustion, we shall find that these are together equal in mass, although not in value, to the original bank-note. An equally well-known principle, called the Con¬ servation of Energy ”, asserts that energy can neither be created nor destroyed. The energy of the bullet may seem to disappear when it strikes the target, but both the bullet and the target are heated, and if we could measure the precise amounts of heat generated, the energy represented by their total would be seen to be equal exactly to the original energy of the flying bullet. A third principle called the Conservation of Angular Momentum asserts that rotation can neither be created nor destroyed; the total rotation, measured in terms of angular momentum, remains constant. It should be explained that in measuring the angular momentum of a system of rotating bodies we have to take account of the direction of the rotation if the



rotation of one body is reckoned as algebraically positive, the rotation of a body rotating in the opposite direction must be reckoned as algebraically negative. The total rotation of two rapidly rotating bodies, as measured by their total angular momentum, may be zero if the bodies are rotating in opposite directions. This principle, like the two others just mentioned, may often appear at first sight to be violated, but never is in actual fact. A boy increases the rotation of his top by lashing it with a whip where does the rotation come from? The answer is that the action of the boy’s arm as he plies his whip would set him rotating backwards if it were not for the friction of his feet on the ground; this friction, acting on the earth, causes it to rotate more or less rapidly, according to direction, than it otherwise would have done, and the algebraic sum of the angular momenta of the earth and top remains absolutely unaltered. If the boy now leaves the top to itself, the friction between the peg of the top and the ground diminishes the speed of rotation of the top, but this again, according to direction, increases or decreases the speed of rotation of the earth, until finally, when the top lies lifeless on the ground, the earth resumes its original speed of rotation. All that the boy has been able to do is to transfer temporarily a certain amount of angular momentum from the earth to the top. He could no more have created rotation in the top out of nothing than he could have created the top itself out of nothing.

Kant violated this principle of the conservation of angular momentum by supposing that the collisions of his atoms generated rotation. The more the atoms of his nebula collided, the faster, according to him, the nebula spun, until finally it spun so fast that rings of matter were thrown off from its equator. The nebula now looked rather like Saturn sur¬ rounded by its rings, a system which Kant adduced as an example of the process he described. Each ring was next supposed to condense into a planet, the remaining central mass forming the sun. In due course each planet, going through similar experiences, surrounded itself with satellites, the final result being a complete solar system of the type we know



The Nebular Hypothesis of Laplace

The next serious cosmogony to appear, which proved to be the most famous and enduring of all, was published by the great French mathematician Laplace in 1796. Although apparently written in complete ignorance of Kant’s cosmogony, the two resembled one another in many respects. Laplace, who naturally did not make Kant’s mistake of supposing that rota¬ tion could be generated out of nothing, arrived at Kant’s hot rotating nebula by the simple expedient of taking it for the raw material out of which his worlds were to be formed. He sup¬ posed this primeval hot rotating nebula to cool and in so doing to contract. Now the angular momentum of a big rotating nebula is greater than that of the same nebula shrunken to a smaller size but still rotating at the same rate, consequently the principle of conservation of angular momentum ”, which requires that the angular momentum shall not diminish, shows that the speed of rotation of the nebula must increase as it contracts. Accordingly Laplace, quite legitimately, supposed his shrinking nebula to rotate ever faster and faster.

As a result of our earth’s rotation, everything on its surface is. acted on by a centrifugal force which tends to drive it away from the axis of rotation. Owing to the slowness of the rotation this force is comparatively feeble, and bodies stay in contact with the earth’s surface because gravity, which tends to retain them there, is far more powerful than centrifugal force. If the earth were to increase its speed of rotation the prepon¬ derance of gravity would lessen; if it rotated once every 85 minutes instead of once every 24 hours as at present, the two forces would be exactly balanced at the earth’s equator. When this balance was exactly struck, objects at the equator would have no weight; they would neither press on the ground nor fly off into space, but would, like Mahomet’s coffin, remain suspended in the air wherever they were placed.

