s Place
Universe
Russel Wallace
EX LIBRIS.
Bertram & Jt
MAN S PLACE IN THE UNIVERSE
MAN S PLACE IN
THE UNIVERSE
A Study of the Results of Scientific Research
in Relation to the Unity or Plurality
of Worlds
BY
ALFRED R. WALLACE
LL.D., D.C.L., F.R.S., ETC.
O, glittering host ! O, golden line !
I would I had an angel s ken,
Your deepest secrets to divine,
And read your mysteries to men. 1
THIRD EDITION"
LONDON
CHAPMAN AND HALL
LIMITED
1904
I said unto my inmost heart,
Shall I don corslet, helm, and shield,
And shall I with a Giant strive,
And charge a Dragon on the field ?
J. H. DELL.
JUN 3 1958
PREFACE
THIS work has been written in consequence of the
great interest excited by my article, under the same
title, which appeared simultaneously in The Fort
nightly Review and the New York Independent.
Two friends who read the manuscript were of
opinion that a volume, in which the evidence could
be given much more fully, would be desirable, and
the result of the publication of the article confirmed
their view.
I was led to a study of the subject when writing
four new chapters on Astronomy for a new edition
of The Wonderful Century. I then found that
almost all writers on general astronomy, from Sir
John Herschel to Professor Simon Newcomb and
Sir Norman Lockyer, stated, as an indisputable
fact, that our sun is situated in the plane of the great
ring of the Milky Way, and also very nearly in the
centre of that ring. The most recent researches also
showed that there was little or no proof of there
being any stars or nebulae very far beyond the
Milky Way, which thus seemed to be the limit, in
that direction, of the stellar universe.
vi MAN S PLACE IN THE UNIVERSE
Turning to the earth and the other planets of the
Solar System, I found that the most recent re
searches led to the conclusion that no other planet
was likely to be the seat of organic life, unless
perhaps of a very low type. For many years I had
paid special attention to the problem of the measure
ment of geological time, and also that of the mild
climates and generally uniform conditions that had
prevailed throughout all geological epochs; and on
considering the number of concurrent causes and
the delicate balance of conditions required to main
tain such uniformity, I became still more convinced
that the evidence was exceedingly strong against the
probability or possibility of any other planet being
inhabited.
Having long been acquainted with most of the
works dealing with the question of the supposed
Plurality of Worlds, I was quite aware of the very
superficial treatment the subject had received, even
in the hands of the most able writers, and this made
me the more willing to set forth the whole of the
available evidence astronomical, physical, and
biological in such a way as to show both what was
proved and what suggested by it.
The present work is the result, and I venture to
think that those who will read it carefully will admit
that it is a book that was worth writing. It is founded
almost entirely on the marvellous body of facts and
PREFACE vii
conclusions of the New Astronomy together with those
reached by modern physicists, chemists, and biolo
gists. Its novelty consists in combining the various
results of these different branches of science into
a connected whole, so as to show their bearing upon
a single problem a problem which is of very great
interest to ourselves.
This problem is, whether or no the logical
inferences to be drawn from the various results
of modern science lend support to the view that our
earth is the only inhabited planet, not only in the
Solar System but in the whole stellar universe.
Of course it is a point as to which absolute demon
stration, one way or the other, is impossible. But
in the absence of any direct proofs, it is clearly
rational to inquire into probabilities ; and these
probabilities must be determined not by our pre
possessions for any particular view, but by an
absolutely impartial and unprejudiced examination
of the tendency of the evidence.
As the book is written for the general, educated
body of readers, many of whom may not be
acquainted with any aspect of the subject or with the
wonderful advance of recent knowledge in that
department often termed the New Astronomy, a
popular account has been given of all those branches
of it which bear upon the special subject here
discussed, This part of the work occupies the first
viii MAN S PLACE IN THE UNIVERSE
six chapters. Those who are fairly acquainted
with modern astronomical literature, as given in
popular works, may begin at my seventh chapter,
which marks the commencement of the considerable
body of evidence and of argument I have been able
to adduce.
To those of my readers who may have been
influenced by any of the adverse criticisms on my
views as set forth in the article already referred to,
I must again urge, that throughout the whole of this
work, neither the facts nor the more obvious con
clusions from the facts are given on my own
authority, but always on that of the best astronomers,
mathematicians, and other men of science to whose
works I have had access, and whose names, with
exact references, I generally give.
What I claim to have done is, to have brought
together the various facts and phenomena they have
accumulated ; to have set forth the hypotheses by
which they account for them, or the results to
which the evidence clearly points ; to have judged
between conflicting opinions and theories ; and
lastly, to have combined the results of the various
widely-separated departments of science, and to have
shown how they bear upon the great problem which
I have here endeavoured, in some slight degree,
to elucidate.
As such a large body of facts and arguments from
PREFACE ix
distinct sciences have been here brought together,
I have given a rather full summary of the whole
argument, and have stated my final conclusions in
six short sentences. I then briefly discuss the two
aspects of the whole problem those from the
materialistic and from the spiritualistic points of view ;
and I conclude with a few general observations on
the almost unthinkable problems raised by ideas
of Infinity- -problems which some of my critics
thought I had attempted in some degree to deal with,
but which, I here point out, are altogether above and
beyond the questions I have discussed, and equally
above and beyond the highest powers of the human
intellect.
BROADSTONE, DORSET,
September 1903.
The wilder d mind is tost and lost,
O sea, in thy eternal tide ;
The reeling brain essays in vain,
O stars, to grasp the vastness wide !
The terrible tremendous scheme
That glimmers in each glancing light,
O night, O stars, too rudely jars
The finite with the infinite !
J. H. DELL
CONTENTS
CHAP. PAGE
I. EARLY IDEAS, . . I
II. MODERN IDEAS, . ... 7
III. THE NEW ASTRONOMY, . . .24
IV. THE DISTRIBUTION OF THE STARS, . . 47
V. DISTANCES OF STARS: THE SUN S MOTION, . 73
VI. UNITY AND EVOLUTION OF THE STAR-SYSTEM, . 99
VII. ARE THE STARS INFINITE?. . . 135
VIII. OUR RELATION TO THE MILKY WAY, . 156
IX. THE UNIFORMITY OF MATTER AND ITS LAWS, 183
X. THE ESSENTIAL CHARACTERS OF ORGANISMS 191
XL PHYSICAL CONDITIONS ESSENTIAL FOR LIFE, 206
XII. THE EARTH IN RELATION TO LIFE, 218 "
XIII. THE ATMOSPHERE IN RELATION TO LIFE, . . 243
XIV. THE OTHER PLANETS ARE NOT HABITABLE, . 262
XV. THE STARS : HAVE THEY PLANETS ? ARE THEY
USEFUL TO Us ? . . . 282
XVI. STABILITY OF THE STAR-SYSTEM : IMPORTANCE OF
CENTRAL POSITION : SUMMARY AND CONCLUSION, 295 S
INDEX, .
EIGHT DIAGRAMS IN THE TEXT AND
TWO STAR CHARTS AT END.
Who is man, and what his place?
Anxious asks the heart, perplext
In this recklessness of space,
Worlds with worlds thus intermixt :
What has he, this atom creature,
In the infinitude of Nature ?
F. T. PALGRAVE.
MAN S PLACE IN THE UNIVERSE
CHAPTER I
EARLY IDEAS AS TO THE UNIVERSE AND ITS
RELATION TO MAN
WHEN men attained to sufficient intelligence for
speculations as to their own nature and that of the
earth on which they lived, they must have been pro
foundly impressed by the nightly pageant of the
starry heavens. The intense sparkling brilliancy of
Sirius and Vega, the more massive and steady lumin
osity of Jupiter and Venus, the strange grouping of
the brighter stars into constellations to which fantastic
names indicating their resemblance to various animals
or terrestrial objects seemed appropriate and were
soon generally adopted, together with the apparently
innumerable stars of less and less brilliancy scattered
broadcast over the sky, many only being visible on
the clearest nights and to the acutest vision, consti
tuted altogether a scene of marvellous and impressive
splendour of which it must have seemed almost im
possible to attain any real knowledge, but which
afforded an endless field for the imagination of the
observer.
The relation of the stars to the sun and moon in
their respective motions was one of the earliest pro-
4
2 MAN S PLACE IN THE UNIVERSE [CHAP.
blems for the astronomer, and it was only solved by
careful and continuous observation, which showed
that the invisibility of the former during the day was
wholly due to the blaze of light, and this is said to
have been proved at an early period by the observed
fact that from the bottom of very deep wells stars
can be seen while the sun is shining. During total
eclipses of the sun also the brighter stars become
visible, and, taken in connection with the fixity of
position of the pole-star, and the course of those
circumpolar stars which never set in the latitudes of
Greece, Egypt, and Chaldea, it soon became possible
to frame a simple hypothesis which supposed the
earth to be suspended in space, while at an unknown
distance from it a crystal sphere revolved upon an
axis indicated by the pole-star, and carried with it
the whole host of heavenly bodies. This was the
theory of Anaximander (540 B.C.), and it served
as the starting-point for the more complex theory
which continued to be held in various forms and
with endless modifications down to the end of the
sixteenth century.
It is believed that the early Greeks obtained some
knowledge of astronomy from the Chaldeans, who
appear to have been the first systematic observers of
the heavenly bodies by means of instruments, and
who are said to have discovered the cycle of eighteen
years and ten days after which the sun and moon
return to the same relative positions as seen from
the earth. The Egyptians perhaps derived their
knowledge from the same source, but there is no
proof that they were great observers, and the accu
rate orientation, proportions, and angles of the Great
I.] EARLY IDEAS 3
Pyramid and its inner passages may perhaps indicate
a Chaldean architect.
The very obvious dependence of the whole life of
the earth upon the sun, as a giver of heat and light,
sufficiently explains the origin of the belief that the
latter was a mere appanage of the former ; and as
the moon also illuminates the night, while the stars
as a whole also give a very perceptible amount of
light, especially in the dry climate and clear atmo
sphere of the East, and when compared with the
pitchy darkness of cloudy nights when the moon
is below the horizon, it seemed clear that the
whole of these grand luminaries sun, moon, stars,
and planets were but parts of the terrestrial system,
and existed solely for the benefit of its inhabitants.
Empedocles (444 B.C.) is said to have been the
first who separated the planets from the fixed stars,
by observing their very peculiar motions, while
Pythagoras and his followers determined correctly
the order of their succession from Mercury to Saturn.
No attempt was made to explain these motions till
a century later, when Eudoxus of Cnidos, a con
temporary of Plato and of Aristotle, resided for some
time in Egypt, where he became a skilful astronomer.
He was the first who systematically worked out and
explained the various motions of the heavenly bodies
on the theory of circular and uniform motion round
the earth as a centre, by means of a series of con
centric spheres, each revolving at a different rate
and on a different axis, but so united that all shared
in the motion round the polar axis. The moon,
for example, was supposed to be carried by three
spheres, che first revolved parallel to the equator
4 MAN S PLACE IN THE UNIVERSE [CHAP.
and accounted for the diurnal motion- -the rising
and setting- -of the moon ; another moved parallel
to the ecliptic and explained the monthly changes of
the moon ; while the third revolved at the same rate
but more obliquely, and explained the inclination of
the moon s orbit to that of the earth. In the same
way each of the five planets had four spheres, two
moving like the first two of the moon, another one
also moving in the ecliptic was required to explain
the retrograde motion of the planets, while a fourth
oblique to the ecliptic was needed to explain the
diverging motions due to the different obliquity of
the orbit of each planet to that of the earth. This
was the celebrated Ptolemaic system in the simplest
form needed to account for the more obvious motions
of the heavenly bodies. But in the course of ages the
Greek and Arabian astronomical observers discovered
small divergences due to the various degrees of
excentricity of the orbits of the moon and planets
and their consequent varying rates of motion ; and to
explain these other spheres were added, together
with smaller circles sometimes revolving excentri-
cally, so that at length about sixty of these spheres,
epicycles and excentrics were required to account
for the various motions observed with the rude
instruments, and the rates of motion determined by
the very imperfect time -measurers of those early
ages. And although a few great philosophers had
at different times rejected this cumbrous system and
had endeavoured to promulgate more correct ideas,
their views had no influence on public opinion even
among astronomers and mathematicians, and the
Ptolemaic system held full sway down to the time of
I.] EARLY IDEAS 5
Copernicus, and was not finally given up till Kepler s
Laws and Galileo s Dialogues compelled the adoption
of simpler and more intelligible theories.
We are now so accustomed to look upon the main
facts of astronomy as mere elementary knowledge
that it is difficult for us to picture to ourselves the
state of almost complete ignorance which prevailed
even among the most civilised nations throughout
antiquity and the Middle Ages. The rotundity of the
earth was held by a few at a very early period, and
was fairly well established in later classical times.
The rough determination of the size of our globe
followed soon after ; and when instrumental observa
tions became more perfect, the distance and size of
the moon were measured with sufficient accuracy to
show that it was very much smaller than the earth.
But this was the farthest limit of the determination
of astronomical sizes and distances before the dis
covery of the telescope. Of the sun s real distance
and size nothing was known except that it was much
farther from us and much larger than the moon ; but
even in the century before the commencement of the
Christian era Posidonius determined the circumference
of the earth to be 240,000 stadia, equal to about 28,600
miles, a wonderfully close approximation considering
the very imperfect data at his command. He is also
said to have calculated the sun s distance, making it
only one-third less than the true amount, but this
must have been a chance coincidence, since he had
no means of measuring angles more accurately than
to one degree, whereas in the determination of the
sun s distance instruments are required which measure
to a second of arc.
6 MAN S PLACE IN THE UNIVERSE [CHAP. i.
Before the discovery of the telescope the sizes of
the planets were quite unknown, while the most that
could be ascertained about the stars was, that they
were at a very great distance from us. This being
the extent of the knowledge of the ancients as to the
actual dimensions and constitution of the visible uni
verse, of which, be it remembered, the earth was held
to be the centre, we cannot be surprised at the almost
universal belief that this universe existed solely for
the earth and its inhabitants. In classical times it
was held to be at once the dwelling-place of the
gods and their gift to man, while in Christian ages
this belief was but slightly, if at all, changed ; and in
both it would have been considered impious to main
tain that the planets and stars did not exist for the
service and delight of mankind alone but in all pro
bability had their own inhabitants, who might in
some cases be even superior in intellect to man him
self. But apparently, during the whole period of
which we are now treating, no one was so daring as
even to suggest that there were other worlds with
other inhabitants, and it was no doubt because of the
idea that we occupied the world, the very centre of
the whole surrounding universe which existed solely
for us, that the discoveries of Copernicus, Tycho
Brahe, Kepler, and Galileo excited so much anta
gonism and were held to be impious and altogether
incredible. They seemed to upset the whole accepted
order of nature, and to degrade man by removing his
dwelling-place, the earth, from the commanding cen
tral position it had always before occupied.
CHAPTER II
MODERN IDEAS AS TO MAN*S RELATION TO THE UNIVERSE
THE beliefs as to the subordinate position held by
sun, moon, and stars in relation to the earth, which
were almost universal down to the time of Coper
nicus, began to give way when the discoveries of
Kepler and the revelations of the telescope demon
strated that our earth was not specially distinguished
from the other planets by any superiority of size or
position. The idea at once arose that the other
planets might be inhabited ; and when the rapidly
increasing power of the telescope, and of astronomical
instruments generally, revealed the wonders of the
solar system and the ever-increasing numbers of the
fixed stars, the belief in other inhabited worlds
became as general as the opposite belief had been in
all preceding ages, and it is still held in modified
forms to the present day.
But it may be truly said that the later like the
earlier belief is founded more upon religious ideas
than upon a scientific and careful examination of the
whole of the facts both astronomical, physical, and
biological, and we must agree with the late Dr.
Whewell, that the belief that other planets are
inhabited has been generally entertained, not in con
sequence of physical reasons but in spite of them.
8 MAN S PLACE IN THE UNIVERSE [CHAP.
And he adds : It was held that Venus, or that
Saturn was inhabited, not because any one could
devise, with any degree of probability, any organised
structure which would be suitable to animal existence
on the surfaces of those planets ; but because it was
conceived that the greatness or goodness of the
Creator, or His wisdom, or some other of His attri
butes, would be manifestly imperfect, if these planets
were not tenanted by living creatures. Those
persons who have only heard that many eminent
astronomers down to our own day have upheld the
belief in a Plurality of Worlds will naturally
suppose that there must be some very cogent argu
ments in its favour, and that it must be supported by
a considerable body of more or less conclusive facts.
They will therefore probably be surprised to hear
that any direct evidence which may be held to
support the view is almost wholly wanting, and that
the greater part of the arguments are weak and
flimsy in the extreme.
Of late years, it is true, some few writers have
ventured to point out how many difficulties there are
in the way of accepting the belief, but even these
have never examined the question from the various
points of view which are essential to a proper
consideration of it ; while, so far as it is still upheld,
it is thought sufficient to show, that in the case of
some of the planets, there seem to be such condi
tions as to render life possible. In the millions of
planetary systems supposed to exist it is held to be
incredible that there are not great numbers as well
fitted to be inhabited by animals of all grades,
including some as high as man or even higher, and
II.] MODERN IDEAS 9
that we must, therefore, believe that they are so
inhabited. As in the present work I propose to
show, that the probabilities and the weight of direct
evidence tend to an exactly opposite conclusion, it
will be well to pass briefly in review the various
writers on the subject, and to give some indication of
the arguments they have used and the facts they
have set forth. For the earlier upholders of the
theory I am indebted to Dr. Whewell, who, in his
Dialogue on the Plurality of Worlds a Supplement
to his well-known volume on the subject refers to
all writers of importance known to him.
The earliest are the great astronomers Kepler and
Huygens, and the learned Bishop Wilkins, who all
believed that the moon was or might probably be
inhabited ; and of these Whewell considers Wilkins
to have been by far the most thoughtful and earnest
in supporting his views. Then we have Sir Isaac
Newton himself who, at considerable length, argued
that the sun was probably inhabited. But the first
regular work devoted to the subject appears to have
been written by M. Fontenelle, Secretary to the
Academy of Sciences in Paris, who in 1686 published
his Conversations on the Plurality of Worlds. The
book consisted of five chapters, the first explaining
the Copernican Theory ; the second maintaining that
the moon is a habitable world ; the third gives
particulars as to the moon, and argues that the other
planets are also inhabited ; the fourth gives details as
to the worlds of the five planets ; while the fifth
declares that the fixed stars are suns, and that each
illuminates a world. This work was so well written,
and the subject proved so attractive, that it was
io MAN S PLACE IN THE UNIVERSE [CHAP,
translated into all the chief European languages,
while the astronomer Lalande edited one of the
French editions. Three English translations were
published, and one of these went through six editions
down to the year 1737. The influence of this work
was very great and no doubt led to that general
acceptance of the theory by such men as Sir William
Herschel, Sir John Herschel, Dr. Chalmers, Dr.
Dick, Dr. Isaac Taylor, and M. Arago, although it
was wholly founded on pure speculation, and there
was nothing that could be called evidence on one
side or the other.
This was the state of public opinion when an
anonymous work appeared (in 1853) under the some
what misleading title of The Plurality of Worlds :
An Essay. This was written, as already stated, by
Dr. Whewell, who, for the first time, ventured to
doubt the generally accepted theory, and showed
that all the evidence at our command led to the con
clusion that some of the planets were certainly not
habitable, that others were probably not so, while in
none was there that close correspondence with
terrestrial conditions which seemed essential for their
habitability by the higher animals or by man. The
book was ably written and showed considerable
knowledge of the science of the time, but it was very
diffuse, and the larger part of it was devoted to
showing that his views were not in any way opposed
to religion. One of his best arguments was founded
on the proposition that the Earth s Orbit is the
Temperate Zone of the Solar System* that there only
is it possible to have those moderate variations of
heat and cold, dryness and moisture, which are suit-
IL] MODERN IDEAS 11
able for animal life. He suggested that the outer
planets of the system consisted mainly of water,
gases, and vapour, as indicated by their low specific
gravity, and were therefore quite unsuitable for
terrestrial life ; while those near the sun were equally
unsuited, because, owing to the great amount of solar
heat, water could not exist on their surfaces. He
devotes a great deal of space to the evidence that
there is no animal life on the moon, and taking this
as proved, he uses it as a counter argument against
the other side. They always urge that, the earth
being inhabited, we must suppose the other planets
to be so too ; to which he replies :- -We know that
the moon is not inhabited though it has all the
advantage of proximity to the sun that the earth has ;
why then should not other planets be equally
uninhabited ?
He then comes to Mars and admits that this
planet is very like the earth so far as we can judge,
and that it may therefore be inhabited, or as the
author expresses it, * may have been judged worthy
of inhabitants by its Maker. But he urges the small
size of Mars, its coldness owing to distance from the
sun, and that the annual melting of its polar ice-caps
will keep it cold all through the summer. If there
are animals they are probably of a low type like the
saurians and iguanodons of our seas during the
Wealden epoch ; but, he argues, as even on our earth
the long process of preparation for man was carried
on for countless millions of years, we need not dis
cuss whether there are intelligent beings on Mars
till we have some better evidence that there are any
living creatures at all.
12 MAN S PLACE IN THE UNIVERSE [CHAP.
Several of the early chapters are devoted to an
attempt to minimise the difficulties of those religious
persons who feel oppressed by the immensity and
complexity of the material universe as revealed by
modern astronomy ; and by the almost infinite insig
nificance of man and his dwelling-place, the earth, in
comparison with it, an insignificance vastly increased
if not only the planets of the solar system, but also
those which circle around the myriads of suns, are
also theatres of life. And these persons are further
disquieted because the very same facts are used by
sceptics of various kinds in their attacks upon
Christianity. Such writers point out the irrationality
and absurdity of supposing that the Creator of all
this unimaginable vastness of suns and systems, fill
ing for all we know endless space, should take any
special interest in so mean and pitiful a creature as
man, the imperfectly developed inhabitant of one of
the smaller worlds attached to a second or third-rate
sun, a being whose whole history is one of war and
bloodshed, of tyranny, torture, and death ; whose
awful record is pictured by himself in such books as
Josephus History of the Jews, the Decline and Fall
of the Roman Empire, and even more forcibly
summarised in that terrible picture of human
fiendishness and misery, The Martyrdom of Man ;
while their character is indicated by one of the
kindest and simplest of their poets in the restrained
but expressive lines :
Man s inhumanity to man
Makes countless thousands mourn.
It is for such a being as this, they say, that God
II.] MODERN IDEAS 13
should have specially revealed His will some
thousands of years ago, and finding that His com
mands were not obeyed, His will not fulfilled, yet
ordained for their benefit the necessarily unique
sacrifice of His Son, in order to save a small portion
of these miserable sinners from the natural and
well-deserved consequence of their stupendous follies,
their unimaginable crimes ? Such a belief they
maintain is too absurd, too incredible, to be held by
any rational being, and it becomes even less credible
and less rational if we maintain that there are count
less other inhabited worlds.
It is very difficult for the religious man to make
any adequate reply to such an attack as this, and as
a result many have felt their position to be untenable
and have accordingly lost all faith in the special
dogmas of orthodox Christianity. They feel them
selves really to be between the horns of a dilemma.
If there are myriads of other worlds, it seems
incredible that they should each be the object of a
special revelation and a special sacrifice. If, on the
other hand, we are the only intelligent beings that
exist in the material universe, and are really the
highest creative product of a Being of infinite wisdom
and power, they cannot but wonder at the vast
apparent disproportion between the Creator and the
created, and are sometimes driven to Atheism from
the hopelessness of comprehending so mean and
petty a result as the sole outcome of infinite power.
Whewell tells us that the great preacher, Dr.
Chalmers, in his Astronomical Discourses, attempted
a reply to these difficulties, but, in his opinion, not
a irery successful one ; and a large part of his own
14 MAN S PLACE IN THE UNIVERSE [CHAP.
work is devoted to the same purpose. His main
point seems to be that we know too little of the
universe to arrive at any definite conclusions on the
question at issue, and that any ideas that we may
have as to the purposes of the Creator in forming the
vast system we see around us are almost sure to be
erroneous. We must therefore be content to remain
ignorant, and must rest satisfied in the belief that
the Creator had a purpose although we are not yet
permitted to know what it was. And to those who
urge that in other worlds there may be other laws of
nature which may render them quite as habitable by
intelligent beings as our world is for us, he replies,
that if we are to suppose new laws of nature in order
to render each planet habitable, there is an end of all
rational inquiry on the subject, and we may maintain
and believe that animals may live on the moon
without air or water, and on the sun exposed to heat
which vaporises earths and metals.
His concluding argument, and perhaps one of his
strongest, is that founded upon the dignity of man,
as conferring a pre-eminence upon the planet which
has produced him. If, he says, man be not merely
capable of Virtue and Duty, of universal Love and
Self- Devotion, but be also immortal ; if his being be
of infinite duration, his soul created never to die ;
then, indeed, we may well say that one soul outweighs
the whole unintelligent creation. 5 And then, addres
sing the religious world, he urges that, if, as they
believe, God has redeemed man by the sacrifice of
His Son, and has given to him a revelation of His
will, then indeed no other conception is possible
than that he is the sole and highest product of the
ii.] MODERN IDEAS 15
universe. The elevation of millions of intellectual,
moral, religious, spiritual creatures, to a destiny so
prepared, consummated, and developed, is no un
worthy occupation of all the capacities of space, time,
and matter. Then with a chapter on The Unity
of the World, and one on The Future/ neither of
which contains anything which adds to the force of
his argument, the book ends.
The publication of this able if rather vague and
diffuse work, contesting popular opinions, was followed
by a burst of indignant criticism on the part of a man
of considerable eminence in some branches of physics
-Sir David Brewster, but who was very inferior,
both in general knowledge of science and in literary
skill, to the writer whose views he opposed. The
purport of the book in which he set forth his objec
tions is indicated by its title More Worlds than
One, the Creed of the Philosopher ctnd the Hope of the
Christian. Though written with much force and
conviction it appeals mainly to religious prejudices,
and assumes throughout that every planet and star
is a special creation, and that the peculiarities of each
were designed for some special purpose. If/ he
says, the moon had been destined to be merely a
lamp to our earth, there was no occasion to variegate
its surface with lofty mountains and extinct volcanoes,
and cover it with large patches of matter that reflect
different quantities of light and give its surface the
appearance of continents and seas. It would have
been a better lamp had it been a smooth piece of
lime or of chalk. It is, therefore, he thinks, prepared
for inhabitants ; and then he argues that all the
other satellites are also inhabited. Again he says
16 MAN S PLACE IN THE UNIVERSE [CHAP.
that when it was found that Venus was about the
same size as the Earth, with mountains and valleys,
days and nights, and years analogous to our own, the
absurdity of believing that she had no inhabitants,
when no other rational purpose could be assigned for
her creation, became an argument of a certain amount
that she was, like the Earth, the seat of animal and
vegetable life. Then, when it was found that Jupiter
was so gigantic as to require four moons to give
him light, the argument from analogy that he was
inhabited became stronger also, because it extended
to two planets/ And thus each successive planet
having certain points of analogy with the others
becomes an additional argument ; so that when we
take account of all the planets, with atmosphere, and
clouds, and arctic snows, and trade-winds, the argu
ment from analogy becomes, he urges, very powerful ;
and the absurdity of the opposite opinion, that
planets should have moons and no inhabitants,
atmospheres with no creatures to breathe in them,
and currents of air without life to be fanned, became
a formidable argument which few minds, if any, could
resist.
The work is full of such weak and fallacious
rhetoric and even, if possible, still weaker. Thus
after describing double stars, he adds But no
person can believe that two suns could be placed in
the heavens for no other purpose than to revolve
round their common centre of gravity ; and he con
cludes his chapter on the stars thus: Wherever
there is matter there must be Life ; Life Physical to
enjoy its beauties- -Life Moral to w r orship its Maker,
and Life Intellectual to proclaim His wisdom and
II.] MODERN IDEAS 17
His power. And again A house without tenants,
a city without citizens, presents to our minds the
same idea as a planet without life, and a universe
without inhabitants. Why the house was built, why
the city was founded, why the planet was made, and
why the universe was created, it would be difficult
even to conjecture. Arguments of this kind, which
in almost every case beg the question at issue, are
repeated ad nauseam. But he also appeals to the
Old Testament to support his views, by quoting the
fine passage in the Psalms When I consider Thy
heavens the work of Thy fingers, the moon and the
stars which Thou hast ordained ; what is man that
Thou art mindful of him ? on which he remarks
We cannot doubt that inspiration revealed to him
[David] the magnitude, the distances, and the final
cause, of the glorious spheres which fixed his admira
tion. And after quoting various other passages from
the prophets, all as he thinks supporting the same
view, he sets forth the extraordinary idea as a con
firmatory argument, that the planets or some of them
are to be the future abode of man. For, he says-
Man in his future state of existence is to consist,
as at present, of a spiritual nature residing in a
corporeal frame. He must live, therefore, upon a
material planet, subject to all the laws of matter.
And he concludes thus : If there is not room, then,
on our globe for the millions of millions of beings
who have lived and died on its surface, we can
scarcely doubt that their future abode must be on
some of the primary or secondary planets of the
solar system, whose inhabitants have ceased to
exist, or upon planets which have long been in a
B
i8 MAN S PLACE IN THE UNIVERSE [CHAP.
state of preparation, as our earth was, for the advent
of intellectual life.
It is pleasant to turn from such weak and trivial
arguments to the only other modern works which
deal at some length with this subject, the late Richard
A. Proctor s Other Worlds than Ours, and a volume
published five years later under the title Our Place
Among Infinities. Written as these were by one
of the most accomplished astronomers of his day,
remarkable alike for the acuteness of his reasoning
and the clearness of his style, we are always inter
ested and instructed even when we cannot agree with
his conclusions. In the first work mentioned above,
he assumes, like Sir David Brewster, the antecedent
probability that the planets are inhabited and on
much the same theological grounds. So strongly
does he feel this that he continually speaks as if the
planets must be inhabited unless we can show very
good reason that they cannot be so, thus throwing
the burden of proving a negative on his opponents,
while he does not attempt to prove his positive con
tention that they are inhabited, except by purely
hypothetical considerations as to the Creator s purpose
in bringing them into existence.
But starting from this point he endeavours to show
how Whewell s various difficulties may be overcome,
and here he always appeals to astronomical or physical
facts, and reasons well upon them. But he is quite
honest ; and, coming to the conclusion that Jupiter
and Saturn, Uranus and Neptune, cannot be habit
able, he adduces the evidence and plainly states the
result. But then he thinks that the satellites of
Jupiter and Saturn may be habitable, and if they may
ii.J MODERN IDEAS 19
be, then he concludes that they must. One great
oversight in his whole argument is, that he is satisfied
with showing the possibility that life may exist now,
but never deals with the question of whether life
could have been developed from its earliest rudiments
up to the production of the higher vertebrates and
man ; and this, as I shall show later, is the crux of
the whole problem.
With regard to the other planets, after a careful
examination of all that is known about them, he
arrives at the conclusion that if Mercury is protected
by a cloud-laden atmosphere of a peculiar kind it
may possibly, but not probably, support high forms
of animal life. But in the case of Venus and Mars
he finds so much resemblance to and so many ana
logies with our earth, that he concludes that they
almost certainly are so.
In the case of the fixed stars, now that we know
by spectroscopic observations that they are true suns,
many of which closely resemble our sun and give out
light and heat as he does, Mr. Proctor argues, that
* The vast supplies of heat thus emitted by the stars
not only suggest the conclusion that there must be
worlds around these orbs for which these heat-
supplies are intended, but point to the existence of
the various forms of force into which heat may be
transmuted. We know that the sun s heat poured
upon our earth is stored up in vegetable and animal
forms of life ; is present in all the phenomena of
nature- -in winds and clouds and rain, in thunder
and lightning, storm and hail ; and that even the
works of man are performed by virtue of the solar
heat-supplies. Thus the fact that the stars send forth
20 MAN S PLACE IN THE UNIVERSE [CHAP.
heat to the worlds which circle around them suggests
at once the thought that on those worlds there must
exist animal and vegetable forms of life. We may
note that in the first part of this passage the presence
of worlds or planets is suggested, while later on
the worlds which circle round them is spoken of as
if it were a proved fact from which the presence of
vegetable and animal life may be inferred. A sug
gestion depending on a preceding suggestion is not
a very firm basis for so vast and wide-reaching a
conclusion.
In the second work referred to above there is one
chapter entitled, A New Theory of Life in other
Worlds, where the author gives his more matured
views of the question, which are briefly stated in the
preface as being that the weight of evidence favours
my theory of the (relative) paucity of worlds/ His
views are largely founded on the theory of probabili
ties, of which subject he had made a special study.
Taking first our earth, he shows that the period
during which life has existed upon it is very small in
comparison with that during which it must have been
slowly forming and cooling, and its atmosphere con
densing so as to form land and water on its surface.
And if we consider the time the earth has been
occupied by man, that is a very minute part, perhaps
not the thousandth part, of the period during which it
has existed as a planet. It follows that even if we
consider only those planets whose physical condition
seems to us to be such as to be able to sustain
life, the chances are perhaps hundreds to one
against their being at that particular stage when
life has begun to be developed, or if it has begun
ii.] MODERN IDEAS 21
has reached as high a development as on our
earth.
With regard to the stars, the argument is still
stronger, because the epochs required for their forma
tion are altogether unknown, while as to the condi
tions required for the formation of planetary systems
around them we are totally ignorant. To this I
would add that we are equally ignorant as to the
probability or even possibility of many of these suns
producing planets which, by their position, size,
atmosphere, or other physical conditions can possibly
become life-producing worlds. And, as we shall see
later, this point has been overlooked by all writers,
including Mr. Proctor himself. His conclusion is,
then, that although the worlds which possess life at
all approaching that of our earth may be relatively
few in number, yet considering the universe as prac
tically infinite in extent, they may be really very
numerous.
It has been necessary to give this sketch of the
views of those who have written specially on the
question of the Plurality of Worlds, because the
works referred to have been very widely read and
have influenced educated opinion throughout the
world. Moreover, Mr. Proctor, in his last work on
the subject, speaks of the theory as being identified
with modern astronomy ; and in fact popular works
still discuss it. But all these follow the same general
line of argument as those already referred to, and
the curious thing is that while overlooking many of
the most essential conditions they often introduce
others which are by no means essential as, for
instance, that the atmosphere must have the same
22 MAN S PLACE IN THE UNIVERSE [CHAP.
proportion of oxygen as our own. They seem to
think that if any of our quadrupeds or birds taken
to another planet could not live there, no animals
of equally high organisation could inhabit it ; entirely
overlooking the very obvious fact that, supposing, as
is almost certain, that oxygen is necessary for life,
then, whatever proportion of oxygen within certain
limits was present, the forms of life that arose would
necessarily be organised in adaptation to that propor
tion, which might be considerably less or greater than
on the earth.
The present volume will show how extremely
inadequate has been the treatment of this question,
which involves a variety of important considerations
hitherto altogether overlooked. These are extremely
numerous and very varied in their character, and the
fact that they all point to one conclusion a conclu
sion which so far as I am aware no previous writer
has reached renders it at least worthy of the careful
consideration of all unbiassed thinkers. The whole
subject is one as to which no direct evidence is
obtainable, but I venture to think that the conver
gence of so many probabilities and indications towards
a single definite theory, intimately connected with
the nature and destiny of man himself, raises this
theory to a very much higher level of probability
than the vague possibilities and theological sugges
tions which are the utmost that have been adduced
by previous writers.
In order to make every step of my argument
clearly intelligible to all educated readers, it will be
necessary to refer continually to the marvellous ex
tension of our knowledge of the universe obtained
IL ] MODERN IDEAS 23
during the last half-century, and constituting what is
termed the New Astronomy. The next chapter will
therefore be devoted to a popular exposition of the
new methods of research, so that the results reached,
which will have to be referred to in succeeding
chapters, may be not only accepted, but clearly un
derstood.
CHAPTER III
THE NEW ASTRONOMY
DURING the latter half of the nineteenth century
discoveries were made which extended the powers of
astronomical research into entirely new and unex
pected regions, comparable to those which were
opened up by the discovery of the telescope more
than two centuries before. The older astronomy for
more than two thousand years was purely mechanical
and mathematical, being limited to observation and
measurement of the apparent motions of the heavenly
bodies, and the attempts to deduce, from these ap
parent motions, their real motions, and thus deter
mine the actual structure of the solar system. This
was first done when Kepler established his three
celebrated laws : and later, when Newton showed
that these laws were necessary consequences of
the one law of gravitation, and when succeeding
observers and mathematicians proved that each fresh
irregularity in the motions of the planets was explic
able by a more thorough and minute application of
the same laws, this branch of astronomy reached its
highest point of efficiency and left very little more to
be desired.
Then, as the telescope became successively im
proved, the centre of interest was shifted to the
CH. in.] THE NEW ASTRONOMY 25
surfaces of the planets and their satellites, which were
watched and scrutinised with the greatest assiduity in
order if possible to attain some amount of knowledge
of their physical constitution and past history. A
similar minute scrutiny was given to the stars and
nebulae, their distribution and grouping, and the
whole heavens were mapped out, and elaborate cata
logues constructed by enthusiastic astronomers in
every part of the world. Others devoted themselves
to the immensely difficult problem of determining
the distances of the stars, and by the middle of the
century a few such distances had been satisfactorily
measured.
Thus, up to the middle of the nineteenth century
it appeared likely that the future of astronomy would
rest almost entirely on the improvement of the tele
scope, and of the various instruments of measurement
by means of which more accurate determinations of
distances might be obtained. Indeed, the author of
the Positive Philosophy, Auguste Comte, felt so sure
of this that he deprecated all further attention to the
stars as pure waste of time that could never lead to
any useful or interesting result. In his Philosophical
Treatise on Popular Astronomy published in 1844,
he wrote very strongly on this point. He there tells
us that, as the stars are only accessible to us by sight
they must always remain very imperfectly known.
We can know little more than their mere existence.
Even as regards so simple a phenomenon as their
temperature this must always be inappreciable to a
purely visual examination. Our knowledge of the
stars is for the most part purely negative, that is,
we can determine only that they do not belong to our
26 MAN S PLACE IN THE UNIVERSE [CHAP.
system. Outside that system there exists, in as
tronomy, only obscurity and confusion, for want of
indispensable facts ; and he concludes thus : It is,
then, in vain that for half a century it has been
endeavoured to distinguish two astronomies, the one
solar the other sidereal. In the eyes of those for
whom science consists of real laws and not of in
coherent facts, the second exists only in name, and
the first alone constitutes a true astronomy ; and
I am not afraid to assert that it will always be
so. And he adds that all efforts directed to this
subject for half a century have only produced an
accumulation of incoherent empirical facts which can
only interest an irrational curiosity/
Seldom has a confident assertion of finality in sci
ence received so crushing a reply as was given to the
above statements of Comte by the discovery in 1860
(only three years after his death) of the method of
spectrum-analysis which, in its application to the stars,
has revolutionised astronomy, and has enabled us to
obtain that very kind of knowledge which he declared
must be for ever beyond our reach. Through it we
have acquired accurate information as to the physics
and chemistry of the stars and nebulae, so that we
now know really more of the nature, constitution, and
temperature of the enormously distant suns which we
distinguish by the general term stars, than we do of
most of the planets of our own system. It has also
enabled us to ascertain the existence of numerous
invisible stars, and to determine their orbits, their
rate of motion, and even, approximately, their mass.
The despised stellar astronomy of the early part of
the century has now taken rank as the most pro-
in.] THE NEW ASTRONOMY 27
foundly interesting department of that grand science,
and the branch which offers the greatest promise of
future discoveries. As the results obtained by means
of this powerful instrument will often be referred to,
a short account of its nature and of the principles on
which it depends must here be given.
The solar spectrum is the band of coloured light
seen in the rainbow and, partially, in the dew-drop,
but more completely when a ray of sunlight passes
through a prism a piece of glass having a triangular
section. The result is, that instead of a spot of white
light we have a narrow band of brilliant colours which
succeed each other in regular order, from violet at
one end through blue, green, and yellow to red at
the other. We thus see that light is not a simple
and uniform radiation from the sun, but is made up
of a large number of separate rays, each of which
produces in our eyes the sensation of a distinct
colour. Light is now explained as being due to
vibrations of ether, that mysterious substance which
not only permeates all matter, but which fills space
at least as far as the remotest of the visible stars and
nebulae. The exceedingly minute waves or vibrations
of the ether produce all the phenomena of heat, light,
and colour, as well as those chemical actions to which
photography owes its wonderful powers. By in
genious experiments the size and rate of vibration of
these waves have been measured, and it is found
that they vary considerably, those forming the red
light, which is least refracted, having- a wave-leneth
<-> o o
of about 320000 of an inch, while the violet rays at
the other end of the spectrum are only about half
that length or 6 3 ^ Q of an inch. The rate at which
28 MAN S PLACE IN THE UNIVERSE [CHAP.
the vibrations succeed each other is from 302 millions
of millions per second for the extreme red rays, to
737 millions of millions for those at the violet end of
the spectrum. These figures are given to show the
wonderful minuteness and rapidity of these heat and
light waves on which the whole life of the world, and
all our knowledge of other worlds and other suns,
directly depends.
But the mere colours of the spectrum are not the
most important part of it. Very early in the nine
teenth century a close examination showed that it
was everywhere crossed by black lines of various
thicknesses, sometimes single, sometimes grouped
together. Many observers studied them and made
accurate drawings or maps showing their positions
and thicknesses, and by combining several prisms,
so that the beam of sunlight had to pass through
them successively, a spectrum could be produced sev
eral feet long, and more than 3000 of these dark lines
were counted in it. But what they were and how
they were caused remained a mystery, till, in the year
1860, the German physicist Kirchhoff discovered the
secret and gave to chemists and astronomers a new
and quite unexpected engine of research.
It had already been observed that the chemical
elements and various compounds, when heated to
incandescence, produced spectra consisting of coloured
lines or bands which were constant for each element,
so that the elements could at once be recognised by
their characteristic spectra ; and it had also been
noticed that some of these bands, especially the
yellow band produced by sodium, corresponded in
position with certain black lines in the solar spectrum.
in.] THE NEW ASTRONOMY 29
KirchhofFs discovery consisted in showing that, when
the light from an incandescent body passes through
the same substance in a state of vapour or gas, so
much of the light is absorbed that the coloured lines
or bands become black. The mystery of more than
half a century was thus solved ; and the thousands of
black lines in the solar spectrum were shown to be
caused by the light from the incandescent matter of
the sun s surface passing through the heated gases
or vapours immediately above it, and thereby having
the bright coloured lines of their spectra changed,
by absorption, to comparative blackness.
Chemists and physicists immediately set to work
examining the spectra of the elements, fixing the
position of the several coloured lines or bands by
accurate measurement, and comparing them with the
dark lines of the solar spectrum. The results were
in the highest degree satisfactory. In a large pro
portion of the elements the coloured bands corre
sponded exactly with a group of dark lines in the
spectrum of the sun, in which, therefore, the same
terrestrial elements were proved to exist. Among
the elements first detected in this manner were
hydrogen, sodium, iron, copper, magnesium, zinc,
calcium, and many others. Nearly forty of the
elements have now been found in the sun, and it
seems highly probable that all our elements really
exist there, but as some are very rare and are present
in very minute quantities they cannot be detected.
Some of the dark lines in the sun were found not to
correspond to any known element, and as this was
thought to indicate an element peculiar to the sun it
was named Helium ; but quite recently it has been
30 MAN S PLACE IN THE UNIVERSE [CHAP.
discovered in a rare mineral. Many of the elements
are represented by a great number of lines, others
by very few. Thus iron has more than 2000, while
lead and potassium have only one each.
The value of the spectroscope both to the chemist
in discovering new elements and to the astronomer in
determining the constitution of the heavenly bodies,
is so great, that it became of the highest importance
to have the position of all the dark lines in the solar
spectrum, as well as the bright lines of all the elements,
determined with extreme accuracy, so as to be able
to make exact comparisons between different spectra.
At first this was done by means of very large-scale
drawings showing the exact position of every dark
or bright line. But this was found to be both in
convenient and not sufficiently exact ; and it was
therefore agreed to adopt the natural scale of the
wave-lengths of the different parts of the spectrum,
which by means of what are termed diffraction-grat
ings can now be measured with great accuracy.
Diffraction-gratings are formed of a polished surface
of hard metal ruled with excessively fine lines, some
times as many as 20,000 to an inch. When sun
light falls upon one of these gratings it is reflected,
and by interference of the rays from the spaces be
tween the fine grooves, it is spread out into a
beautiful and well-defined spectrum, which, when the
lines are very close, is several yards in length. In
these diffraction spectra many dark lines are seen
which can be shown in no other way, and they also
give a spectrum which is far more uniform than that
produced by glass prisms in which minute differences
in the composition of the glass cause some rays
in.] THE NEW ASTRONOMY 31
to be refracted more and others less than the normal
amount.