Laplace imagines his nebula to continue to shrink and so to rotate ever faster, until this stage is reached. The ring of particles which form the equator then exert no pressure on



the rest of the nebula, and as this shrinks from under them they are left suspended in space. Ring after ring of particles is left behind in this way, and, just as in Kant’s cosmogony, the matter of these rings in time aggregates into planets, the central mass ultimately forming the sun. A repetition of the process surrounds the planets with satellites.

Although this hypothesis held the stage as the central figure in cosmogonic theory for well over a century, it has by no means escaped criticism, and the present general opinion of astronomers is that these criticisms compel its abandonment. The criticism which perhaps has done most to undermine the theory, that put forward by Babinet in 1861, is based on the insufficiency of the angular momentum of the present solar system. To have broken up in the way imagined by Laplace, the sun must have had a certain calculable minimum amount of angular momentum. Where has it gone to? It no longer resides in the solar system; the total present angular momentum of the whole solar system, sun, planets, satellites, and all, if concentrated in the sun would cause it to rotate about once in ten hours and to show about the same amount of flattening as is shown by Jupiter. But Jupiter is still very far from breaking up through excess of rotation, and so would the sun have been in the past if endowed with only the present total angular momentum of his system. It is, of course, not absolutely im¬ possible, although it is certainly very improbable, that at some time a strange star, wandering into the solar system from outside, may in some way have carried off the missing angular momentum in disentangling himself. For this and other reasons, Babinet’s criticism cannot be regarded as absolutely unanswerable. A far more deadly criticism is suggested by recent research; mathematical theory predicts that a stellar body breaking up from excess of rotation ought almost cer¬ tainly to break into two approximately equal masses, and observation reveals countless examples in the sky of precisely the type predicted. In fact the end of the Laplacean process, at any rate on a stellar scale, is a binary star two stars of nearly equal mass revolving about one another and nothing in the



least resembling a solar system. We have another instance of abstract thought creating a theory, and abstract research and observation combining to destroy it.

Modern Cosmogony

This last reflection suggests that cosmogony may most profitably attempt to advance by keeping abstract thought and observation, so far as possible, hand in hand. At present, perhaps, theory has rather outrun observation, so that a tele¬ scope will prove more profitable than a library, and the sky will tell us more than the writings of theoretical cosmogonists. The cosmogonist who attempts to learn directly from the sky cannot expect to live long enough to watch the fulfilment of evolutionary changes, but he finds samples of stars in all stages of their careers and a bit of wit enables him to construct the evolutionary chain from these. So a traveller entering a forest of trees in a strange land cannot stay to watch the growth of the individual trees from seedlings to maturity, but if he can find trees in all stages of growth, the knowledge he will obtain by arranging these in order is only one degree less certain than that he could obtain by staying to observe the life-histories of individual trees. From the labours of generations of travellers in the forest of the sky it has recently emerged that, broadly speaking, all the stars are trees of one species. Terrestrial trees may be deciduous or evergreen, male or female, and so on, but stars are just stars. There is only one evolutionary chain to be pieced together, so that, for instance, every star in the sky gives us a picture of what our sun either has been at some time in the past or will be at some time in the future.

So sweeping a generalization as this naturally requires some limitation, and must in any case be explained with care. To superficial observation the most noticeable difference, indeed almost the only difference, in the stars is a difference in apparent brightness. This difference of course results in large part from differences of distances. Modern astronomical methods, however, make it possible to determine the distances of great numbers of stars and, after allowing for the effect of



variations of distance, the stars are still found to show enormous differences in intrinsic brightness, or, to use the scientific term, in luminosity. Canopus, which appears as the second brightest star in the sky, is too far off for his distance to be measured with any accuracy. But we know that he is so far off that he must be at least 10,000 times, and more probably 50,000 times, as luminous as our sun. The eclipsing star V Puppis, whose distance can be measured with fair certainty, shines with just about 10,000 times the luminosity of our sun. If either Canopus or V Puppis were put in place of our sun we should be scorched to cinders within a few moments; the seas would boil away and the solid rocks be raised to a red heat. An example of the other extreme of luminosity is provided by our nearest known neighbour in the sky, Proxima Centauri, a star so faint that, in spite of his extreme nearness, he has only quite recently been discovered. His luminosity is about a ten-thousandth part of that of the sun. If he were put in place of our sun we should receive less than 100 times the illumina¬ tion we receive from the full moon; in the intense cold which would result, the seas, the earth, and probably the atmosphere itself would rapidly freeze solid. There is no reason to think that Proxima Centauri represents in any sense an absolute end to the scale of luminosity; he was discovered only because of his nearness to us, and there are probably great numbers of still fainter stars which, being both fainter and remoter than Proxima Centauri, have not yet been discovered at all. There are good reasons for thinking that at least half of the stars in the universe are quite dark, and there is probably a continuous gradation of luminosity from these up to the stars of highest luminosity, such as Canopus and V Puppis, with 10,000 or more times the luminosity of our sun.

With the aid of the spectroscope, the astronomer analyses the light of a star into its constituent parts, and from this analysis he can deduce inter alia the temperature of the star’s surface. From analogy with glowing coals one might perhaps have expected that the most luminous stars would be the hottest, but this does not prove to be the case. Often enough they are



rather cool, and it is the enormous size of their radiating surface, rather than any specially high temperature, that is responsible for their high luminosity. By a triumph of instrumental skill, for which the credit must be given mainly to Professor Michel- son of Chicago, it has recently been found possible to obtain direct measurement of the diameters of certain bright stars to give some idea of the difficulty of such measurements it may be mentioned that none of these stars subtends an angle as great as is subtended by a pin-head at a distance of six miles. The star a Orionis (Betelguese) was found to have a diameter of 215,000,000 miles, and so must have a radiating surface about 80,000 times that of our sun, but, because his surface is so much cooler, he only emits 5000 times as much light and heat.

Stars which are somewhat less luminous than this are found, in general, to have hotter surfaces, and the rise of temperature continues until we come to stars whose luminosity is only 100 or 50 times that of our sun. Here we encounter surface tem¬ peratures of 10,000° C. or more, ranging up to perhaps 30,000° C. These are the highest surface temperatures met with; on passing to stars of still lower luminosity, luminosity and tem¬ perature are found to decrease together. Stars whose lumin¬ osity is only about equal to that of our sun have surface tem¬ peratures of about 6ooo° C., a temperature somewhat higher than that of the electric arc. Finally the stars at the end of the scale as known to us, the faintest visible stars, have surface temperatures of only from 2000° C. to 3000° C., not much hotter than a hot coal fire. And there is very little doubt that the series descends, could we but see it, to still colder stars whose lumino¬ sity is not far from zero.

The masses of the stars vary much less than their lumino¬ sities; the most massive star known has not much more than 70 times the mass of the sun, while the least massive has about a seventh of the sun’s mass. Thus although the star a Orionis already mentioned has about 20 million times the volume of the sun, it is tolerably certain that it has not got 20 million times the mass; its enormous area of surface merely indicates that the matter inside is of very low density. Of the quantities



we have been discussing, masses and temperatures show a far smaller range than do luminosities and densities. To a first, although very rough, approximation, it is permissible to think of all the stars as having the same masses and the same tem¬ peratures, so that variations of luminosity merely represent variations of density, high luminosity indicating a vast expanse of surface and therefore low density, and conversely.

In actual fact, this law of low density accompanying high luminosity and vice versa, which we have obtained by such rough and ready methods, is fully confirmed by a precise study of the densities of the stars. The most luminous stars are found to have densities ranging down to less than a thousandth of that of ordinary air, and so must of course be in the gaseous state; the least luminous are fully as dense as the solid earth and there is a continuous transition between the two. The sequence of luminosity, temperature, and density may perhaps be suggested by the following table, in which certain prominent stars are taken as landmarks:

Type of Star.

Luminosity in Terms of Sun.


Tempera¬ ture (Cen¬ tigrade).

Density in Terms of Water.

Most lumin¬ ous of all a Orionis a Persei


y Cassiopeia?


Proxima Cen- tauri

Least lumin¬ ous of all

10,000 or






\ - 1

1 10,000