The spectra produced by diffraction-gratings are
double ; that is, they are spread out on both sides of
the central line of the ray which remains white, and
the several coloured or dark lines are so clearly
defined that they can be thrown on a screen at a
considerable distance, giving a great length to the
spectrum. The data for obtaining the wave-lengths
are the distance apart of the lines, the distance of the
screen, and the distance apart of the first pair of
dark lines on each side of the central bright line.
All these can be measured with extreme accuracy by
means of telescopes with micrometers and other
contrivances, and the result is an accuracy of deter
mination of wave-lengths which can probably not be
equalled in any other kind of measurement.
As the wave-lengths are so excessively minute, it
has been found convenient to fix upon a still smaller
unit of measurement, and as the millimetre is the
smallest unit of the metric system, the ten-millionth
of a millimetre (technically termed tenth meter ) is
the unit adopted for the measurement of wave
lengths, which is equal to about the 250 millionth
of an inch. Thus the wave-lengths of the red and
blue lines characteristic of hydrogen are 6563 07 and
4861-51 respectively. This excessively minute scale
of wave-lengths, once determined by the most refined
measurement, is of very great importance. Having
the wave-lengths of any two lines of a spectrum so
determined, the space between them can be laid down
on a diagram of any length, and all the lines that
occur in any other spectrum between these two lines
32 MAN S PLACE IN THE UNIVERSE [CHAP.
can be marked in their exact relative positions. Now,
as the visible spectrum consists of about 300,000 rays
of light, each of different wave-lengths and therefore
of different refrangibilities, if it is laid down on such
a scale as to be of a length of 3000 inches (250 feet),
each wave-length will be j-^j of an inch long, a space
easily visible by the naked eye.
The possession of an instrument of such wonder
ful delicacy, and with powers which enable it to
penetrate into the inner constitution of the remotest
orbs of space, rendered it possible, within the next
quarter of a century, to establish what is practically
a new science Astrophysics often popularly termed
the New Astronomy. A brief outline of the main
achievements of this science must now be given.
The first great discovery made by Spectrum-
analysis, after the interpretation of the sun s spectrum
had been obtained, was, the real nature of the fixed
stars. It is true they had long been held by astro
nomers to be suns, but this was only an opinion
of the accuracy of which it did not seem possible to
obtain any proof. The opinion was founded on two
facts their enormous distance from us, so great that
the whole diameter of the earth s orbit did not lead
to any apparent change of their relative positions,
and their intense brilliancy which at such distances
could only be due to an actual size and splendour
comparable with our sun. The spectroscope at once
proved the correctness of this opinion. As one after
another was examined, they were found to exhibit
spectra of the same general type as that of the sun
a band of colours crossed by dark lines. The very
first stars examined by Sir William Huggins showed
ill.] THE NEW ASTRONOMY 33
the existence of nine or ten of our elements. Very
soon all the chief stars of the heavens were spectro-
scopically examined, and it was found that they
could be classed in three or four groups. The first
and largest group contains more than half the visible
stars, and a still larger proportion of the most
brilliant, such as Sirius, Vega, Regulus, and Alpha
Crucis in the Southern Hemisphere. They are
characterised by a white or bluish light, rich in the
ultra-violet rays, and their spectra are distinguished
by the breadth and intensity of the four dark bands
due to the absorption of hydrogen, while the various
black lines which indicate metallic vapours are com
paratively few, though hundreds of them can be
discovered by careful examination.
The next group, to which Capella and Arcturus
belong, is also very numerous, and forms the solar
type of stars. Their light is of a yellowish colour,
and their spectra are crossed throughout by innumer
able fine dark lines more or less closely correspon
ding with those in the solar spectrum.
The third group consists of red and variable stars,
which are characterised by fluted spectra. Such
spectra show like a range of Doric columns seen in
perspective, the red side being that most illumi
nated.
The last group, consisting of few and com
paratively small stars, has also fluted spectra, but
the light appears to come from the opposite direc
tion.
These groups were established by Father Secchi,
the Roman astronomer, in 1867, and have been
adopted with some modifications by Vogel of the
c
34 MAN S PLACE IN THE UNIVERSE [CHAP.
Astrophysical Observatory at Potsdam. The exact
interpretation of these different spectra is somewhat
uncertain, but there can be little doubt that they
coincide primarily with differences of temperature
and with corresponding differences in the composition
and extent of the absorptive atmospheres. Stars
with fluted spectra indicate the presence of vapours
of the metalloids or of compound substances, while
the reversed flutings indicate the presence of carbon.
These conclusions have been reached by careful
laboratory experiments which are now carried on at
the same time as the spectral examination of the
stars and other heavenly bodies, so that each
peculiarity of their spectra, however puzzling and
apparently unmeaning, has been usually explained,
by being shown to indicate certain conditions of
chemical constitution or of temperature.
But whatever difficulty there may be in explaining
details, there remains no doubt whatever of the
fundamental fact that all the stars are true suns,
differing no doubt in size, and their stage of develop
ment as indicated by the colour or intensity of their
light or heat, but all alike possessing a photosphere
or light-emitting surface, and absorptive atmospheres
of various qualities and density.
Innumerable other details, such as the often con
trasted colours of double stars, the occasional varia
bility of their spectra, their relations to the nebulae,
the various stages of their development and other
problems of equal interest, have occupied the con
tinued attention of astronomers, spectroscopists, and
chemists ; but further reference to these difficult
questions would be out of place here. The present
III.] THE NEW ASTRONOMY 35
sketch of the nature of spectrum-analysis applied to
the stars is for the purpose of making its principle
and method of observation intelligible to every
educated reader, and to illustrate the marvellous
precision and accuracy of the results attained by it.
So confident are astronomers of this accuracy that
nothing less than perfect correspondence of the various
bright lines in the spectrum of an element in the
laboratory with the dark lines in the spectrum of the
sun or of a star is required before the presence of
that element is accepted as proved. As Miss Clerke
tersely puts it Spectroscopic coincidences admit of
no compromise. Either they are absolute or they
are worthless.
MEASUREMENT OF MOTION IN THE LINE OF SIGHT
We must now describe another and quite distinct
application of the spectroscope, which is even more
marvellous than that already described. It is the
method of measuring the rate of motion of any of the
visible heavenly bodies in a direction either directly
towards us, or directly away from us, technically
described as radial motion, or by the expression
1 in the line of sight. And the extraordinary thing is
that this power of measurement is altogether inde
pendent of distance, so that the rate of motion in
miles per second of the remotest of the fixed stars, if
sufficiently bright to show a distinct spectrum, can be
measured with as much certainty and accuracy as in
the case of a much nearer star or a planet.
In order to understand how this is possible we
36 MAN S PLACE IN THE UNIVERSE [CHAP.
have again to refer to the wave-theory of light ; and
the analogy of other wave-motions will enable us
better to grasp the principle on which these calcula
tions depend. If on a nearly calm day we count the
waves that pass each minute by an anchored steam
boat, and then travel in the direction the waves come
from, we shall find that a larger number pass us in
the same time. Again, if we are standing near
a railway, and an engine comes towards us whistling,
we shall notice that it changes its tone as it passes
us ; and as it recedes the sound will be in a lower
key, although the engine may be at exactly the same
distance from us as when it was approaching. Yet
the sound does not change to the ear of the engine-
driver, the cause of the change being that the
sound-waves reach us in quicker succession as the
source of the waves is approaching us than when it
is retreating from us. Now, just as the pitch of
a note depends upon the rapidity with which the
successive air-vibrations reach our ear, so does the
colour of a particular part of the spectrum depend
upon the rapidity with which the ethereal waves
which produce colour reach our eyes ; and as this
rapidity is greater when the source of the light is
approaching than when it is receding from us, a
slight shifting of the position of the coloured bands,
and therefore of the dark lines, will occur, as com
pared with their position in the spectrum of the sun
or of any stationary source of light, if there is any
motion sufficient in amount to produce a perceptible
shift.
That such a change of colour would occur was
o
pointed out by Professor Doppler of Prague in 1842,
in.] THE NEW ASTRONOMY 37
and it is hence usually spoken of as the Doppler
principle ; but as the changes of colour were so
minute as to be impossible of measurement it was not
at that time of any practical importance in astronomy.
But when the dark lines in the spectrum were care
fully mapped, and their positions determined with
minute accuracy, it was seen that a means of measur
ing the changes produced by motion in the line of
sight existed, since the position of any of the dark
or coloured lines in the spectra of the heavenly bodies
could be compared with those of the corresponding
lines produced artificially in the laboratory. This
was first done in 1868 by Sir William Huggins, who,
by the use of a very powerful spectroscope constructed
for the purpose, found that such a change did occur
in the case of many stars, and that their rate of
motion towards us or away from us the radial
motion could be calculated. As the actual distance
of some of these stars had been measured, and their
change of position annually (their proper motion)
determined, the additional factor of the amount of
motion in the direction of our line of sight completed
the data required to fix their true line of motion
among the other stars. The accuracy of this method
under favourable conditions and with the best instru
ments is very great, as has been proved by those
cases in which we have independent means of calcu
lating the real motion. The motion of Venus towards
or away from us can be calculated with great accuracy
for any period, being a resultant of the combined
motions of the planet and of our earth in their re
spective orbits. The radial motions of Venus were
determined at the Lick Observatory in August and
38 MAN S PLACE IN THE UNIVERSE [CHAP.
September 1890, by spectroscopic observations, and
also by calculation, to be as follows :
By Observation. By Calculation.
Aug. 1 6th. 7 3 miles per second. 8 i miles per second.
22nd. 8-9 8-2 ,, ,,
3oth. 7-3 8-3
Sep. 3rd. 8-3 8-3
,, 4th. 8 2 ,, ,, ,, 8*3 ,, ,, ,,
showing that the maximum error was only one mile
per second, while the mean error was about a quarter
of a mile. In the case of the stars the accuracy of
the method has been tested by observations of the
same star at times when the earth s motion in its
orbit is towards or away from the star, whose
apparent radial velocity is, therefore, increased or
diminished by a known amount. Observations of
this kind were made by Dr. Vogel, Director of the
Astrophysical Observatory at Potsdam, showing, in
the case of three stars, of which ten observations
were taken, a mean error of about two miles per
second ; but as the stellar motions are more rapid
than those of the planets, the proportionate error is
no greater than in the example given above.
The great importance of this mode of determining
the real motion of the stars is, that it gives us a
knowledge of the scale on which such motions are
progressing ; and when in the course of time we
discover whether any of their paths are rectilinear
or curved, we shall be in a position to learn something
of the nature of the changes that are going on and of
the laws on which they depend.
in.] THE NEW ASTRONOMY 39
INVISIBLE STARS AND IMPERCEPTIBLE MOTIONS
But there is another result of this power of deter
mining radial motion which is even more unexpected
and marvellous, and which has extended our know
ledge of the stars in quite a new direction. By its
means it is possible to determine the existence of
invisible stars and to measure the rate of otherwise
imperceptible motions ; that is of stars which are
invisible in the most powerful modern telescopes, and
whose motions have such a limited range that no
telescope can detect them.
Double or binary stars forming systems which
revolve around their common centre of gravity were
discovered by Sir William Herschel, and very great
numbers are known ; but in most cases their periods
of revolution are long, the shortest being about
twelve years, while many extend to several hundred
years. These are, of course, all visible binaries, but
many are now known of which one star only is
visible while the other is either non-luminous or is so
close to its companion that they appear as a single
star in the most powerful telescopes. Many of the
variable stars belong to the former class, a good
example of which is Algol in the constellation
Perseus, which changes from the second to the fourth
magnitude in about four and a half hours, and in
about four and a half hours more regains its bril
liancy till its next period of obscuration which occurs
regularly every two days and twenty-one hours.
The name Algol is from the Arabic Al Ghoul, the
familiar ghoul of the Arabian Nights, so named
The Demon from its strange and weird behaviour.
40 MAN S PLACE IN THE UNIVERSE [CHAP.
It had long been conjectured that this obscuration
was due to a dark companion which partially eclipsed
the bright star at every revolution, showing that the
plane of the orbit of the pair was almost exactly
directed towards us. The application of the spectro
scope made this conjecture a certainty. At an equal
time before and after the obscuration, motion in the
line of sight was shown, towards and away from us,
at a rate of twenty-six miles per second. From these
scanty data and the laws of gravitation which fix the
period of revolution of planets at various distances
from their centres of revolution, Professor Pickering
of the Harvard Observatory was able to arrive at
the following figures as highly probable, and they
may be considered to be certainly not far from the
truth.
Diameter of Algol, . . . 1,061,000 miles.
Diameter of dark companion, 830,000 ,,
Distance between their centres, 3,230,000 ,,
Orbital speed of Algol, . . . 26-3 miles per sec.
Orbital speed of companion, . 55-4 ,, ,,
Mass of Algol, mass of our Sun.
Mass of companion, . . f ,, ,,
When it is considered that these figures relate
to a pair of stars only one of which has ever been
seen, that the orbital motion even of the visible star
cannot be detected in the most powerful telescopes,
when, further, we take into account the enormous dis
tance of these objects from us, the great results of
spectroscopic observation will be better appreciated.
But besides the marvel of such a discovery by such
simple means, the facts discovered are themselves in
the highest degree marvellous. All that we had
known of the stars through telescopic observation
in.] THE NEW ASTRONOMY 41
indicated that they were at very great distances from
each other however thickly they may appear scattered
over the sky. This is the case even with close
telescopic double stars, owing to their enormous
remoteness from us. It is now estimated that even
stars of the first magnitude are, on a general average,
about eighty millions of millions of miles distant;
while the closest double stars that can be distinctly
separated by large telescopes are about half a second
apart. These, if at the above distance, will be about
1500 millions of miles from each other. But in the
case of Algol and its companion, we have two bodies
both larger than our sun, yet with a distance of only
i\ millions of miles between their surfaces, a distance
not much exceeding their combined diameters. We
should not have anticipated that such huge bodies
could revolve so closely to each other, and as we
now know that the neighbourhood of our sun and
probably of all suns- -is full of meteoric and cometic
matter, it would seem probable that in the case of
two suns so near together the quantity of such matter
would be very great, and would lead probably by
continued collisions to increase of their bulk, and
perhaps to their final coalescence into a single giant
orb. It is said that a Persian astronomer in the
tenth century calls Algol a red star, while it is now
white or somewhat yellowish. This would imply an
increase of temperature caused by collisions or friction,
and increasing proximity of the pair of stars.
A considerable number of double stars with dark
companions have been discovered by means of the
spectroscope, although their motion is not directly in
the line of sight, and therefore there is no obscura-
42 MAN S PLACE IN THE UNIVERSE [CHAP.
tion. In order to discover such pairs the spectra of
large numbers of stars are taken on photographic
plates every night and for considerable periods-
for a year or for several years. These plates are
then carefully examined with a high magnifying
power to discover any periodical displacement of the
lines, and it is astonishing in how large a number of
cases this has been found to exist and the period of
revolution of the pair determined.
But besides discovering double stars of which one
is dark and one bright, many pairs of bright stars
have been discovered by the same means. The
method in this case is rather different. Each com
ponent star, being luminous, will give a separate
spectrum, and the best spectroscopes are so powerful
that they will separate these spectra when the stars
are at their maximum distance although no telescope
in existence, or ever likely to be made, can separate
the component stars. The separation of the spectra
is usually shown by the most prominent lines becom
ing double and then after a time single, indicating
that the plane of revolution is more or less obliquely
towards us, so that the two stars if visible would
appear to open out and then get nearer together
every revolution. Then, as each star alternately
approaches and recedes from us the radial velocity
of each can be determined, and this gives the relative
mass. In this way not only doubles, but triple and
multiple systems, have been discovered. The stars
proved to be double by these two methods are so
numerous that it has been estimated by one of the
best observers that about one star in every thirteen
shows inequality in its radial motion and is therefore
really a double star.
III.] THE NEW ASTRONOMY 43
THE NEBULAE
One other great result of spectrum-analysis, and
in some respects perhaps the greatest, is its demon
stration of the fact that true nebulae exist, and that
they are not all star-clusters so remote as to be
irresolvable, as was once supposed. They are shown
to have gaseous spectra, or sometimes gaseous and
stellar spectra combined, and this, in connection with
the fact that nebulae are frequently aggregated around
nebulous stars or groups of stars, renders it certain
that the nebulae are in no way separated in space
from the stars, but that they constitute essential parts
of one vast stellar universe. There is, indeed, good
reason to believe that they are really the material out
of which stars are made, and that in their forms,
aggregations, and condensations, we can trace the
very process of evolution of stars and suns.
PHOTOGRAPHIC ASTRONOMY
But there is yet another powerful engine of re
search which the new astronomy possesses, and
which, either alone or in combination with the spec
troscope, had produced and will yet produce in the
future an amount of knowledge of the stellar universe
which could never be attained by any other means.
It has already been stated how the discovery of new
variable and binary stars has been rendered possible
by the preservation of the photographic plates on
which the spectra are self-recorded, night after night,
with every line, whether dark or coloured, in true
position, so as to bear magnification, and, by com-
44 MAN S PLACE IN THE UNIVERSE [CHAP.
parison with others of the series, enabling the most
minute changes to be detected and their amount
accurately measured. Without the preservation of
such comparable records, which is in no other way
possible, by far the larger portion of spectroscopic
discoveries could never have been made.
But there are two other uses of photography of
quite a different nature which are equally and
perhaps in their final outcome may be far more
important. The first is, that by the use of the photo
graphic plate the exact positions of scores, hundreds,
or even thousands of stars can be self-mapped simul
taneously with extreme accuracy, while any number
of copies can be made of these star-maps. This en
tirely obviates the necessity for the old method of fix
ing the position of each star by repeated measurement
by means of very elaborate instruments, and their
registration in laborious and expensive catalogues.
So important is this now seen to be, that specially
constructed cameras are made for stellar photography,
and by means of the best kinds of equatorial mount
ing are made to revolve slowly so that the image of
each star remains stationary upon the plate for
several hours.
Arrangements have been now made among all the
chief observatories of the world to carry out a photo
graphic survey of the heavens with identical instru
ments, so as to produce maps of the whole star-
system on the same scale. These will serve as fixed
data for future astronomers, who will thus be able to
determine the movements of stars of all magnitudes
with a certainty and accuracy hitherto unattainable.
The other important use of photography depends
in.] THE NEW ASTRONOMY 45
upon the fact that with a longer exposure within cer
tain limits we increase the light-collecting power. It
will surprise many persons to learn that an ordinary
good portrait-camera with a lens three or four inches
in diameter, if properly mounted so that an exposure
of several hours can be made, will show stars so
minute that they are invisible even in the great Lick
telescope. In this way the camera will often reveal
double-stars or small groups which can be made
visible in no other way.
Such photographs of the stars are now constantly
reproduced in works on Astronomy and in popular
magazine articles, and although some of them are
very striking, many persons are disappointed with
them, and cannot understand their great value, be
cause each star is represented by a white circle often
of considerable size and with a somewhat undefined
outline, not by a minute point of light as stars appear
in a good telescope. But the essential matter in all
such photographs is not so much the smallness, as
the roundness, of the star - images, as this proves
the extreme precision with which the image of every
star has been kept by the clockwork motion of the
instrument on the same point of the plate during the
whole exposure. For example, in the fine photo
graph of the Great Nebula in Andromeda, taken
2Qth December 1888, by Dr. Isaac Roberts, with an
exposure of four hours, there are probably over a
thousand stars large and small to be seen, every one
represented by an almost exactly circular white dot
of a size dependent on the magnitude of the star.
These round dots can be bisected by the cross hairs
of a micrometer with very great accuracy, and thus
46 MAN S PLACE IN THE UNIVERSE [CHAP.III.
the distance between the centres of any of the pairs,
as well as the direction of the line joining their
centres, can be determined as accurately as if each
was represented by a point only. But as a minute
white speck would be almost invisible on the maps,
and would convey no information as to the approxi
mate magnitude of the star, mistakes would be much
more easily made, and it would probably be found
necessary to surround each star with a circle to
indicate its magnitude, and to enable it to be easily
seen. It is probable, therefore, that the supposed
defect is really an important advantage. The above-
mentioned photograph is beautifully reproduced in
Proctor s Old and New Astronomy, published after
his greatly lamented death.
But besides the amount of altogether new know
ledge obtained by the methods of research here
briefly explained, a great deal of light has been
thrown on the distribution of the stars as a whole,
and hence on the nature and extent of the stellar
universe, by a careful study of the materials obtained
by the old methods, and by the application of the
doctrine of probabilities to the observed facts. In
this way alone some very striking results have been
reached, and these have been supported and strength
ened by the newer methods, and also by the use of
new instruments in the measurement of stellar dis
tances. Some of these results bear so closely and
directly upon the special subject of the present
volume, that our next chapter must be devoted to a
consideration of them.
CHAPTER IV
THE DISTRIBUTION OF THE STARS
IF we look at the heavens on a clear, moonless night
in winter, and from a position embracing the entire
horizon, the scene is an inexpressibly grand one.
The intense sparkling brilliancy of Sirius, Capella,
Vega, and other stars of the first magnitude ; their
striking arrangement in constellations or groups, of
which Orion, the Great Bear, Cassiopeia, and the
Pleiades, are familiar examples ; and the filling up
between these by less and less brilliant points down
to the limit of vision, so as to cover the whole sky
with a scintillating tracery of minute points of light,
convey together an idea of such confused scattering
and such enormous numbers, that it seems impossible
to count them or to reduce them to systematic order.
Yet this was done for all except the faintest stars by
Hipparchus, 134 B.C., who catalogued and fixed the
positions of more than 1000 stars, and this is about
the number, down to the fifth magnitude, visible in
the latitude of Greece. A recent enumeration of all
the stars visible to the naked eye, under the most
favourable conditions and by the best eyesight, has
been made by the American astronomer, Pickering.
His numbers are for the Northern Hemisphere
2509, and for the Southern Hemisphere 2824, thus
48 MAN S PLACE IN THE UNIVERSE [CHAP.
showing a somewhat greater richness in the southern
celestial hemisphere. But as this difference is due
entirely to a preponderance of stars between mag
nitudes 5^ and 6, that is, just on the limits of vision,
while those down to magnitude 5^- are more numerous
by 85 in the Northern Hemisphere, Professor New-
comb is of opinion that there is no real superiority of
numbers of visible stars in one hemisphere over the
other. Again, the total number of the visible stars
by the above enumeration is 5333. But this includes
stars down to 6*2 magnitude, while it is generally
considered that magnitude 6 marks the limit of
visibility. On a re-examination of all the materials,
the Italian astronomer Schiaparelli concludes that
the total number of stars down to the sixth magnitude
is 4303 ; and they seem to be about equally divided
between the northern and southern skies.
THE MILKY WAY
But besides the stars themselves, a most con
spicuous object both in the northern and southern
hemisphere is that wonderful irregular belt of faintly
diffused light termed the Milky Way or Galaxy.
This forms a magnificent arch across the sky, best
seen in the autumn months in our latitude. This
arch, while following the general course of a great
circle round the heavens, is extremely irregular in
detail, sometimes being single, sometimes double,
sending off occasional branches or offshoots, and
also containing in its very midst dark rifts, spots,
or patches, where the black background of almost
starless sky can be seen through it. When examined
IV.] THE DISTRIBUTION OF THE STARS 49
through an opera-glass or small telescope quantities
of stars are seen on the luminous background, and
with every increase in the size and power of the
telescope more and more stars become visible, till
with the largest and best modern instruments the
whole of the Galaxy seems densely packed with them,
though still full of irregularities, wavy streams of
stars, and dark rifts and patches, but always showing
a faint nebulous background as if there remained
other myriads of stars which a still higher optical
power would reveal.
The relations of this great belt of telescopic stars
to the rest of the star-system have long interested
astronomers, and many have attempted its solution.
By a system of gauging, that is counting all the stars
that passed over the field of his telescope in a certain
time, Sir William Herschel was the first who made
a systematic effort to determine the shape of the
stellar universe. From the fact that the number of
stars increased rapidly as the Milky Way was ap
proached from whatever direction, while in the
Galaxy itself the numbers visible were at once more
than doubled, he formed the idea that the shape of
the entire system must be that of a highly compressed
very broad mass or ring rather less dense towards the
centre where our sun was situated. Roughly speak
ing, the form was likened to a flat disc or grindstone,
but of irregular thickness, and split in two on one
side where it appears to be double. The immense
quantity of the stars which formed it was supposed
to be due to the fact that we looked at it edgewise
through an immense depth of stars ; while at right
angles to its direction when looking towards what is
D
50 MAN S PLACE IN THE UNIVERSE [CHAP.
termed the pole of the Galaxy, and also in a less
degree when looking obliquely, we see out into
space through a much thinner stratum of stars, which
thus seem on the average to be very much farther
apart.
But, in the latter part of his life, Sir William
Herschel realised that this was not the true explana
tion of the features presented by the Galaxy. The
brilliant spots and patches in it, the dark rifts and
openings, the narrow streams of light often bounded
by equally narrow streams or rifts of darkness, render
it quite impossible to conceive that this complex
luminous ring has the form of a compressed disc
extending in the direction in which we see it to a
distance many times greater than its thickness. In
one very luminous cluster Herschel thought that
his telescope had penetrated to regions twenty times
as far off as the more brilliant stars forming the
nearer portions of the same object. Now, in the case
of the Magellanic clouds, which are two roundish
nebular patches of large size some distance from the
Milky Way in the Southern Hemisphere and looking
like detached portions of it, Sir John Herschel him
self has shown that any such interpretation of its
form is impossible ; because it requires us to suppose
that in both these cases we see, not rounded masses of
a roughly globular shape, but immensely long cones or
cylinders, placed in such a direction that we see only
the ends of them. He remarks that one such object
so situated would be an extraordinary coincidence,
but that there should be two or many such is alto
gether out of the question. But in the Milky Way
there are hundreds or even thousands of such spots
IV.] THE DISTRIBUTION OF THE STARS 51
or masses of exceptional brilliancy or exceptional
darkness ; and, if the form of the Galaxy is that of a
disc many times broader than thick, and which we
see edgewise, then every one of these patches and
clusters, and all the narrow winding streams of bright
light or intense blackness, must be really excessively
long cylinders, or tunnels, or deep curving laminae,
or narrow fissures. And every one of these, which
are to be found in every part of this vast circle of
luminosity, must be so arranged as to be exactly
turned towards our sun. The weight of this argu
ment, which has been most forcibly and clearly set
forth by the late Mr. R. A. Proctor, in his very
instructive volume Our Place among Infinities, is
now generally admitted by astronomers, and the
natural conclusion is that the form of the Milky
Way is that of a vast irregular ring, of which the
section at any part is, roughly speaking, circular ;
while the many narrow rifts or lanes or openings
where we seem to be able to see completely through
it to the darkness of outer space beyond, render it
probable that in those directions its thickness is less
instead of greater than its apparent width, that is,
that we see the broader side rather than the narrow
edge of it.
Before entering on the consideration of the rela
tions which the bulk of the stars we see scattered
over the entire vault of heaven bear to this great
belt of telescopic stars, it will be advisable to give
a somewhat full description of the Galaxy itself, both
because it is not often delineated on star-maps with
sufficient accuracy, or so as to show its wonderful
intricacies of structure, and also because it constitutes
52 MAN S PLACE IN THE UNIVERSE [CHAP.
the fundamental phenomenon upon which the argu
ment set forth in this volume primarily rests. For
this purpose I shall use the description of it given by
Sir John Herschel in his Outlines of Astronomy,
both because he, of all the astronomers of the last
century, had studied it most thoroughly, in the
northern and in the southern hemispheres, by eye-
observation and with the aid of telescopes of great
power and admirable quality ; and also because, amid
the throng of modern works and the exciting novel
ties of the last thirty years, his instructive volume is,
comparatively speaking, very little known. This
precise and careful description will also be of service
to any of my readers who may wish to form a closer
personal acquaintance with this magnificent and in
tensely interesting object, by examining its peculi
arities of form and beauties of structure either with
the naked eye, or with the aid of a good opera-glass,
or with a small telescope of good defining power.
A DESCRIPTION OF THE MILKY WAY
Sir John Herschel s description is as follows :-
The course of the Milky Way as traced through the
heavens by the unaided eye, neglecting occasional
deviations and following the line of its greatest
brightness as well as its varying breadth and inten
sity will permit, conforms, as nearly as the indefinite-
ness of its boundary will allow it to be fixed, to that of
a great circle inclined at an angle of about 63 to the
equinoctial, and cutting that circle in Right Ascen
sion 6h. 47m. and i8h. 47m., so that its northern and
southern poles respectively are situated in Right
IV.] THE DISTRIBUTION OF THE STARS 53
Ascension I2h. 47m., North Polar Distance 63, and
R.A. oh. 47m., NPD. 117. Throughout the region
where it is so remarkably subdivided, this great circle
holds an intermediate situation between the two great
streams ; with a nearer approximation however to the
brighter and continuous stream than to the fainter
and interrupted one. If we trace its course in order
of right ascension, we find it traversing the constella
tion Cassiopeia, its brightest part passing about two
degrees to the north of the star Delta of that con
stellation. Passing thence between Gamma and
Epsilon Cassiopeise, it sends off a branch to the
south-preceding side, towards Alpha Persei, very con
spicuous as far as that star, prolonged faintly towards
Eta of the same constellation, and possibly traceable
towards the Hyades and Pleiades as remote outliers.
The main stream, however (which is here very faint),
passes on through Auriga, over the three remarkable
stars, Epsilon, Zeta, Eta, of that constellation called
the Haedi, preceding Capella, between the feet of
Gemini and the horns of the Bull (where it intersects
the ecliptic nearly in the Solstitial Colure) and thence
over the club of Orion to the neck of Monoceros,
intersecting the equinoctial in R.A. 6h. 54m. Up to
this point, from the offset in Perseus, its light is feeble
and indefinite, but thenceforward it receives a gradual
accession of brightness, and where it passes through
the shoulder of Monoceros and over the head of
Canis Major it presents a broad, moderately bright,
very uniform, and to the naked eye, starless stream
up to the point where it enters the prow of the ship
Argo, nearly on the southern tropic. Here it again
subdivides (about the star m Puppis), sending off a
54 MAN S PLACE IN THE UNIVERSE [CHAP.
narrow and winding branch on the preceding side as
far as Gamma Argus, where it terminates abruptly.
The main stream pursues its southward course to
the 1 23rd parallel of NPD., where it diffuses itself
broadly and again subdivides, opening out into a wide
fan-like expanse, nearly 20 in breadth, formed of
interlacing branches, which all terminate abruptly,
in a line drawn nearly through Lambda and Gamma
Argus.
* At this place the continuity of the Milky Way is
interrupted by a wide gap, and where it recommences
on the opposite side it is by a somewhat similar fan-
shaped assemblage of branches which converge upon
the bright star Eta Argus. Thence it crosses the
hind feet of the Centaur, forming a curious and
sharply-defined semicircular concavity of small radius,
and enters the Cross by a very bright neck or isthmus
of not more than three or four degrees in breadth,
being the narrowest portion of the Milky Way. After
this it immediately expands into a broad and bright
mass, enclosing the stars Alpha and Beta Crucis and
Beta Centauri, and extending almost up to Alpha of
the latter constellation. In the midst of this bright
mass, surrounded by it on all sides, and occupying
about half its breadth, occurs a singular dark pear-
shaped vacancy, so conspicuous and remarkable as
to attract the notice of the most superficial gazer
and to have acquired among the early southern
navigators the uncouth but expressive appellation
of the coal- sack. In this vacancy, which is about
8 in length and 5 broad, only one very small star
visible to the naked eye occurs, though it is far
from devoid of telescopic stars, so that its striking
iv.] THE DISTRIBUTION OF THE STARS 55
blackness is simply due to the effect of contrast with
the brilliant ground with which it is on all sides sur
rounded. This is the place of nearest approach of
the Milky Way to the South Pole. Throughout all
this region its brightness is very striking, and when
compared with that of its more northern course
already traced, conveys strongly the impression of
greater proximity, and would almost lead to a belief
that our situation as spectators is separated on all
sides by a considerable interval from the dense body
of stars composing the Galaxy, which in this view of
the subject would come to be considered as a flat ring
or some other re-entering form of immense and irre
gular breadth and thickness, within which we are
o
excentrically situated, nearer to the southern than to
the northern part of its circuit.
At Alpha Centauri the Milky Way again sub
divides, sending off a great branch of nearly half its
breadth, but which thins off rapidly, at an angle of
about 20 with its general direction to Eta and d Lupi,
beyond which it loses itself in a narrow and faint
streamlet. The main stream passes on increasing in
breadth to Gamma Normse, where it makes an
abrupt elbow and again subdivides into one principal
and continuous stream of very irregular breadth and
brightness, and a complicated system of interlaced
streaks and masses, which covers the tail of Scorpio,
and terminates in a vast and faint effusion over the
whole extensive region occupied by the preceding
leg of Ophiuchus, extending northward to the
parallel of 103 NPD., beyond which it cannot be
traced ; a wide interval of 14, free from all appear
ance of nebulous light, separating it from the great
56 MAN S PLACE IN THE UNIVERSE [CHAP.
branch on the north side of the equinoctial of which
it is usually represented as a continuation.
Returning to the point of separation of this great
branch from the main stream, let us now pursue the
course of the latter. Making an abrupt bend to the
following side, it passes over the stars Iota Arae,
Theta and Iota Scorpii, and Gamma Tubi to
Gamma Sagittarii, where it suddenly collects into
a vivid oval mass about 6 in length and 4 in
breadth, so excessively rich in stars that a very
moderate calculation makes their number exceed
100,000. Northward of this mass, this stream
crosses the ecliptic in longitude about 276, and
proceeding along the bow of Sagittarius into
Antinous has its course rippled by three deep con
cavities, separated from each other by remarkable
protuberances, of which the larger and brighter forms
the most conspicuous patch in the southern portion
of the Milky Way visible in our latitudes.
Crossing the equinoctial at the iQth hour of
R.A., it next runs in an irregular, patchy, and wind
ing stream through Aquila, Sagitta, and Vulpecula
up to Cygnus ; at Epsilon of which constellation its
continuity is interrupted, and a very confused and
irregular region commences, marked by a broad dark
vacuity, not unlike the southern "coal-sack," occupy
ing the space between Epsilon, Alpha, and Gamma
Cygni, which serves as a kind of centre for the
divergence of three great streams ; one, which we
have already traced ; a second, the continuation of
the first (across the interval) from Alpha northward,
between Lacerta and the head of Cepheus to the
point in Cassiopeia whence we set out, and a third
iv.] THE DISTRIBUTION OF THE STARS 57
branching off from Gamma Cygni, very vivid and
conspicuous, running off in a southern direction
through Beta Cygni, and s Aquilae almost to the
equinoctial, where it loses itself in a region thinly
sprinkled with stars, where in some maps the modern
constellation Taurus Poniatowski is placed. This is
the branch which, if continued across the equinoctial,
might be supposed to unite with the great southern
effusion in Ophiuchus already noticed. A consider
able offset, or protuberant appendage, is also thrown
off by the northern stream from the head of Cepheus
directly towards the pole, occupying the greater part
of the quartile formed by Alpha, Beta, Iota, and
Delta of that constellation.
To complete this careful, detailed description of
the Milky Way, it will be well to add a few passages
from the same work as to its telescopic appearance
and structure.
When examined with powerful telescopes, the
constitution of this wonderful zone is found to be no
less various than its aspect to the naked eye is
irregular. In some regions the stars of which it is
composed are scattered with remarkable uniformity
over immense tracts, while in others the irregularity
of their distribution is quite as striking, exhibiting a
rapid succession of closely clustering rich patches
separated by comparatively poor intervals, and in
deed in some instances by spaces absolutely dark
and completely void of any star, even of the smallest
telescopic magnitude. In some places not more than
40 or 50 stars on an average occur in a gauge-field
of 15 , while in others a similar average gives a
result of 400 or 500. Nor is less variety observable
58 MAN S PLACE IN THE UNIVERSE [CHAP.
in the character of its different regions in respect of
the magnitudes of the stars they exhibit, and the
proportional numbers of the larger and smaller
magnitudes associated together, than in respect of
their aggregate numbers. In some, for instance,
extremely minute stars occur in numbers so moderate
as to lead us irresistibly to the conclusion that in
these regions we see fairly through the starry
stratum, since it is impossible otherwise that the
numbers of the smaller magnitudes should not go on
continually increasing ad infinitum. In such cases,
moreover, the ground of the heavens is for the most
part perfectly dark, which again would not be the
case if innumerable multitudes of stars, too minute
to be individually discernible, existed beyond. In
other regions we are presented with the phenomenon
of an almost uniform degree of brightness of the
individual stars, accompanied with a very even dis
tribution of them over the ground of the heavens,
both the larger and smaller magnitudes being
strikingly deficient. In such cases it is equally
impossible not to perceive that we are looking
through a sheet of stars nearly of a size, and of
no great thickness compared with the distance which
separates them from us. Were it otherwise we
should be driven to suppose the more distant stars
uniformly the larger, so as to compensate by their
greater intrinsic brightness for their greater distance,
a supposition contrary to all probability. . . .
* Throughout by far the larger portion of the
extent of the Milky Way in both hemispheres, the
general blackness of the ground of the heavens on
which its stars are projected, and the absence of that
iv.] THE DISTRIBUTION OF THE STARS 59
innumerable multitude and excessive crowding of the
smallest visible magnitudes, and of glare produced by
the aggregate light of multitudes too small to affect
the eye singly, must, we think, be considered un
equivocal indications that its dimensions in directions
where these conditions obtain are not only not infinite,
but that the space-penetrating power of our telescopes
suffices fairly to pierce through and beyond it.
In the above-quoted passages the italics are those
of Sir John Herschel himself, and we see that he
drew the very same conclusions from the facts he
describes, and for much the same reasons, as Mr.
Proctor has drawn from the observations of Sir
William Herschel ; and, as we shall see, the best
astronomers to-day have arrived at a similar result,
from the additional facts at their disposal, and in
some cases from fresh lines of arument.
THE STARS IN RELATION TO THE MILKY WAY
Sir John Herschel was so impressed with the
form, structure, and immensity of the Galactic Circle,
as he sometimes terms it, that he says (in a footnote
p. 575, loth ed.), This circle is to sidereal what
the invariable ecliptic is to planetary astronomy- -a
plane of ultimate reference, the ground-plane of the
sidereal system. We have now to consider what are
the relations of the whole body of the stars to this
Galactic Circle this plane of ultimate reference for
the whole stellar universe.
If we look at the heavens on a starry night, the
whole vault appears to be thickly strewn with stars
of various degrees of brightness, so that we could
60 MAN S PLACE IN THE UNIVERSE [CHAP.
hardly say that any extensive region the north,
east, south, or west, or the portion vertically above
us is very conspicuously deficient or superior in
numbers. In every part there are to be found a fair
proportion of stars of the first two or three magni
tudes, while where these may seem deficient a crowd
of smaller stars takes their place.
But an accurate survey of the visible stars shows
that there is a large amount of irregularity in their
distribution, and that all magnitudes are really more
numerous in or near the Milky Way, than at a dis
tance from it, though not in so large a degree as to
be very conspicuous to the naked eye. The area
of the whole of the Milky Way cannot be estimated
at more than one-seventh of the whole sphere, while
some astronomers reckon it at only one-tenth. If
stars of any particular size were uniformly distributed,
at most one-seventh of the whole number should be
found within its limits. But Mr. Gore finds that of
32 stars brighter than the second magnitude 12 lie
upon the Milky Way, or considerably more than
twice as many as there should be if they were
uniformly distributed. And in the case of the 99
stars which are brighter than the third magnitude 33
lie upon the Milky Way, or one-third instead of one-
seventh. Mr. Gore also counted all the stars in Heis s
Atlas which lie upon the Milky Way, and finds there
are 1186 out of a total of 5356, a proportion of
between a fourth and a fifth instead of a seventh.
The late Mr. Proctor in 1871 laid down on a chart
two feet diameter all the stars down to magnitude
9i given in Agrelander s forty large charts of the
stars visible in the northern hemisphere. They were
IV.] THE DISTRIBUTION OF THE STARS 61
324,198 in number, and they distinctly showed by
their greater density not only the whole course of the
Milky Way but also its more luminous portions and
many of the curious dark rifts and vacuities, which
latter are almost wholly avoided by these stars.
Later on Professor Seeliger of Munich made an
investigation of the relation of more than 135,000
stars down to the ninth magnitude to the Milky Way,
by dividing the whole of the heavens into nine
regions, one and nine being circles of 20 wide (equal
to 40 diameter) at the two poles of the Galaxy ; the
middle region, five, is a zone 20 wide including the
Milky Way itself, and the other six intermediate
zones are each 20 wide. The following table shows
the results as given by Professor Newcomb, who has
made some alterations in the last column of Density
of Stars in order to correct differences in the estimate
of magnitudes by the different authorities.
Regions. Area in Degrees. Number of Stars. Density.
I. 1,3987 4,277 278
3,146-9 10,185 3 3
III. 5,126-6 19,488 3-54
IV. 4,5 8 9 8 24,492 5-32
v - 4,519 S 33,267 8-17
VI. 3,97i-5 23,580 6-07
2,954-4 11,790 37i
VIII. 1,796-6 6,375 3-21
IX. 468-2 1,644 3*14
.#.-#.- -The inequality of the N. and S. areas is because the
enumeration of the stars only went as far as 24 S. Decl., and there
fore included only a part of Regions vn., vm., and ix.
Upon this table of densities Professor Newcomb
remarks as follows : The star-density in the several
regions increases continuously from each pole (regions
62 MAN S PLACE IN THE UNIVERSE [CHAP.
i. and ix.) to the Galaxy itself (region v.). If the
latter were a simple ring of stars surrounding a
DIAGRAM OF STAR-DENSITY
n m JT v vi
From Herschel s Gauges (as given by Professor Newcomb, p. 251).
spherical system of stars, the star-density would be
about the same in regions i., IL, and in., and also
in vii., VIIL, and ix., but would suddenly increase
in iv. and vi. as the boundary of the ring was
approached. Instead of such being the case, the
numbers 278, 3*03, and 3*54 in the north, and 3*14,
3 *2 1, and 371 in the south, show a progressive
increase from the galactic pole to the Galaxy itself.
iv.] THE DISTRIBUTION OF THE STARS 63
The conclusion to be drawn is a fundamental one.
The universe, or at least the denser portion of it, is
really flattened between the galactic poles, as supposed
by Herschel and Struve/
But looking at the series of figures in the table, and
again as quoted by Professor Newcomb, they seem
to me to show in some measure what he says they
do not show. I therefore drew out the above diagram
from the figures in the table, and it certainly shows
that the density in regions i., n., and in., and in
regions VIL, vni., and ix., may be said to be
about the same/ that is, they increase very slowly,
and that they do suddenly increase in iv. and vi.
as the boundary of the Galaxy is approached. This
may be explained either by a flattening towards the
poles of the Galaxy, or by the thinning out of stars
in that direction.
In order to show the enormous difference of star-
density in the Galaxy and at the galactic poles,
Professor Newcomb gives the following table of the
Herschelian gauges, on which he only remarks that
they show an enormously increased density in the
galactic region due to the Herschels having counted
so many more stars there than any other observers.
Region,
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
IX.
Density,
107
J 54
281
560
2,019
672
261
154
Ill
But an important characteristic of these figures is,
that the Herschels alone surveyed the whole of the
heavens from the north to the south pole, that they
did this with instruments of the same size and quality,
6 4
MAN S PLACE IN THE UNIVERSE [CHAP.
and that from almost life-long experience in this
particular work they were unrivalled in their power
of counting rapidly and accurately the stars that
passed over each field of view of their telescopes.
DIAGRAM OF STAR-DENSITY
it ur nr v
From a table in The Stars (p. 249).
Their results, therefore, must be held to have a com
parative value far above those of any other observer
or combination of observers. I have therefore thought
it advisable to draw a diagram from their figures,
and it will be seen how strikingly it agrees with the
former diagram in the very slow increase of star-
richness in the first three regions north and south,
the sudden increase in regions iv. and vi. as we
approach the Galaxy, while the only marked differ
ence is in the enormously greater richness of the
IV.] THE DISTRIBUTION OF THE STARS 65
Galaxy itself, which is an undoubtedly real pheno
menon, and is brought out here by the unrivalled
observing power of the two greatest astronomers in
this special department that have ever lived.
We shall find later on that Professor Newcomb
himself, as the result of a quite different inquiry
arrives at a result in accordance with these diagrams
which will then be again referred to. As this is a
very interesting subject, it will be well to give another
diagram from two tables of star-density in Sir John
Herschel s volume already quoted. The tables are
as follows :
Zones of Galactic Average number of Stars
North Polar Distance. per Field of 15 .
o to 15 . . . . 4-32
15 to 30 .... 5-42
30 to 45 . . . . 8 2i
45 to 60 .... 13*61
60 to 75 .... 24*09
75 to 90 . . . . 53-43
Zones of Galactic Average number of Stars
South Polar Distance. per Field of 15 ,
to 15 . . . . 6*05
15 to 30 .... 6 62
30 to 45 . . . , 9 o8
45 to 60 .... *3 49
60 to 75 .... 26-29
75 to 90 .... 59 o6
In these tables the Milky Way itself is taken as
occupying two zones of 15 each, instead of one of
20 as in Professor Newcomb s tables, so that the
excess in the number of stars over the other zones is
not so large. They show also a slight preponderance
in all the zones of the southern hemisphere, but this
E
66
MAN S PLACE IN THE UNIVERSE [CHAP.
is not great, and may probably be due to the clearer
atmosphere of the Cape of Good Hope as compared
with that of England.
DIAGRAM OF STAR-DENSITY.
Galactic
ZLune
From Table in Sir J. Herschel s Outlines of Astronomy
(loth ed., pp. 577-578).
It need only be noted here that this diagram shows
the same general features as those already given, of
a continuous increase of star-density from the poles
of the Galaxy, but more rapidly as the Galaxy itself
IV.] THE DISTRIBUTION OF THE STARS 67
is more nearly approached. This fact must, there
fore, be accepted as indisputable.
CLUSTERS AND NEBULAE IN RELATION TO THE GALAXY
An important factor in the structure of the heavens
is afforded by the distribution of the two classes of
objects known as clusters and nebulae. Although we
can form an almost continuous series from double stars
which revolve round their common centre of gravity,
through triple and quadruple stars, to groups and
aggregations of indefinite extent of which the
Pleiades form a good example, since the six stars
visible to the naked eye are increased to hundreds
by high telescopic powers, while photographs with
three hours exposure show more than 2000 stars
yet none of these correspond to the large class known
as clusters, whether globular or irregular, which
are very numerous, about 600 having been re
corded by Sir John Herschel more than fifty years
ago. Many of these are among the most beautiful
and striking objects in the heavens even with a very
small telescope or good opera-glass. Such is the
luminous spot called Praesepe, or the Beehive in
the constellation Cancer, and another in the sword-
handle of Perseus.
In the southern hemisphere there is a hazy star
of about the fourth magnitude, Omega Centauri,
which with a good telescope is seen to be really a
magnificent cluster nearly two-thirds the diameter of
the moon, and described by Sir John Herschel as very
gradually increasing in brightness to the centre, and
composed of innumerable stars of the thirteenth and
68 MAN S PLACE IN THE UNIVERSE [CHAP.
fifteenth magnitudes, forming the richest and largest
object of the kind in the heavens. He describes it
as having rings like lace-work formed of the larger
stars. By actual count, on a good photograph,
there are more than 6000 stars, while other
observers consider that there are at least 10,000.
In the northern hemisphere one of the finest is that
in the constellation Hercules, known as 13 Messier.
It is just visible to the naked eye or with an opera-
glass as a hazy star of the sixth magnitude, but a
good telescope shows it to be a globular cluster, and
the great Lick telescope resolves even the densest
central portion into distinct stars, of which Sir John
Herschel considered there were many thousands.
These two fine clusters are figured in many of the
modern popular works on astronomy, and they afford
an excellent idea of these beautiful and remarkable
objects, which, when more thoroughly studied, will
probably aid in elucidating some of the obscure
problems connected with the constitution and de
velopment of the stellar universe.
But for the purpose of the present work the most
interesting fact connected with star-clusters is their
remarkable distribution in the heavens. Their special
abundance in and near the Milky Way had often
been noted, but the full importance of the fact could
not be appreciated till Mr. Proctor and, later, Mr.
Sidney Waters marked down, on maps of the two
hemispheres, all the star-clusters and nebulae in the
best catalogues. The result is most interesting. The
clusters are seen to be thickly strewn over the entire
course of the Milky Way, and along its margins,
while in every other part of the heavens they are
iv.] THE DISTRIBUTION OF THE STARS 69
thinly scattered at very distant intervals, with the one
exception of the Magellanic clouds of the southern
hemisphere where they are again densely grouped ;
and if anything were needed to prove the physical
connection of these clusters with the Galaxy it would
be their occurrence in these extensive nebulous
patches which seem like outlying portions of the
Milky Way itself. With these two exceptions pro
bably not one-twentieth part of the whole number of
star-clusters are found in any part of the heavens
remote from the Milky Way.
Nebulae were for a long time confounded with star-
clusters, because it was thought that with sufficient
telescopic power they would all be resolvable into
stars as in the case of the Milky Way itself. But
when the spectroscope showed that many of the
nebulae consisted wholly or mainly of glowing gases,
while neither the highest powers of the best telescopes
nor the still greater powers of the photographic plate
gave any indications of resolvability, although a few
stars were often found to be, as it were, entangled in
them, and evidently forming part of them, it was
seen that they constituted a distinct stellar pheno
menon, a view which was enforced and rendered
certain by their quite unique mode of distribution.
A few of the larger and irregular type, as in the case
of the grand Orion nebula visible to the naked eye,
the great spiral nebula in Andromeda, and the won
derful Keyhole nebula round Eta Argus, are situated
in or near the Milky Way ; but with these and a few
other exceptions the overwhelming majority of the
smaller irresolvable nebulae appear to avoid it, there
being a space almost wholly free from nebulae along
70 MAN S PLACE IN THE UNIVERSE [CHAP-.
its borders, both in the northern and southern hemi
spheres ; while the great majority are spread over the
sky, far away from it in the southern hemisphere, and
in the north clustering in a very marked degree
around the galactic pole. The distribution of nebulae
is thus seen to be the exact opposite to that of the
star-clusters, while both are so distinctly related to
the position of the Milky Way the ground-plane of
the sidereal system, as Sir John Herschel termed it
-that we are compelled to include them all as con
nected portions of one grand and, to some extent,
symmetrical universe, whose remarkable and opposite
mode of distribution over the heavens may probably
afford a clue to the mode of development of that
universe and to the changes that are even now tak
ing place within it. The maps referred to above are
of such great importance, and are so essential to a
clear comprehension of the nature and constitution
of the vast sidereal system which surrounds us, that
I have, with the permission of the Royal Astronomical
Society, reproduced them here. (See end of volume.)
A careful examination of them will give a clearer
idea of the very remarkable facts of distribution of
star-clusters and nebulae than can be afforded by any
amount of description or of numerical statements.
The forms of many of the nebulae are very curious.
Some are quite irregular, as the Orion nebula, the
Keyhole nebula in the southern hemisphere, and
many others. Some show a decidedly spiral form, as
those in Andromeda and Canes Venatici ; others
again are annular or ring-shaped, as those in Lyra
and Cygnus, while a considerable number are termed
planetary nebulae, from their exhibiting a faint circular
IV.] THE DISTRIBUTION OF THE STARS 71
disc like that of a planet. Many have stars or groups
of stars evidently forming parts of them, and this is
especially the case with those of the largest size.
But all these are comparatively few in number and
more or less exceptional in type, the great majority
being minute cloudy specks only visible with good
telescopes, and so faint as to leave much doubt as to
their exact shape and nature. Sir John Herschel
catalogued 5000 in 1864, and more than 8000 were
discovered up to 1890; while the application of the
camera has so increased the numbers that it is thought
there may really be many hundreds of thousands of
them.
The spectroscope shows the larger irregular nebulae
to be gaseous, as are the annular and planetary
nebulae as well as many very brilliant white stars ;
and all these objects are most frequent in or near the
Milky Way. Their spectra show a green line not
produced by any terrestrial element. With the great
Lick telescope several of the planetary nebulae have
been found to be irregular and sometimes to be
formed of compressed or looped rings and other
curious forms.
Many of the smaller nebulae are double or triple,
but whether they really form revolving systems is
not yet known. The great mass of the small nebulae
that occupy large tracts of the heavens remote from
the Galaxy are often termed irresolvable nebulae,
because the highest powers of the largest telescopes
show no indication of their being star-clusters, while
they are too faint to give any definite indications of
structure in the spectroscope. But many of them
resemble comets in their forms, and it is thought not
72 MAN S PLACE IN THE UNIVERSE [CHAP.IV.
impossible that they may be not very dissimilar in
constitution.
We have now passed in review the main features
presented to us in the heavens outside the solar
system, so far as regards the numbers and distribu
tion of the lucid stars (those visible to the naked eye)
as well as those brought to view by the telescope ;
the form and chief characteristics of the Milky Way
or Galaxy ; and lastly, the numbers and distribution
of those interesting objects star-clusters and nebulae
in their special relations to the Milky Way. This
examination has brought clearly before us the unity
of the whole visible universe ; that everything we can
see, or obtain any knowledge of, with all the resources
of modern gigantic telescopes, of the photographic
plate, and of the even more marvellous spectroscope,
forms parts of one vast system which may be shortly
and appropriately termed the Stellar universe.
In our next chapter we shall carry the investigation
a step further, by sketching in outline what is known
of the motions and distances of the stars, and thus
obtain some important information bearing upon our
special subject of inquiry.
CHAPTER V
DISTANCE OF THE STARS THE SUN*S MOTION
THROUGH SPACE
IN early ages, before any approximate idea was
reached of the great distances of the stars from us,
the simple conception of a crystal sphere to which
these luminous points were attached and carried
round every day on an axis near which our
pole-star is situated, satisfied the demands for an
explanation of the phenomena. But when Copernicus
set forth the true arrangement of the heavenly bodies,
earth and planets alike revolving round the sun at
distances of many millions of miles, and when this
scheme was enforced by the laws of Kepler and the
telescopic discoveries of Galileo, a difficulty arose
which astronomers were unable satisfactorily to over
come. If, said they, the earth revolves round the
sun at a distance which cannot be less (according to
Kepler s measurement of the distance of Mars at
opposition) than 13^ millions of miles, then how is
it that the nearer stars are not seen to shift their
apparent places when viewed from opposite sides of
this enormous orbit ? Copernicus, and after him
Kepler and Galileo, stoutly maintained that it was
because the stars were at such an enormous distance
from us that the earth s orbit was a mere point in
74 MAN S PLACE IN THE UNIVERSE [CHAP.
comparison. But this seemed wholly incredible, even
to the great observer Tycho Brahe, and hence the
Copernican theory was not so generally accepted as
it otherwise would have been.
Galileo always declared that the measurement
would some day be made, and he even suggested
the method of effecting it which is now found to be
the most trustworthy. But the sun s distance had to
be first measured with greater accuracy, and that was
only done in the latter part of the eighteenth century
by means of transits of Venus ; and by later obser
vations with more perfect instruments it is now
pretty well fixed at about 92,780,000 miles, the
limits of error being such that Q2f millions may
perhaps be quite as accurate.
With such an enormous base-line as twice this
distance, which is available by making observations
at intervals of about six months when the earth is
at opposite points in its orbit, it seemed certain that
some parallax or displacement of the nearer stars
could be found, and many astronomers with the best
instruments devoted themselves to the work. But
the difficulties were enormous, and very few really
satisfactory results were obtained till the latter half
of the nineteenth century. About forty stars have
now been measured with tolerable certainty, though
of course with a considerable margin of possible or
probable error ; and about thirty more, which are
found to have a parallax of one-tenth of a second or
less, must be considered to leave a very large margin
of uncertainty.
The two nearest fixed stars are Alpha Centauri
and 6 1 Cygni. The former is one of the brightest
V.] DISTANCE OF THE STARS 75
stars in the southern hemisphere, and is about
275,000 times as far from us as the sun. The light
from this star will take 4^- years to reach us, and this
light-journey/ as it is termed, is generally used by
astronomers as an easily remembered mode of record
ing the distances of the fixed stars, the distance in
miles in this case about 25 millions of millions
being very cumbrous. The other star, 61 Cygni, is
only of about the fifth magnitude, yet it is the second
nearest to us, with a light-journey of about 7^ years.
If we had no other determinations of distance than
these two, the facts would be of the highest import
ance. They teach us, first, that magnitude or bright
ness of a star is no proof of nearness to us, a fact of
which there is much other evidence ; and in the
second place, they furnish us with a probable mini
mum distance of independent suns from one another,
which, in proportion to their sizes, some being known
to be many times larger than our sun, is not more
than we might expect. This remoteness may be
partly due to those which were once nearer together
having coalesced under the influence of gravitation.
As this measurement of the distance of the nearer
stars should be clearly understood by every one who
wishes to obtain some real comprehension of the
scale of this vast universe of which we form a part, the
method now adopted and found to be most effectual
will be briefly explained.
Every one who is acquainted with the rudiments
of trigonometry or mensuration, knows that an in
accessible distance can be accurately determined if
we can measure a base-line from both ends of which
the inaccessible object can be seen, and if we have a
76 MAN S PLACE IN THE UNIVERSE [CHAP.
good instrument with which to measure angles. The
accuracy will mainly depend upon our base-line being
not excessively short in comparison with the distance
to be measured. If it is as much as half or even a
quarter as long the measurement may be as accurate
as if directly performed over the ground, but if it is
only one-hundredth or one-thousandth part as long, a
very small error either in the length of the base or in
the amount of the angles will produce a large error
in the result.
In measuring the distance of the moon, the earth s
diameter, or a considerable portion of it, has served
as a base-line. Either two observers at great dis
tances from each other, or the same observer after an
interval of nine or ten hours, may examine the moon
from positions six or seven thousand miles apart, and
by accurate measurements of its angular distance
from a star, or by the time of its passage over the
meridian of the place as observed with a transit
instrument, the angular displacement can be found
and the distance determined with very great accuracy,
although that distance is more than thirty times the
length of the base. The distance of the planet Mars
when nearest to us has been found in the same way.
His distance from us even when at his nearest point
during the most favourable oppositions is about
36 million miles, or more than four thousand times
the earth s diameter, so that it requires the most
delicate observations many times repeated and with
the finest instruments to obtain a tolerably approxi
mate result. When this is done, by Kepler s law of
the fixed proportion between the distances of planets
from the sun and their times of revolution, the propor-
v.] DISTANCE OF THE STARS 77
tionate distance of all the other planets and that of
the sun can be ascertained. This method, however,
is not sufficiently accurate to satisfy astronomers,
because upon the sun s distance that of every other
member of the solar system depends. Fortunately
there are two other methods by which this important
measurement has been made with much greater
approach to certainty and precision.
The first of these methods is by means of the rare
occasions when the planet Venus passes across the
sun s disc as seen from the earth. When this takes
place, observations of the transit, as it is termed, are
made at remote parts of the earth, the distance
between which places can of course easily be calcu
lated from their latitudes and longitudes. The
diagram here given illustrates the simplest mode of
determining the sun s distance by this observation,
Diagram illustrating the transit of Venus.
and the following description from Proctor s Old and
New Astronomy is so clear that I copy it verbally :
V represents Venus passing between the Earth E
and the Sun S ; and we see how an observer at E
will see Venus as at v , while an observer at E will
see her as at v. The measurement of the distance
v v , as compared with the diameter of the sun s disc,
determines the angle v V v or EVE ; whence the
distance E V can be calculated from the known
length of the base-line E E . For instance, it is
78 MAN S PLACE IN THE UNIVERSE [CHAP.
known (from the known proportions of the Solar
System as determined from the times of revolution
by Kepler s third law) that E V bears to V v the
proportion 28 to 72, or 7 to 18 ; whence E E bears
to v v the same proportion. Suppose, now, that the
distance between the two stations is known to be
7000 miles, so that vv is 18,000 miles ; and that v v
is found by accurate measurement to be A part of
the sun s diameter. Then the sun s diameter, as
determined by this observation, is 48 times 18,000
miles, or 864,000 miles ; whence from his known
apparent size, which is that of a globe 107^ times
farther away from us than its own diameter, his
distance is found to be 92,736,000 miles.
Of course, there being two observers, the propor
tion of the distance v v to the diameter of the sun s
disc cannot be measured directly, but each of them
can measure the apparent angular distance of the
planet from the sun s upper and lower margins as it
passes across the disc, and thus the angular distance
between the two lines of transit can be obtained.
The distance vv can also be found by accurately
noting the times of the upper and lower passage of
Venus, which, as the line of transit is considerably
shorter in one than the other, gives by the known
properties of the circle the exact proportion of the
distance between them to the sun s diameter ; and as
this is found to be the most accurate method, it is
the one generally adopted. For this purpose the
stations of the observers are so chosen that the
length of the two chords, v and v , may have a con
siderable difference, thus rendering the measurement
more easy.
v.] DISTANCE OF THE STARS 79
The other method of determining the sun s dis
tance is by the direct measurement of the velocity of
light. This was first done by the French physicist,
Fizeau, in 1849, by the use of rapidly revolving
mirrors, as described in most works on physics. This
method has now been brought to such a degree of
perfection that the sun s distance so determined is
considered to be equally trustworthy with that derived
from the transits of Venus. The reason that the
determination of the velocity of light leads to a deter
mination of the sun s distance is, because the time
taken by light to pass from the sun to the earth is
independently known to be 8 min. 13^ sec. This
was discovered so long ago as 1675 by means of the
eclipses of Jupiter s satellites. These satellites re
volve round the planet in from if to 16 days, and,
owing to their moving very nearly in the plane of
the ecliptic and the shadow of Jupiter being so large,
the three which are nearest to the planet are eclipsed
at every revolution. This rapid revolution of the
satellites and frequency of the eclipses enabled their
periods of recurrence to be determined with extreme
accuracy, especially after many years of careful obser
vation. It was then found that when Jupiter was at
its farthest distance from the earth the eclipses of the
satellites took place a little more than eight minutes
later than the time calculated from the mean period
of revolution, and when the planet was nearest to us
the eclipses occurred the same amount earlier. And
when further observation showed that there was no
difference between calculation and observation when
the planet was at its mean distance from us, and that
the error arose and increased exactly in proportion to
8o MAN S PLACE IN THE UNIVERSE [CHAP.
our varying distance from it, then it became clear
that the only cause adequate to produce such an
effect was, that light had not an infinite velocity but
travelled at a certain fixed rate. This however, though
a highly probable explanation, was not absolutely
proved till nearly two centuries later, by means of
two very difficult measurements that of the actual
distance of the sun from the earth, and that of the
actual speed of light in miles per second ; the latter
corresponding almost exactly with the speed deduced
from the eclipses of Jupiter s satellites and the sun s
distance as measured by the transits of Venus.
But this problem of measuring the sun s distance,
and through it the dimensions of the orbits of all the
planets of our system, sinks into insignificance when
compared with the enormous difficulties in the way of
the determination of the distance of the stars. As
a great many people, perhaps the majority of the
readers of any popular scientific book, have little
knowledge of mathematics and cannot realise what
an angle of a minute or a second really means, a
little explanation and illustration of these terms will
not be out of place. An angle of one degree (i) is
the 36oth part of a circle (viewed from its centre), the
9<Dth part of a right angle, the 6oth part of either of
the angles of an equilateral triangle. To see exactly
how much is an angle of one degree we draw a short
line (B C) one-tenth of an inch long, and from a point
(A) 5f inches from it (accurately 572957795 inches)
v.] DISTANCE OF THE STARS 81
we draw straight lines to B and C. Then the angle
at A is one degree.
Now, in all astronomical work, one degree is con
sidered to be quite a large angle. Even before the
invention of the telescope the old observers fixed the
position of the stars and planets to half or a quarter
of a degree, while Mr. Proctor thinks that Tycho
Brahe s positions of the stars and planets were correct
to about one or two minutes of arc. But a minute of
arc is obtained by dividing the line B C into sixty
equal parts and seeing the distance between two of
these with the naked eye from the point A. But as
very long-sighted people can see very minute objects
at 10 or 12 inches distance, we may double the dis
tance A B, and then making the line B C one three-
hundredth part of an inch long, we shall have the
angle of one minute which Tycho Brahe was perhaps
able to measure. How very large an amount a
minute is to the modern astronomer is, however,
well shown by the fact that the maximum difference
between the calculated and observed positions of
Uranus, which led Adams and Leverrier to search
for and discover Neptune, was only i|- minutes, a
space so small as to be almost invisible to the average
eye, so that if there had been two planets, one in the
calculated, the other in the observed place, they
would have appeared as one to unassisted vision.
In order now to realise what one second of arc
really means, let us look at the circle here shown,
which is as nearly as possible one-tenth of an inch
in diameter (one - O - tenth of an inch). If we
remove this circle to a distance of 28 feet 8 inches
it will subtend an angle of one minute, and we shall
F
82 MAN S PLACE IN THE UNIVERSE [CHAP.
have to place it at a distance of nearly 1730 feet
almost one-third of a mile to reduce the angle to
one second. But the very nearest to us of the fixed
stars, Alpha Centauri, has a parallax of only three-
fourths of a second ; that is, the distance of the earth
from the sun about 92f millions of miles would
appear no wider, seen from the nearest star, than does
three-fourths of the above small circle at one-third of
a mile distance. To see this circle at all at that
distance would require a very good telescope with a
power of at least 100, while to see any small part
of it and to measure the proportion of that part to
the whole would need very brilliant illumination and
a large and powerful astronomical telescope.
WHAT is A MILLION ?
But when we have to deal with millions, and even
with hundreds and thousands of millions, there is
another difficulty- -that few people can form any
clear conception of what a million is. It has been
suggested that in every large school the walls of one
room or hall should be devoted to showing a million
at one view. For this purpose it would be necessary
to have a hundred large sheets of paper each about
4 feet 6 inches square, ruled in quarter inch squares.
In each alternate square a round black wafer or circle
should be placed a little overlapping the square,
thus leaving an equal amount of white space between
the black spots. At each tenth spot a double width
should be left so as to separate each hundred spots
(10 x 10). Each sheet would then hold ten thousand
spots, which would all be distinctly visible from the
V.] DISTANCE OF THE STARS 83
middle of a room 20 feet wide, each horizontal or
vertical row containing a thousand. One hundred
such sheets would contain a million spots, and they
would occupy a space 450 feet long in one row, or
90 feet long in five rows, so that they would entirely
cover the walls of a room, about 30 feet square and
25 feet high, from floor to ceiling, allowing space for
doors but not for windows, the hall or gallery being
lighted from above. Such a hall would be in the
highest degree educational in a country where
millions are spoken of so glibly and wasted so
recklessly ; while no one can really appreciate
modern science, dealing as it does with the un
imaginably great and little, unless he is enabled to
realise by actual vision, and summing up, what a
vast number is comprised in one of those millions,
which, in modern astronomy and physics, he has
to deal with not singly only, but by hundreds and
thousands or even by millions. In every consider
able town, at all events, a hall or gallery should have
a million thus shown upon its walls. It would in no
way interfere with the walls being covered when
required with maps, or ornamental hangings, or
pictures ; but when these were removed, the visible
and countable million would remain as a permanent
lesson to all visitors ; and I believe that it would
have widespread beneficial effects in almost every
department of human thought and action. On a
small scale any one can do this for himself by getting
a hundred sheets of engineer s paper ruled in small
squares, and making the spots very small ; and even
this would be impressive, but not so much so as on
the larger scale.
84 MAN S PLACE IN THE UNIVERSE [CHAP.
In order to enable every reader of this volume at
once to form some conception of the number of units
in a million, I have made an estimate of the number
of letters contained in it, and I find them to amount
to about 420,000- considerably less than half a
million. Try and realise, when reading it, that if
every letter were a pound sterling, we waste as many
pounds as there are letters in two such volumes
whenever we build a battleship.
Having thus obtained some real conception of the
immensity of a million, we can better realise what it
must be to have every one of the dots above de
scribed, or every one of the letters in two such
volumes as this lengthened out so as to be each a mile
long, and even then we should have reached little
more than a hundredth part of the distance from our
earth to the sun. When, by careful consideration of
these figures, we have even partially realised this
enormous distance, we may take the next step, which
is, to compare this distance with that of the nearest
fixed star. We have seen that the parallax of that star
is three-fourths of a second, an amount which implies
that the star is 271,400 times as far from us as our
sun is. If after seeing what a million is, and knowing
that the sun is 92^ times this distance from us in
milesa distance which itself is almost inconceivable
to us- -we find that we have to multiply this almost
inconceivable distance 271,400 times more than a
quarter of a million times- -to reach the nearest of
the fixed stars, we shall begin to realise, however
imperfectly, how vast is the system of suns around
us, and on what a scale of immensity the material
universe, which we see so gloriously displayed in the
V.] DISTANCE OF THE STARS 85
starry heavens and the mysterious galaxy, is con
structed.
This somewhat lengthy preliminary discussion is
thought necessary in order that my readers may form
some idea of the enormous difficulty of obtaining
any measurement whatever of such distances. I
now propose to point out what the special diffi
culties are, and how they have been overcome ; and
thus I hope to be able to satisfy them that the
figures astronomers give us of the distances of the
stars are in no way mere guesses or probabilities,
but are real measurements which, within certain not
very wide limits of error, may be trusted as giving
us correct ideas of the magnitude of the visible
universe.
MEASUREMENT OF STELLAR DISTANCES
The fundamental difficulty of this measurement is,
of course, that the distances are so vast that the
longest available base-line, the diameter of the
earth s orbit, only subtends an angle of little more
than a second from the nearest star, while for all
the rest it is less than one second and often only
a small fraction of it. But this difficulty, great as
it is, is rendered far greater by the fact that there
is no fixed point in the heavens from which to
measure, since many of the stars are known to be
HI motion, and all are believed to be so in varying
degrees, while the sun itself is now known to be
moving among the stars at a rate which is not yet
accurately determined, but in a direction which is
fairly well known. As the various motions of the
86 MAN S PLACE IN THE UNIVERSE [CHAP.
earth while passing round the sun, though extremely
complex, are very accurately known, it was first
attempted to determine the changed position of stars
by observations, many times repeated at six months
intervals, of the moment of their passage over the
meridian and their distance from the zenith ; and
then by allowing for all the known motions of the
earth, such as precession of the equinoxes and nuta
tion of the earth s axis, as well as for refraction and
for the aberration of light, to determine what residual
effect was due to the difference of position from
which the star was viewed ; and a result was thus
obtained in several cases, though almost always a
larger one than has been found by later observations
and by better methods. These earlier observations,
however perfect the instruments and however skilful
the observer, are liable to errors which it seems im
possible to avoid. The instruments themselves are
subject in all their parts to expansion and con
traction by changes of temperature ; and when these
changes are sudden, one part of the instrument may
be affected more than another, and this will often
lead to minute errors which may seriously affect
the amount to be measured when that is so small.
Another source of error is due to atmospheric re
fraction, which is subject to changes both from hour
to hour and at different seasons. But perhaps most
important of all are minute changes in level of the
foundations of the instruments even when they are
carried down to solid rock. Both changes of tern-
o
perature and changes of moisture of the soil produce
minute alterations of level ; while earth-tremors and
slow movements of elevation or depression are now
v.] DISTANCE OF THE STARS 87
known to be very frequent. Owing to all these
causes, actual measurements of differences of position
at different times of the year, amounting to small
fractions of a second, are found to be too uncertain
for the determination of such minute angles with the
required accuracy.
But there is another method which avoids almost
all these sources of error, and this is now generally
preferred and adopted for these measurements. It
is, that of measuring the distance between two stars
situated apparently very near each other, one of
which has large proper motion, while the other has
none which is measurable. The proper motions of
the stars was first suspected by Halley in 1717, from
finding that several stars, whose places had been
given by Hipparchus, 130 B.C., were not in the
positions where they now ought to be ; and other
observations by the old astronomers, especially those
of occultations of stars by the moon, led to the same
result. Since the time of Halley very accurate ob
servations of the stars have been made, and in many
cases it is found that they move perceptibly from
year to year, while others move so slowly that it
is only after forty or fifty years that the motion can
be detected. The greatest proper motions yet deter
mined amount to between 7" and 8" in a year, while
other stars require twenty, or even fifty or a hundred
years to show an equal amount of displacement. At
first it was thought that the brightest stars would
have the largest proper motion, because it was sup
posed they were nearest to us, but it was soon found
that many small and quite inconspicuous stars moved
as rapidly as the most brilliant, while in many very
MAN S PLACE IN THE UNIVERSE [CHAP.
bright stars no proper motion at all can be detected
That which moves most rapidly is a small star of
less than the sixth magnitude.
It is a matter of common observation that the
motion of things at a distance cannot be perceived
so well as when near, even though the speed may
be the same. If a man is seen on the top of a hill
several miles off, we have to observe him closely
for some time before we can be sure whether he is
walking or standing still. But objects so enor
mously distant as we now know that the stars are,
may be moving at the rate of many miles in a second
and yet require years of observation to detect any
movement at all.
The proper motions of nearly a hundred stars
have now been ascertained to be more than one
second of arc annually, while a large number have
ess than this, and the majority have no perceptible
motion, presumably due to their enormous distance
from us. It is therefore not difficult in most cases
to find one or two motionless stars sufficiently close
to a star having a large proper motion (anything
more than one-tenth of a second is so called) to
serve as fixed points of measurement. All that is
then required is, to measure with extreme accuracy
the angular distance of the moving from the fixed
stars at intervals of six months. The measurements
can be made, however, on every fine night, each
one being compared with one at nearly an interval
of six months from it. In this way a hundred or
more measurements of the same star may be made
in a year, and the mean of the whole, allowance
being made for proper motion in the interval, will
v.] DISTANCE OF THE STARS 89
give a much more accurate result than any single
measurement. This kind of measurement can be
made with extreme accuracy when the two stars
can be seen together in the field of the telescope ;
either by the use of a micrometer, or by means of
an instrument called a heliometer, now often con
structed for the purpose. This is an astronomical
telescope of rather large size, the object glass of
which is cut in two straight across the centre, and
the two halves made to slide upon each other by
means of an exceedingly fine and accurate screw-
motion, so adjusted and tested as to measure the
angular distance of two objects with extreme ac
curacy. This is done by the number of turns of
the screw required to bring the two stars into contact
with each other, the image of each one being formed
by one of the halves of the object glass.
But the greatest advantage of this method of de
termining parallax is, as Sir John Herschell points
out, that it gets rid of all the sources of error which
render the older methods so uncertain and inaccu
rate. No corrections are required for precession,
nutation, or aberration, since these affect both stars
alike, as is the case also with refraction ; while
alterations of level of the instrument have no pre
judicial effect, since the measures of angular distance
taken by this method are quite independent of such
movements. A test of the accuracy of the deter
mination of parallax by this instrument is the very
close agreement of different observers, and also their
agreement with the new and perhaps even superior
method by photography. This method was first
adopted by Professor Pritchard of the Oxford Ob-
90 MAN S PLACE IN THE UNIVERSE [CHAP.
servatory, with a fine reflector of thirteen inches
aperture. Its great advantage is, that all the small
stars in the vicinity of the star whose parallax is
sought are shown in their exact positions upon the
plate, and the distances of all of them from it can
be very accurately measured, and by comparing plates
taken at six months intervals, each of these stars
gives a determination of parallax, so that the mean
of the whole will lead to a very accurate result.
Should, however, the result from any one of these
stars differ considerably from that derived from the
rest, it will be due in all probability to that star
having a proper motion of its own, and it may there
fore be rejected. To illustrate the amount of labour
bestowed by astronomers on this difficult problem,
it may be mentioned that for the photographic
measurement of the star 61 Cygni, 330 separate
plates were taken in 1886-7, an< ^ on these 30,000
measurements of distances of the pairs of star-images
were made. The result agreed closely with the best
previous determination by Sir Robert Ball, using the
micrometer, and the method was at once admitted by
astronomers as being of the greatest value.
Although, as a rule, stars having large proper
motions are found to be comparatively near us, there
is no regular proportion between these quantities,
indicating that the rapidity of the motion of the stars
varies greatly. Among fifty stars whose distances
have been fairly well determined, the rate of actual
motion varies from one or two up to more than a
hundred miles per second. Among six stars with
less than a tenth of a second of annual proper motion
there is one with a parallax of nearly half a second,
v.] DISTANCE OF THE STARS 91
and another of one-ninth of a second, so that they
are nearer to us than many stars which move several
seconds a year. This may be due to actual slowness
of motion, but is almost certainly caused in part by
their motion being either towards us or away from
us, and therefore only measurable by the spectro
scope ; and this had not been done when the lists
of parallaxes and proper motions from which these
facts are taken were published. It is evident that
the actual direction and rate of motion of a star
cannot be known till this radial movement, as it
is termed- -that is, towards or away from us- -has
been measured ; but as this element always tends
to increase the visually observed rate of motion, we
cannot, through its absence, exaggerate the actual
motions of the stars.
THE SUN S MOVEMENT THROUGH SPACE
But there is yet another important factor which
affects the apparent motions of all the stars the
movement of our sun, which, being a star itself, has
a proper motion of its own. This motion was sus
pected and sought for by Sir William Herschel
a century ago, and he actually determined the
direction of its motion towards a point in the con
stellation Hercules, not very far removed from that
fixed upon as the average of the best observations
since made. The method of determining this motion
is very simple, but at the same time very difficult.
When we are travelling in a railway carriage near
objects pass rapidly out of sight behind us, while
those farther from us remain longer in view, and very
92 MAN S PLACE IN THE UNIVERSE [CHAP.
distant objects appear almost stationary for a con
siderable time. For the same reason, if our sun is
moving in any direction through space, the nearer
stars will appear to travel in an opposite direction to
our movement, while the more distant will remain
quite stationary. This movement of the nearest stars
is detected by an examination and comparison of
their proper motions, by which it is found that in one
part of the heavens there is a preponderance of the
proper motions in one direction and a deficiency in
the opposite direction, while in the directions at right
angles to these the proper motions are not on the
average greater in one direction than in the opposite.
But the proper motions of the stars being themselves
so minute, and also so irregular, it is only by a most
elaborate mathematical investigation of the motions
of hundreds or even of thousands of stars, that the
direction of the solar motion can be determined.
Till quite recently astronomers were agreed that the
motion was towards a point in Hercules near the out
stretched arm in the figure of that constellation. But
the latest inquiries into this problem, involving the
comparison of the motions of several thousand stars
in all parts of the heavens, have led to the conclusion
that the most probable direction of the solar apex
(as the point towards which the sun is moving is
termed), is in the adjacent constellation Lyra, and
not far from the brilliant star Vega. This is the
position which Professor Newcomb of Washington
thinks most probable, though there is still room for
further investigation. To determine the rate of the
motion is very much more difficult than to fix its
direction, because the distances of so few stars have
v.] DISTANCE OF THE STARS 93
been determined, and very few indeed of these lie in
the directions best adapted to give accurate results.
The best measurements down to 1890 led to a
motion of about 15 miles a second. But more
recently the American astronomer, Campbell, has
determined by the spectroscope the motion in the
line of sight of a considerable number of stars
towards and away from the solar apex, and by com
paring the average of these motions, he derives
a motion for the sun of about 12^ miles a second,
and this is probably as near as we can yet reach
towards the true amount.
SOME NUMERICAL RESULTS OF THE ABOVE
MEASUREMENTS
The measurements of distances and proper motions
of a considerable number of the stars, of the motion
of our sun in space (its proper motion), together with
accurate determinations of the comparative brilliancy
of the brightest stars as compared with our sun and
with each other, have led to some very remarkable
numerical results which serve as indications of the
scale of magnitude of the stellar universe.
The parallaxes of about fifty stars have now been
repeatedly measured with such consistent results that
Professor Newcomb considers them to be fairly
trustworthy, and these vary from one-hundredth to
three-quarters of a second. Three more, all stars of
the first magnitude Rigel, Canopus, and Alpha
Cygni--have no measurable parallax, notwithstand
ing the long-continued efforts of many astronomers,
affording a striking example of the fact that brilliancy
94 MAN S PLACE IN THE UNIVERSE [CHAP.
alone is no test of proximity. Six more stars have
a parallax of only one-fiftieth of a second, and five of
these are either of the first or second magnitudes.
Of these nine stars having very small parallax or
none, six are situated in or near to the Milky Way,
another indication of exceeding remoteness, which is
further shown by the fact that they all have a very
small proper motion or none at all. These facts
support the conclusion, which had been already
reached by astronomers from a careful study of the
distribution of the stars, that the larger portion of the
stars of all magnitudes scattered throughout the
Milky Way or along its borders really belong to the
same great system, and may be said to form a part
of it. This is a conclusion of extreme importance
because it teaches us that the grandest of the suns,
such as Rigel and Betelgeuse in the constellation
Orion, Antares in the Scorpion, Deneb in the Swan
(Alpha Cygni), and Canopus (Alpha Argus), are in
all probability as far removed from us as are the
innumerable minute stars which give the nebulous or
milky appearance to the Galaxy.
It is well to consider for a moment what these
facts mean. Professor S. Newcomb, one of the
highest authorities on these problems, tells us that
the long series of measurements to discover the
parallax of Canopus, the brightest star in the southern
hemisphere, would have shown a parallax of one-
hundredth of a second, had such existed. Yet the
results always seemed to converge to a mean of
o^ ooo! Suppose, then, we assume the parallax of
this star to be somewhat less than the hundredth of a
second let us say T |^ of a second. At the distance
v.] DISTANCE OF THE STARS 95
this gives, light would take almost exactly 400 years
to reach us, so that if we suppose this very brilliant
star to be situated a little on this side of the Galaxy,
we must give to that great luminous circle of stars a
distance of about 500 light years. We shall now
perceive the advantage of being able to realise what
a million really is. A person who had once seen
a wall-space more than 100 feet long and 20 feet
high completely covered with quarter - inch spots
a quarter of an inch apart ; and then tried to imagine
every spot to be a mile long and to be placed end to
end in one row, would form a very different con
ception of a million miles than those who almost
daily read of millions, but are quite unable to
visualise even one of them. Having really seen one
million, we can partially realise the velocity of light,
which travels over this million miles in a little less
than 5^ seconds ; and yet light takes more than 4^
years at this inconceivable speed to come to us from
the very nearest of the stars. To realise this still
more impressively, let us take the distance of this
nearest star, which is 26 millions of millions of miles.
Let us look in imagination at this large and lofty hall
covered from floor to ceiling with quarter-inch spots
-only one million. Let all these be imagined as
miles. Then repeat this number of miles in a
straight line, one after the other, as many times as
there are spots in this hall ; and even then you have
reached only one twenty-sixth part of the distance to
the nearest fixed star ! This million times a million
miles has to be repeated twenty-six times to reach
the nearest fixed star ; and it seems probable that this
gives us a good indication of the distance from each
96 MAN S PLACE IN THE UNIVERSE [CHAP.
other of at least all the stars down to the sixth
magnitude, perhaps even of a large number of the
telescopic stars. But as we have found that the
bright stars of the Milky Way must be at least one
hundred times farther from us than these nearest
stars, we have found what may be termed a mini
mum distance for that vast star-ring. It may be
immensely farther, but it is hardly possible that it
should be anything less.
THE PROBABLE SIZE OF THE STARS
Having thus obtained an inferior limit for the
o
distance of several stars of the first magnitude, and
their actual brilliancy or light-emission as compared
with our sun having been carefully measured, we
have afforded us some indication of size though
perhaps an uncertain one. By these means it has
been found that Rigel gives out about ten thousand
times as much light as our sun, so that if its surface
is of the same brightness, it must be a hundred times
the diameter of the sun. But as it is one of the
white or Sirian type of stars it is probably very much
more luminous, but even if it were twenty times
brighter it would still have to be twenty-two and a
half times the diameter of the sun ; and as the stars
of this type are probably wholly gaseous and much
less dense than our sun, this enormous size may not
be far from the truth. It is believed that the Sirian
stars generally have a greater surface brilliancy than
our sun. Beta Aurigse, a star of the second magni
tude but of the Sirian type, is one of the double stars
whose distance has been measured, and this has
v.] DISTANCE OF THE STARS 97
enabled Mr. Gore to find the mass of the binary
system to be five times that of the sun, and their
light one hundred and seventeen times greater. Even
if the density is much less than the sun s, the intrinsic
brilliancy of the surface will be considerably greater.
Another double star, Gamma Leonis, has been found
to be three hundred times more brilliant than the sun
if of the same density, but it would require to be
seven times rarer than air to have the extent of
surface needed to give the same amount of light if
its surface emitted no more light than our sun from
equal areas.
It is clear, therefore, that many of the stars are
much larger than our sun as well as more luminous ;
but there are also large numbers of small stars whose
large proper motions, as well as the actual measure
ment of some, prove them to be comparatively near
to us which yet are only about one-fiftieth part as
bright as the sun. These must, therefore, be either
comparatively small, or if large must be but slightly
luminous. In the case of some double stars it has
been proved that the latter is the case ; but it seems
probable that others are very much smaller than the
average. Up to the present time no means of deter
mining the size of a star by actual measurement has
been discovered, since their distances are so enormous
that the most powerful telescopes show only a point
of light. But now that we have really measured the
distance of a good many stars we are able to deter
mine an upper limit for their actual dimensions. As
the nearest fixed star, Alpha Centauri, has a parallax
f " 75> tms means that if this star has a diameter as
great as our distance from the sun (which is not much
G
98 MAN S PLACE IN THE UNIVERSE [CHAP. v.
more than a hundred times the sun s diameter) it
would be seen to have a distinct disc about as large
as that of Jupiter s first satellite. If it were even
one-tenth of the size supposed it would probably be
seen as a disc in our best modern telescopes. The
late Mr. Ranyard remarks that if the Nebular Hypo
thesis is true, and our sun once extended as far as
the orbit of Neptune, then, among the millions of
visible suns there ought to be some now to be found
in every stage of development. But any sun having
a diameter at all approaching this size, and situated
as far off as a hundred times the distance of Alpha
Centauri, would be seen by the Lick telescope to have
a disc half a second in diameter. Hence the fact
that there are no stars with visible discs proves that
there are no suns of the required size, and adds
another argument, though not perhaps a strong one,
against the acceptance of the Nebular Hypothesis.
CHAPTER VI
THE UNITY AND EVOLUTION OF THE STAR SYSTEM
THE very condensed sketch now given of such of the
discoveries of recent Astronomy as relate to the
subject we are discussing will, it is hoped, give some
idea both of the work already done and of the number
of interesting problems yet remaining to be solved.
The most eminent astronomers in every part of the
world look forward to the solution of these problems
not, perhaps, as of any great value in themselves,
but as steps towards a more complete knowledge of
our universe as a whole. Their aim is to do for the
star-system what Darwin did for the organic world,
to discover the processes of change that are at work
in the heavens, and to learn how the mysterious
nebulae, the various types of stars, and the clusters
and systems of stars are related to each other. As
Darwin solved the problem of the origin of organic
species from other species, and thus enabled us to
understand how the whole of the existing forms of
life have been developed out of pre-existing forms,
so astronomers hope to be able to solve the problem
of the evolution of suns from some earlier stellar
types, so as to be able, ultimately, to form some
intelligible conception of how the whole stellar
universe has come to be what it is. Volumes have
99
too MAN S PLACE IN THE UNIVERSE [CHAP.
already been written on this subject, and many
ingenious suggestions and hypotheses have been
advanced. But the difficulties are very great; the
facts to be co-ordinated are excessively numerous,
and they are necessarily only a fragment of an
unknown whole. Yet certain definite conclusions
have been reached ; and the agreement of many
independent observers and thinkers on the funda
mental principles of stellar evolution seems to assure
us that we are progressing, if slowly yet with some
established basis of truth, towards the solution of
this, the most stupendous scientific problem with
which the human intellect has ever attempted to
grapple.
THE UNITY OF THE STELLAR UNIVERSE
During the latter half of the nineteenth century
the opinion of astronomers has been tending more
and more to the conception that the whole of the
visible universe of stars and nebulae constitutes one
complete and closely-related system ; and during the
last thirty years especially the vast body of facts
accumulated by stellar research has so firmly estab
lished this view that it is now hardly questioned by
any competent authority.
The idea that the nebulae were far more remote
from us than the stars long held sway, even after it
had been given up by its chief supporter. When Sir
William Herschel, by means of his then unapproached
telescopic power, resolved the Milky Way more or
less completely into stars, and showed that numerous
objects which had been classed as nebulae were really
vi.] EVOLUTION OF THE STAR SYSTEM 101
clusters of stars, it was natural to suppose that those
which still retained their cloudy appearance under
the highest telescopic powers were also clusters or
systems of stars, which only needed still higher powers
to show their true nature. This idea was supported
by the fcct that several nebulae were found to be
more or less ring-shaped, thus corresponding on a
smaller scale to the form of the Milky Way ; so that
when Herschel discovered thousands of telescopic
nebulae, he was accustomed to speak of them as so
many distinct universes scattered through the im
measurable depths of space.
Now, although any real conception of the immen
sity of the one stellar universe, of which the Milky
Way with its associated stars is the fundamental
feature, is, as I have shown, almost unattainable, the
idea of an unlimited number of other universes,
almost infinitely remote from our own and yet dis
tinctly visible in the heavens, so seized upon the
imagination that it became almost a commonplace of
popular astronomy and was not easily given up even
by astronomers themselves. And this was in a large
part due to the fact that Sir William Herschel s
voluminous writings, being almost all in the Philoso
phical Transactions of the Royal Society, were very
little read, and that he only indicated his change of
view by a few brief sentences which might easily be
overlooked. The late Mr. Proctor appears to have
been the first astronomer to make a thorough study
of the whole of Herschel s papers, and he tells us
that he read them all over five times before he was
able thoroughly to grasp the writer s views at different
periods.
102 MAN S PLACE IN THE UNIVERSE [CHAP.
But the first person to point out the real teaching
of the facts as to the distribution of the nebulae was
not an astronomer, but our greatest philosophical
student of science in general, Herbert Spencer. In
a remarkable essay on The Nebular Hypothesis in
the Westminster Review of July, 1858, he maintained
that the nebulae really formed a part of our own
Galaxy and of our own stellar universe. A single
passage from his paper will indicate his line of
argument, which, it may be added, had already been
partially set forth by Sir John Herschel in his Out
lines of Astronomy.
If there were but one nebula, it would be a
curious coincidence were this one nebula so placed
in the distant regions of space as to agree in
direction with a starless spot in our own sidereal
system. If there were but two nebulae, and both
were so placed, the coincidence would be excessively
strange. What, then, shall we say on finding that
there are thousands of nebulae so placed ? Shall
we believe that in thousands of cases these far-
removed galaxies happen to agree in their visible
positions with the thin places in our own galaxy?
Such a belief is impossible.
He then applies the same argument to the distri
bution of the nebulae as a whole : In that zone of
celestial space where stars are excessively abundant,
nebulae are rare, while in the two opposite celestial
spaces that are farthest removed from this zone,
nebulae are abundant. Scarcely any nebulae lie near
the galactic circle (or plane of the Milky Way) ; and
the great mass of them lie round the galactic poles.
Can this also be mere coincidence ? And he con-
vi.] EVOLUTION OF THE STAR SYSTEM 103
eludes, from the whole mass of the evidence, that
the proofs of a physical connection become over
whelming/
Nothing could be more clear or more forcible ; but
Spencer not being an astronomer, and writing in a
comparatively little read periodical, the astronomical
world hardly noticed him ; and it was from ten to
fifteen years later, when Mr. R. A. Proctor, by his
laborious charts and his various papers read before
the Royal and Royal Astronomical Societies from
1869 to 1875, compelled the attention of the scientific
world, and thus did more perhaps than any other
man to establish firmly the grand and far-reaching
principle of the essential unity of the stellar
universe, which is now accepted by almost every
astronomical writer of eminence in the civilised
world.
THE EVOLUTION OF THE STELLAR UNIVERSE
Amid the enormous mass of observations and of
suggestive speculation upon this great and most
interesting problem, it is difficult to select what is
most important and most trustworthy. But the
attempt must be made, because, unless my readers
have some knowledge of the most important facts
bearing upon it (besides those already set forth), and
also learn something of the difficulties that meet the
inquirer into causes at every step of his way, and of
the various ideas and suggestions which have been
put forth to account for the facts and to overcome
the difficulties, they will not be in a position to
estimate, however imperfectly, the grandeur, the
104 MAN S PLACE IN THE UNIVERSE [CHAP.
marvel, and the mystery of the vast and highly com
plex universe in which we live and of which we are
an important, perhaps the most important, if not the
only permanent outcome.
THE SUN A TYPICAL STAR
It being now a recognised fact that the stars are
suns, some knowledge of our own sun is an essential
preliminary to an inquiry into their nature, and into
the probable changes they have undergone.
The fact that the sun s density is only one-fourth
that of the earth, or less than one and a half times
that of water, demonstrates that it cannot be solid,
since the force of gravity at its surface being twenty-
six and a half times that at the earth s surface, the
materials of a solid globe would be so compressed
that the resulting density would be at least twenty
times greater instead of four times less than that of
the earth. All the evidence goes to show that the
body of the sun is really gaseous, but so compressed
by its gravitative force as to behave more like a
liquid. A few figures as to the vast dimensions of
the sun and the amount of light and heat emitted
by it will enable us better to understand the
phenomena it presents, and the interpretation of
those phenomena.
Proctor estimated that each square inch of the
sun s surface emitted as much light as twenty-five
electric arcs ; and Professor Langley has shown by
experiment that the sun is 5300 times brighter, and
eighty-seven times hotter than the white-hot metal
in a Bessemer converter. The actual amount of
vi.] EVOLUTION OF THE STAR SYSTEM 105
solar heat received by the earth is sufficient, if
wholly utilised, to keep a three-horse-power engine
continually at work on every square yard of the sur
face of our globe. The size of the sun is such, that
if the earth were at its centre, not only would there
be ample space for the moon s orbit, but sufficient for
another satellite 190,000 miles beyond the moon, all
revolving inside the sun. The mass of matter in
the sun is 745 times greater than that of all the
planets combined ; hence the powerful gravitative
force by which they are retained in their distant
orbits.
What we see as the sun s surface is the photo
sphere or outer layer of gaseous or partially liquid
matter kept at a definite level by the power of
gravitation. The photosphere has a granular texture
implying some diversity of surface or of luminosity ;
although the even contour of the sun s margin shows
o o
that these irregularities are not on a very large scale.
This surface is apparently rent asunder by what are
termed sun-spots, which were long supposed to be
cavities, showing a dark interior ; but are now
thought to be due to downpours of cooled materials
driven out from the sun, and forming the promi
nences seen during solar eclipses. They appear to
be black, but around their margin is a shaded border
or penumbra formed of elongated shining patches
crossing and over-lapping, something like heaps of
straw. Sometimes brilliant portions overhang the
dark spots, and often completely bridge them over ;
and similar patches, called faculse, accompany spots,
and in some cases almost surround them.
Sun-spots are sometimes numerous on the sun s
io6 MAN S PLACE IN THE UNIVERSE [CHAP.
disc, sometimes very few, and they are of such
enormous size that when present they can easily be
seen with the naked eye, protected by a piece of
smoked glass ; or, better still, with an ordinary
opera-glass similarly protected. They are found to
increase in number for several years, and then to
decrease ; the maxima recurring after an average
period of eleven years, but with no exactness, since
the interval between two maxima or minima is some
times only nine and sometimes as much as thirteen
years ; while the minima do not occur midway
between two maxima, but much nearer to the suc
ceeding than to the preceding one. What is more
interesting is, that variations in terrestial magnetism
follow them with great accuracy ; while violent com
motions in the sun, indicated by the sudden appear
ance of faculae, sun-spots, or prominences on the
sun s limb, are always accompanied by magnetic dis
turbances on the earth.
WHAT SURROUNDS THE SUN
It has been well said that what we commonly term
the sun is really the bright spherical nucleus of a
nebulous body. This nucleus consists of matter in
the gaseous state, but so compressed as to resemble
a liquid or even a viscous fluid. About forty of the
elements have been detected in the sun by means of
the dark lines in its spectrum, but it is almost certain
that all the elements, in some form or other, exist
there. This semi-liquid glowing surface is termed
the photosphere, since from it are given out the light
and heat which reach our earth.
VI.] EVOLUTION OF THE STAR SYSTEM 107
Immediately above this luminous surface is what
is termed the reversing layer* or absorbing layer,
consisting of dense metallic vapours only a few
hundred miles thick, and, though glowing, somewhat
cooler than the surface of the photosphere. Its
spectrum, taken, at the moment when the sun is
totally darkened, through a slit which is directed
tangentially to the sun s limb, shows a mass of bright
lines corresponding in a large degree to the dark
lines in the ordinary solar spectrum. It is thus
shown to be a vaporous stratum which absorbs the
special rays emitted by each element and forming its
characteristic coloured lines, changing them into
black lines. But as coloured lines are not found in
this layer corresponding to all the black lines in the
solar spectrum, it is now held that special absorption
must also occur in the chromosphere and perhaps
in the corona itself. Sir Norman Lockyer, in his
volume on Inorganic Evolution, even goes so far as
to say, that the true reversing layer of the sun
that which by its absorption produced the dark lines
in the solar spectrum is now shown to be not the
chromosphere itself but a layer above it, of lower
temperature.
Above the reversing layer comes the chromosphere,
a vast mass of rosy or scarlet emanations sur
rounding the sun to a depth of about 4000 miles.
When seen during eclipses it shows a serrated
waving outline, but subject to great changes of
form, producing the prominences already mentioned.
These are of two kinds : the quiescent/ which are
something like clouds of enormous extent, and which
keep their forms for a considerable time ; and the
io8 MAN S PLACE IN THE UNIVERSE [CHAP.
Eruptive, which shoot out in towering tree-like
flames or geyser-like eruptions, and while doing so
have been proved to reach velocities of over 300
miles a second, and subside again with almost
equal rapidity. The chromosphere and its quiescent
prominences appear to be truly gaseous, consisting of
hydrogen, helium, and coronium, while the eruptive
prominences always show the presence of metallic
vapours, especially of calcium. Prominences increase
in size and number in close accordance with the
increase of sun-spots. Beyond the red chromosphere
and prominences is the marvellous white glory of the
corona, which extends to an enormous distance round
the sun. Like the prominences of the chromosphere,
it is subject to periodical changes in form and size,
corresponding to the sun-spot period, but in inverse
order, a minimum of sun-spots going with a
maximum extension of the corona. At the total
eclipse of July 1878, when the sun s surface was
almost wholly clear, a pair of enormous equatorial
streamers stretched east and west of the sun to a
distance of ten millions of miles, and less extensions
of the corona occurred at the poles. At the eclipses
of 1882 and 1883, on the other hand, when sun-
spots were at a maximum, the corona was regularly
stellate with no great extensions, but of high
brilliancy. This correspondence has been noted at
every eclipse, and there is therefore an undoubted
connection between the two phenomena.
The light of the corona is believed to be derived
from three sources from incandescent solid or liquid
particles thrown out from the sun, from sunlight
reflected from these particles, and from gaseous
vi.] EVOLUTION OF THE STAR SYSTEM 109
emissions. Its spectrum possesses a green ray, which
is peculiar to it, and is supposed to indicate a gas
named coronium ; in other respects the spectrum is
more like that of reflected sunlight. The enormous
extensions of the corona into great angular streamers
seem to indicate electrical repulsive forces analogous
to those which produce the tails of comets.
Connected with the sun s corona is that strange
phenomenon, the zodiacal light. This is a delicate
nebulosity, which is often seen after sunset in spring
and before sunrise in autumn, tapering upwards from
the sun s direction along the plane of the ecliptic.
Under very favourable conditions it has been traced
in the eastern sky in spring to 180 from the sun s
position, indicating that it extends beyond the earth s
orbit. Lono--continued observations from the summit
o
of the Pic du Midi show that this is really the case,
and that it lies almost exactly in the plane of the
sun s equator. It is therefore held to be produced
by the minute particles thrown off the sun, through
those coronal wings and streamers which are visible
only during solar eclipses.
The careful study of the solar phenomena has very
clearly established the fact that none of the sun s
envelopes, from the reversing layer to the corona
itself, is in any sense an atmosphere. The com
bination of enormous gravitative force with an
amount of heat which turns all the elements into
the liquid or gaseous state, leads to consequences
which it is difficult for us to follow or comprehend.
There is evidently constant internal movement or
circulation in the interior of the sun, resulting in
the faculae, the sun-spots, the intensely luminous
no MAN S PLACE IN THE UNIVERSE [CHAP.
photosphere, and the chromosphere with its vast
flaming coruscations and eruptive protuberances.
But it seems impossible that this incessant and
violent movement can be kept up without some
great and periodical or continuous inrush of fresh
materials to renew the heat, keep up the internal
circulation, and supply the waste. Perhaps the
movement of the sun through space may bring him
into contact with sufficiently large masses of matter
to continually excite that internal movement with
out which the exterior surface would rapidly become
cool and all planetary life cease. The various solar
envelopes are the result of this internal agitation,
uprushes, and explosions, while the vast white corona
is probably of little more density than comets tails,
probably even of less density, since comets not un-
frequently rush through its midst without suffering
any loss of velocity. The fact that none of the
solar envelopes are visible to us until the light
of the photosphere is completely shut off, and that
they all vanish the very instant the first gleam of
direct sunlight reaches us, is another proof of their
extreme tenuity, as is also the sharply defined edge
of the sun s disc. The envelopes therefore consist
partly of liquid or vaporous matter, in a very finely
divided state, driven off by explosions or by electrical
forces, and this matter, rapidly cooling, becomes
solidified into minutest particles, or even physical
molecules. Much of this matter continually falls
back on the sun s surface, but a certain quantity
of the very finest dust is continually driven away
by electrical repulsion, so as to form the corona
and the zodiacal light. The vast coronal streamers
vi.] EVOLUTION OF THE STAR SYSTEM in
and the still more extensive ring of the zodiacal
lio-ht are therefore in all probability due to the same
causes, and have a similar physical constitution with
the tails of comets.
As the whole of our sunlight must pass through
both the reversing layer and the red chromosphere,
its colour must be somewhat modified by them.
Hence it is believed that, if they were absent, not
only would the light and heat of the sun be con
siderably greater, but its colour would be a purer
white, tending towards bluish rather than towards the
yellowish tinge it actually possesses.
THE NEBULAR AND METEORITIC HYPOTHESES
As the constitution of the sun, and its agency in
producing magnetism and electricity in the matter
and orbs around it, afford us our best guide to the
constitution of the stars and nebulae, and to their
possible action on each other, and even upon our
earth, so the mode of evolution of the sun and solar
system, from some pre-existing condition, is likely to
help us towards gaining some knowledge of the con
stitution of the stellar universe and the processes of
change going on there.
At the very commencement of the nineteenth
century the great mathematician Laplace published
his Nebular Theory of the Origin of the Solar
System ; and although he put it forth merely as a
suggestion, and did not support it with any numerical
or physical data, or by any mathematical processes,
his great reputation, and its apparent probability
and simplicity, caused it to be almost universally
ii2 MAN S PLACE IN THE UNIVERSE [CHAP.
accepted, and to be extended so as to apply to the
evolution of the stellar universe. This theory, very
briefly stated, is, that the whole of the matter of the
solar system once formed a globular or spheroidal
mass of intensely heated gases, extending beyond
the orbit of the outermost planet, and having a slow
motion of revolution about an axis. As it cooled
and contracted, its rate of revolution increased, and
this became so great that at successive epochs it
threw off rings, which, owing to slight irregulari
ties, broke up, and, gravitating together, formed the
planets. The contraction continuing, the sun, as we
now see it, was the result.
For about half a century this nebular hypothesis
was generally accepted, but during the last thirty
years so many objections and difficulties have been
suggested, that it has been felt impossible to retain
it even as a working hypothesis. At the same time
another hypothesis has been put forth which seems
more in accordance with the facts of nature as we
find them in our own solar system, and which is not
open to any of the objections against the nebular
theory, even if it introduces a few new ones.
A fundamental objection to Laplace s theory is,
that in a gas of such extreme tenuity as the solar
nebula must have been, even when it extended only
to Saturn or Uranus, it could not possibly have had
any cohesion, and therefore could not have given
off whole rings at distant intervals, but only small
fragments continuously as condensation went on, and
these, rapidly cooling, would form solid particles, a
kind of meteoric dust, which might aggregate into
numerous small planets, or might persist for in-
vi.J EVOLUTION OF THE STAR SYSTEM 113
definite periods, like the rings of Saturn or the
great ring of the Asteroids.
Another equally vital objection is, that, as the
nebula when extending beyond the orbit of Neptune
could have had a mean density of only about the
two-hundred millionth of our air at sea level, it must
have been many hundred times less dense than this
at and near its outer surface, and would there be ex
posed to the cold of stellar space a cold that would
solidify hydrogen. It is thus evident that the gases
of all the metallic and other solid elements could not
possibly exist as such, but would rapidly, perhaps
almost instantaneously, become first liquid and then
solid, forming meteoric dust even before contrac
tion had gone far enough to produce such increased
rotation as would throw off any portion of the
gaseous matter.
Here we have the foundations of the meteoritic
hypothesis which is now steadily making its way.
It is supported by the fact that we everywhere
find proofs of such solid matter in the planetary
spaces around us. It falls continually upon the earth.
It can be collected on the Arctic and Alpine snows.
It occurs everywhere in the deepest abysses of the
ocean where there are not sufficient organic de
posits to mask it. It constitutes, as has now been
demonstrated, the rings of Saturn. Thousands of
vast rings of solid particles circulate around the sun,
and when our earth crosses any of these rings, and
their particles enter our atmosphere with planetary
velocity, the friction ignites them and we see fall
ing stars. Comets tails, the sun s corona, and the
zodiacal light are three strange phenomena, which,
H
ii4 MAN S PLACE IN THE UNIVERSE [CHAP.
though wholly insoluble on any theory of gaseous
formation, receive their intelligible explanation by
means of excessively minute solid particles micro
scopic cosmic dust driven outward by the tre
mendous electrical repulsions that emanate from the
sun.
Having these and other proofs that solid matter,
ranging in size, perhaps, from the majestic orbs of
Jupiter and Saturn down to the inconceivably minute
particles driven millions of miles into space to form
a comet s tail, does actually exist everywhere around
us, and by collisions between the particles or with
planetary atmospheres can produce heat and light
and gaseous emanations, we find a basis of fact
and observation for the meteoritic hypothesis which
Laplace s nebular, and essentially gaseous, theory
does not possess.
During the latter half of the nineteenth century
several writers suggested this idea of the possible
formation of the Solar System, but so far as I am
aware, the late R. A. Proctor was the first to discuss
it in any detail, and to show that it explained many
of the peculiarities in the size and arrangement of the
planets and their satellites which the nebular hypo
thesis did not explain. This he does at some length
in the chapter on meteors and comets in his Other
Worlds than Ours, published in 1870. He assumed,
instead of the fire-mist of Laplace, that the space
now occupied by the solar system, and for an un
known distance around it, was occupied by vast
quantities of solid particles of all the kinds of matter
which we now find in the earth, sun, and stars. This
matter was dispersed somewhat irregularly, as we
vi.] EVOLUTION OF THE STAR SYSTEM 115
see that all the matter of the universe is now dis
tributed; and he further assumed that it was all
in motion, as we now know that all the stars and
other cosmical masses are, and must be, in motion
towards or around some centre.
Under these conditions, wherever the matter was
most aggregated, there would be a centre of attrac
tion through gravitation, which would necessarily lead
to further aggregation, and the continual impacts of
such aggregating matter would produce heat. In
course of time, if the supply of cosmic matter was
ample (as the result shows that it must have been,
whatever theory we adopt), our sun, thus formed,
would approximate to its present mass and acquire
sufficient heat by collision and gravitation to convert
its whole body into the liquid or gaseous condition.
While this was going on, subordinate centres of
aggregation might form, which would capture a
certain proportion of the matter flowing in under the
attraction of the central mass, while, owing to the
nearly uniform direction and velocity with which the
whole system was revolving, each subordinate centre
would revolve around the central mass, in somewhat
different planes, but all in the same direction.
Mr. Proctor shows the probability that the largest
outside aggregation would be at a great distance
from the central mass, and this having once been
formed, any centres farther away from the sun would
be both smaller and very remote, while those inside
the first would, as a rule, become smaller as they
were nearer the centre. The heated condition of
the earth s interior would thus be due, not to the
primitive heat of matter in a gaseous state out of which
n6 MAN S PLACE IN THE UNIVERSE [CHAP.
it was formed a condition physically impossible
but would be acquired in the process of aggregation
by the collisions of meteoric masses falling on it, and
by its own gravitative force producing continuous
condensation and heat.
On this view Jupiter would probably be formed
first, and after him at very great distances, Saturn,
Uranus, and Neptune ; while the inner aggregations
would be smaller, as the much greater attractive
power of the sun would give them comparatively
little opportunity of capturing the meteoric matter
that was continuously flowing towards him.
THE METEORITIC NATURE OF THE NEBULA
Having thus reached the conclusion that wherever
apparently nebulous matter exists within the limits of
the solar system it is not gaseous but consists of
solid particles, or, if heated gases are associated with
the solid matter they can be accounted for by the
heat due to collisions either with other solid particles
or with accumulations of gases at a low temperature,
as when meteorites enter our atmosphere, it was an
easy step to consider whether the cosmic nebulae
and stars may not have had a similar origin.
From this point of view the nebulae are supposed
to be vast aggregations of meteorites or cosmic
dust, or of the more persistent gases, revolving with
circular or spiral motions, or in irregular streams,
and so sparsely scattered that the separate particles
of dust may be miles perhaps hundreds of miles-
apart ; yet even those nebulae, only visible by the
telescope, may contain as much matter as the whole
vi.] EVOLUTION OF THE STAR SYSTEM 117
solar system. From this simple origin, by steps
which can be observed in the skies, almost all the
forms of suns and systems can be traced by means of
the known laws of motion, of heat-production, and of
chemical action. The chief English advocate of this
view at the present time is Sir Norman Lockyer,
who, in numerous papers, and in his works on
The Meteoritic Hypothesis and Inorganic Evolution,
has developed it in detail, as the result of many years
continuous research, aided by the contributory work
of continental and American astronomers. These
views are gradually spreading among astronomers
and mathematicians, as will be seen by the very
brief outline which will now be given of the explana
tions they afford of the main groups of phenomena
presented by the stellar universe.
DR. ROBERTS ON SPIRAL NEBULAE
Dr. Isaac Roberts, who possesses one of the finest
telescopes constructed for photographing stars and
nebulae, has given his views on stellar evolution, in
Knowledge of February 1897, illustrated by four
beautiful photographs of spiral nebulae. These
curious forms were at first thought to be rare, but
are now found to be really very numerous when
details are brought out by the camera. Many of the
very large and apparently quite irregular nebulae,
like the Magellanic Clouds, are found to have faint
indications of spiral structure. As more than ten
thousand nebulae are now known, and new ones are
continually being discovered, it will be a long time
before these can all be carefully studied and photo-
n8 MAN S PLACE IN THE UNIVERSE [CHAP.
graphed, but present indications seem to show that a
considerable proportion of them will exhibit spiral
forms.
Dr. Roberts tells us that all the spiral nebulae
he has photographed are characterised by having
a nucleus surrounded by dense nebulosity, most of
them being also studded with stars. These stars
are always arranged more or less symmetrically,
following the curves of the spiral, while outside
the visible nebula are other stars arranged in curves
strongly suggesting a former greater extension of the
nebulous matter. This is so marked a feature that it
at once leads to a possible explanation of the numerous
slightly curved lines of stars found in every part of
the heavens, as being the result of their origin from
spiral nebulae whose material substance has been
absorbed by them.
Dr. Roberts proposes several problems in relation
to these bodies : Of what materials are spiral nebulae
composed? Whence comes the vortical motion
which has produced their forms ? The material
he finds in those faint clouds of nebulous matter,
often of vast extent, that exist in many parts of the
sky, and these are so numerous that Sir William
Herschel alone recorded the positions of fifty-two
such regions, many of which have been confirmed
by recent photographs. Dr. Roberts considers these
to be either gaseous or with discrete solid particles
intermixed. He also enumerates smaller nebulous
masses undergoing condensation and segregation
into more regular forms ; spiral nebulae in various
stages of condensation and of aggregation ; elliptic
nebulae; and globular nebulae. In the last three
vi.] EVOLUTION OF THE STAR SYSTEM 119
classes there is clear evidence, on every photograph
that has been taken, that condensation into stars
or starlike forms is now going on.
He adopts Sir Norman Lockyer s view that
collisions of meteorites within each swarm or cloud
would produce luminous nebulosity ; so also would
collisions between separate swarms of meteorites
produce the conditions required to account for the
vortical motions and the peculiar distribution of
the nebulosity in the spiral nebulae. Almost any
collision between unequal masses of diffused matter
would, in the absence of any massive central body
round which they would be forced to revolve, lead to
spiral motions. It is to be noted that, although the
stars formed in the spiral convolutions of the
nebulae follow those curves, and retain them after
the nebulous matter has been all absorbed by them,
yet, whenever such a nebula is seen by us edge
wise, the convolutions with their enclosed stars will
appear as straight lines ; and thus not only numbers
of star groups arranged in curves, but also those
which form almost perfect straight lines, may
possibly be traced back to an origin from spiral
nebulae.
Motion being a necessary result of gravitation, we
know that every star, planet, comet, or nebula must
be in motion through space, and these motions
except in systems physically connected or which
have had a common origin are, apparently, in all
directions. How these motions originated and are
o
now regulated we do not know ; but there they are,
and they furnish the motive power of the collisions,
which, when affecting large bodies or masses of
120 MAN S PLACE IN THE UNIVERSE [CHAP.
diffused matter, lead to the formation of the various
kinds of permanent stars ; while when smaller masses
of matter are concerned those temporary stars are
formed which have interested astronomers in all
ages. It must be noted that although the motions
of the single stars appear to be in straight lines, yet
the spaces through which they have been observed
to move are so small that they may really be moving
in curved orbits around some central body, or the
centre of gravity of some aggregation of stars bright
and dark, which may itself be comparatively at rest.
There may be thousands of such centres around us,
and this may sufficiently explain the apparent motions
of stars in all directions.
A SUGGESTION AS TO THE FORMATION or
SPIRAL NEBULA
In a remarkable paper in the Astrophysical Journal
(July 1901), Mr. T. C. Chamberlin suggests an
origin for the spiral nebulae, as well as of swarms of
meteorites and comets, which seems likely to be a
true, although perhaps not the only one.
There is a well-known principle which shows that
when two bodies in space, of stellar size, pass within
a certain distance of each other, the smaller one will
be liable to be torn into fragments by the differential
attraction of the larger and denser body. This was
originally proved in the case of gaseous and liquid
bodies, and the distance within which the smaller one
will be disrupted (termed the Roche limit) is cal
culated on the supposition that the disrupted body is
a liquid mass. Mr. Chamberlin shows, however,
vi.] EVOLUTION OF THE STAR SYSTEM 121
that a solid body will also be disrupted at a lesser
distance dependent on its size and cohesive strength ;
but, as the size of the two bodies increases, the dis
tance at which disruption will occur increases also,
till with very large bodies, such as suns, it becomes
almost as large as in the case of liquids or gases.
The disruption occurs from the well-known law of
differential gravitation on the two sides of a body
leading to tidal deformation in a liquid, and to un
equal strain in a solid. When the changes of gravita-
tive force take place slowly, and are also small in
amount, the tides in liquids or strains in solids are
very small, as in the case of our earth when acted on
by the sun and moon, the result is a small tide in the
ocean and atmosphere, and no doubt also in the
molten interior, to which the comparatively thin crust
may partially adjust itself. But if we suppose two
dark or luminous suns whose proper motions are in
such a direction as to bring them near each other,
then, as they approach, each will be deflected towards
the other, and will pass round their common centre
of gravity with immense velocity, perhaps hundreds
of miles in a second. At a considerable distance
they will begin to produce tidal elongation towards
and away from each other, but when the disruptive
limit is nearly reached, the gravitative forces will be
increasing so rapidly that even a liquid mass could
not adjust its shape with sufficient quickness and the
tremendous internal strains would produce the effects
of an explosion, tearing the whole mass (of the
smaller of the two) into fragments and dust.
But it is also shown that, during the entire process,
the two elongated portions of the originally spherical
122 MAN S PLACE IN THE UNIVERSE [CHAP.
mass would be so acted upon by gravity as to produce
increasing rotation, which as the crisis approached
would extend the elongation, and aid in the ex
plosive result. This rapid rotation of the elongated
mass would, when the disruption occurred, neces
sarily give to the fragments a whirling or spiral
motion, and thus initiate a spiral nebula of a size and
character dependent on the size and constitution of
the two masses, and on the amount of the explosive
forces set up by their approach.
There is one very suggestive phenomenon which
seems to prove that this is one of the modes of
formation of spiral nebulae. When the explosive
disruption occurs the two protuberances or elonga
tions of the body will fly apart, and having also a
rapid rotatory movement, the resulting spiral will
necessarily be a double one. Now, it is the fact that
almost all the well-developed spiral nebulae have two
such arms opposite to each other, as beautifully
shown in M. 100 Comae, M. 51 Canum, and others
photographed by Dr. I. Roberts. It does not seem
likely that any other origin of these nebulae should
give rise to a double rather than to a single spiral.
THE EVOLUTION OF DOUBLE STARS
The advance in knowledge of double and multiple
stars has been wonderfully rapid, numerous observers
having devoted themselves to this special branch.
Many thousands were discovered during the first
half of the nineteenth century, and as telescopic
power increased new ones continued to flow in by
hundreds and thousands, and there has been recently
vi.] EVOLUTION OF THE STAR SYSTEM 123
published by the Yerkes Observatory a catalogue of
1290 such stars, discovered between 1871 and 1899
by one observer, Mr. S. W. Burnham. All these
have been found by the use of the telescope, but
during the last quarter of a century the spectroscope
has opened up a new world of double stars of enormous
extent and the highest interest.
The telescopic binaries which have been observed
for a sufficient time to determine their orbits, range
from periods of about eleven years as a minimum up
to hundreds and even more than a thousand years.
But the spectroscope reveals the fact that the many
thousands of telescopic binaries form only a very
small part of the binary systems in existence. The
overwhelming importance of this discovery is, that it
carries the times of revolution from the minimum of
the telescopic doubles downward in unbroken series
through periods of a few years, to those reckoned by
months, by days, and even by hours. And with this
reduction of period there necessarily follows a corre
sponding reduction of distance, so that sometimes the
two stars must be in contact, and thus the actual
birth or origin of a double star has been observed to
occur, even though not actually seen. This mode of
origin was indeed anticipated by Dr. Lee of Chicago
in 1892, and it has been confirmed by observation in
the short space of ten years.
In a remarkable communication to Nature
(September i2th, 1901) Mr. Alexander W. Roberts
of Lovedale, South Africa, gives some of the main
results of this branch of inquiry. Of course all the
variable stars are to be found among the spectroscopic
binaries. They consist of that portion of the class
124 MAN S PLACE IN THE UNIVERSE [CHAP.
in which the plane of the orbit is directed towards us,
so that during their revolution one of the pair either
wholly or partially eclipses the other. In some of
these cases there are irregularities, such as double
maxima and minima of unequal lengths, which may
be due to triple systems or to other causes not yet
explained, but as they all have short periods and
always appear as one star in the most powerful
telescopes, they form a special division of the spectro-
scopic binary systems.
There are known at present twenty-two variables
of the Algol type, that is, stars having each a dark
companion very close to it which obscures it either
wholly or partially during every revolution. In these
cases the density of the systems can be approximately
determined, and they are found to be, on the average,
only one-fifth that of water, or one-eighth that of our
sun. But as many of them are as large as our sun, or
even considerably larger, it is evident that they must
be wholly gaseous, and, even if very hot, of a less
complex constitution than our luminary. Mr. A. W.
Roberts tells us that five out of these twenty-two
variables revolve in absolute contact forming systems
of the shape of a dumb-bell. The periods vary from
twelve days to less than nine hours ; and, starting
from these, we now have a continuous series of length
ening periods up to the twin stars of Castor which
require more than a thousand years to complete their
revolution.
During his observations of the above five stars,
Mr. Roberts states that one, X Carinae, was found
to have parted company, so that instead of being
actually united to its companion the two are now
vi.] EVOLUTION OF THE STAR SYSTEM 125
at a distance apart equal to one-tenth of their
diameters, and he may thus be said to have been
almost a witness of the birth of a stellar system.
A year later we find the record (in Knowledge,
October 1902) of Professor Campbell s researches at
the Lick Observatory. He states that, out of 350
stars observed spectroscopically, one in eight is
a spectroscopic binary ; and so impressed is he with
their abundance that, as accuracy of measurement
increases, he believes that the star that is not a
spectroscopic binary will prove to be the rare exception !
Professor G. Darwin had already shown that the
1 dumb-bell was a figure of equilibrium in a rotating
mass of fluid; and we now find proofs that such
figures exist, and that they form the starting-point
for the enormous and ever-increasing quantities of
spectroscopic binary star-systems that are now known.
The origin of these binary stars is also of especial
interest as giving support to Professor Darwin s well-
known explanation of the origin of the moon by disrup
tion from the earth, owing to the very rapid rotation
of the parent planet. It now appears that suns often
subdivide in the same manner, but, owing perhaps to
their intensely heated gaseous state they seem usually
to form nearly equal globes. The evolution of this
special form of star-system is therefore now an ob
served fact ; though it by no means follows that all
double stars have had the same mode of origin.
CLUSTERS OF STARS AND VARIABLES
The clusters of stars, which are tolerably abun
dant in the heavens and offer so many strange and
126 MAN S PLACE IN THE UNIVERSE [CHAP.
beautiful forms to the telescopist, are yet among the
most puzzling phenomena the philosophic astronomer
has to deal with.
Many of these clusters which are not very crowded
and of irregular forms, strongly suggest an origin
from the equally irregular and fantastic forms of
nebulae by a process of aggregation like that which
Dr. Roberts describes as developing within the spiral
nebulae. But the dense globular clusters which form
o
such beautiful telescopic objects, and in some of
which more than six thousand stars have been counted
besides considerable numbers so crowded in the
centre as to be uncountable, are more difficult to
explain. One of the problems suggested by these
clusters is as to their stability. Professor Simon
Newcomb remarks on this point as follows : Where
thousands of stars are condensed into a space so
small, what prevents them from all falling together
into one confused mass ? Are they really doing so,
and will they ultimately form a single body ? These
are questions which can be satisfactorily answered
only by centuries of observation ; they must there
fore be left to the astronomers of the future.
There are, however, some remarkable features in
these clusters which afford possible indications of
their origin and essential constitution. When closely
examined most of them are seen to be less regular
than they at first appear. Vacant spaces can be
noted in them ; even rifts of definite forms. In some
there is a radiated structure ; in others there are
curved appendages ; while some have fainter centres.
These features are so exactly like what are found,
in a more pronounced form, in the larger nebulae,
VL] EVOLUTION OF THE STAR SYSTEM 127
that we can hardly help thinking that in these
clusters we have the result of the condensation of
very large nebulae, which have first aggregated
towards numerous centres, while these agglomera
tions have been slowly drawn towards the common
centre of gravity of the whole mass. It is suggestive
of this origin that while the smaller telescopic nebulae
are far removed from the Milky Way, the larger
ones are most abundant near its borders ; while the
star-clusters are excessively abundant on and near
the Milky Way, but very scarce elsewhere, except in
or near vast nebulae like the Magellanic Clouds. We
thus see that the two phenomena may be comple
mentary to each other, the condensation of nebulae
having gone on most rapidly where material was
most abundant, resulting in numerous star-clusters
O
where there are now few nebulae.
There is one striking feature of the globular
clusters which calls for notice ; the presence in
some of them of enormous quantities of variable stars,
while in others few or none can be found. The
Harvard Observatory has for several years devoted
much time to this class of observations, and the
results are given in Professor Newcomb s recent
volume on The Stars/ It appears that twenty-
three clusters have been observed spectroscopically,
the number of stars examined in each cluster vary
ing from 145 up to 3000, the total number of stars
thus minutely tested being 19,050. Out of this total
number 509 were found to be variable ; but the
curious fact is, the extreme divergence in the propor
tion of variables to the whole number examined in
the several clusters. In two clusters, though 1279
128 MAN S PLACE IN THE UNIVERSE [CHAP.
stars were examined, not a single variable was found.
In three others the proportion was from one in 1050
to one in 500. Five more ranged up to one in 100,
and the remainder showed from that proportion up to
one in seven, 900 stars being examined in the last
mentioned cluster of which 132 were variable !
When we consider that variable stars form only
a portion, and necessarily a very small proportion, of
binary systems of stars, it follows that in all the clusters
which show a large proportion of variables, a very
much larger proportion in some cases perhaps all,
must be double or multiple stars revolving round
each other. With this remarkable evidence, in
addition to that adduced for the prevalence of double
stars and variables among the stars in general, we
can understand Professor Newcomb adding his
testimony to that of Professor Campbell already
quoted, that it is probable that among the stars
in general, single stars are the exception rather than
the rule. If such be the case, the rule should hold
yet more strongly among the stars of a condensed
cluster.
THE EVOLUTION OF THE STARS
So long as astronomers were limited to the use of
the telescope only, or even the still greater powers of
the photographic plate, nothing could be learnt of the
actual constitution of the stars or of the process of
their evolution. Their apparent magnitudes, their
movements, and even the distances of a few could
be determined ; while the diversity of their colours
offered the only clue (a very imperfect one) even to
vi.] EVOLUTION OF THE STAR SYSTEM 129
their temperature. But the discovery of spectrum
analysis has furnished the means of obtaining some
definite knowledge of the physics and chemistry of
the stars, and has thus established a new branch of
science Astro-physics- -which has already attained
large proportions, and which furnishes the materials
for a periodical and some important volumes. This
branch of the subject is very complex, and as it is
not directly connected with our present inquiry, it is
only referred to again in order to introduce such of
its results as bear upon the question of the classifica
tion and evolution of the stars.
By a long series of laboratory experiments it has
been shown that numerous changes occur in the
spectra of the elements when subjected to different
temperatures, ranging upwards to the highest attain
able by means of a battery producing an electric
spark several feet long. These changes are not in
the relative position of the bands or dark lines, but
in their number, breadth, and intensity. Other
changes are due to the density of the medium in
which the elements are heated, and to their chemical
condition as to purity ; and from these various modi
fications and their comparison with the solar spectrum
and those of its appendages, it has become possible
to determine, from the spectrum of a star, not only its
temperature as compared with that of the electric
spark and of the sun, but also its place in a develop
mental series.
The first general result obtained by this research
is, that the bluish white or pure white stars, having a
spectrum extending far towards the violet end, and
which exhibits the coloured bands of gases only,
i
130 MAN S PLACE IN THE UNIVERSE [CHAP.
usually hydrogen and helium, are the hottest. Next
come those with a shorter spectrum not extending so
far towards the violet end, and whose light is there
fore more yellow in tint. To this group our sun
belongs ; and they are all characterised like it by dark
lines due to absorption, and by the presence of metals,
especially iron, in a gaseous state. The third group
have the shortest spectra and are of a red colour,
while their spectra contain lines denoting the presence
of carbon. These three groups are often spoken of
as * gaseous stars, metallic stars, and carbon stars.
Other astronomers call the first group * Sirian stars,
because Sirius, though not the hottest, is a character
istic type; the second being termed solar stars ;
others again speak of them as stars of Class i.,
Class ii., etc., according to the system of classification
they have adopted. It was soon perceived, however,
that neither the colour nor the temperature of stars
gave much information as to their nature and state
of development, because, unless we supposed the
stars to begin their lives already intensely hot (and
all the evidence is against this), there must be a
period during which heat increases, then one of
maximum heat, followed by one of cooling and final
loss of light altogether. The meteoritic theory of
the origin of all luminous bodies in the heavens, now
very widely adopted, has been used, as we have seen,
to explain the development of stars from nebulae,
and its chief exponent in this country, Sir Norman
Lockyer, has propounded a complete scheme of
stellar evolution and decay which may be here briefly
outlined :
Beginning with nebulae, we pass on to stars
vi.] EVOLUTION OF THE STAR SYSTEM 131
having banded or fluted spectra, indicating compara
tively low temperatures and showing bands or lines
of iron, manganese, calcium, and other metals. They
are more or less red in colour, Antares in the Scor
pion being one of the most brilliant red stars known.
These stars are supposed to be in the process of
aggregation, to be continually increasing in size and
heat, and thus to be subject to great disturbances.
Alpha Cygni has a similar spectrum but with more
hydrogen, and is much hotter. The increase of heat
goes on through Rigel and Beta Crucis, in which we
find mainly hydrogen, helium, oxygen, nitrogen, and
also carbon, but only faint traces of metals. Reach
ing the hottest of all Epsilon Orionis and two stars
in Argo- -hydrogen is predominant, with traces of a
few metals and carbon. The cooling series is indi
cated by thicker lines of hydrogen and thinner lines
of the metallic elements, through Sirius, to Arcturus
and our sun, thence to 19 Piscium, which shows
chiefly flutings of carbon, with a few faint metallic
lines. The process of further cooling brings us to
the dark stars.
We have here a complete scheme of evolution,
carrying us from those ill-defined but enormously
diffused masses of gas and cosmic dust we know as
nebulae, through planetary nebulae, nebulous stars,
variable and double-stars, to red and white stars and
on to those exhibiting the most intense blue-white
lustre. We must remember, however, that the most
brilliant of these stars, showing a gaseous spectrum
and forming the culminating point of the ascending
series, are not necessarily hotter than, or even so hot
as, some of those far down on the descending scale ;
132 MAN S PLACE IN THE UNIVERSE [CHAP.
since it is one of the apparent paradoxes of physics
that a body may become hotter during the very pro
cess of contraction through loss of heat. The reason
is that by cooling it contracts and thus becomes denser,
that a portion of its mass falls towards its centre,
and in doing so produces an amount of heat which,
though absolutely less than the heat lost in cooling,
will under certain conditions cause the reduced sur
face to become hotter. The essential point is, that
the body in question must be wholly gaseous, allowing
of free circulation from surface to centre. The law,
as given by Professor S. Newcomb, is as follows :-
* When a spherical mass of incandescent gas con
tracts through the loss of its heat by radiation into
space, its temperature continually becomes higher as
long as the gaseous condition is retained.
To put it in another way : if the compression was
caused by external force and no heat was lost, the
globe would get hotter by a calculable amount for
each unit of contraction. But the heat lost in causing
a similar amount of contraction is so little more than
the increase of heat produced by contraction, that
the slightly diminished total heat in a smaller bulk
causes the temperature of the mass to increase.
But if, as there is reason to believe, the various
types of stars differ also in chemical constitution,
some consisting mainly of the more permanent gases,
while in others the various metallic and non-metallic
elements are present in very different proportions,
there should really be a classification by constitution
as well as by temperature, and the course of evolu
tion of the differently constituted groups may be to
some extent dissimilar.
VI.] EVOLUTION OF THE STAR SYSTEM 133
With this limitation, the process of evolution and
decay of suns through a cycle of increasing and
decreasing temperature, as suggested by Sir Norman
Lockyer, is clear and suggestive. During the ascend
ing series the star is growing both in mass and heat,
by the continual accretion of meteoritic matter either
drawn to it by gravitation or falling towards it
through the proper motions of independent masses.
This goes on till all the matter for some distance
around the star has been utilised, and a maximum of
size, heat, and brilliancy attained. Then the loss of
heat by radiation is no longer compensated by the
influx of fresh matter, and a slow contraction occurs
accompanied by a slightly increased temperature.
But owing to the more stable conditions continuous
envelopes of metals in the gaseous state are formed,
which check the loss of heat and reduce the brilliancy
of colour ; whence it follows that bodies like our sun
may be really hotter than the most brilliant white
stars, though not giving out quite so much heat. The
loss of heat is therefore reduced ; and this may serve
to account for the undoubted fact that during the
enormous epochs of geological time there has been
very little diminution in the amount of heat we have
received from the sun.
On the general question of the meteoritic hypo
thesis one of our first mathematicians, Professor
George Darwin, has thus expressed his views : * The
conception of the growth of the planetary bodies by
the aggregation of meteorites is a good one, and
perhaps seems more probable than the hypothesis
that the whole solar system was gaseous. I may add,
that one of the chief objections made to it, that
134 MAN S PLACE IN THE UNIVERSE [CHAP.VI.
meteorites are too complex to be supposed to be the
primitive matter out of which suns and worlds have
been made, does not seem to me valid. The primi
tive matter, whatever it was, may have been used up
again and again, and if collisions of large solid globes
ever occur and it is assumed by most astronomers
that they must sometimes occur then meteoric par
ticles of all sizes would be produced which might
exhibit any complexity of mineral constitution. The
material universe has probably been in existence long
enough for all the primitive elements to have been
again and again combined into the minerals found
upon the earth and many others. It cannot be too
often repeated that no explanation no theory can
ever take us to the beginning of things, but only one
or two steps at a time into the dim past, which may
enable us to comprehend, however imperfectly, the
processes by which the world, or the universe, as it
is, has been developed out of some earlier and
simpler condition.
CHAPTER VII
ARE THE STARS INFINITE IN NUMBER?
MOST of the critics of my first short discussion of
this subject laid great stress upon the impossibility
of proving that the universe, a part of which we
see, is not infinite ; and a well-known astronomer
declared that unless it can be demonstrated that our
universe is finite the entire argument founded upon
our position within it falls to the ground. I had
laid myself open to this objection by rather in
cautiously admitting that if the preponderance of
evidence pointed in this direction any inquiry as to
our place in the universe would be useless, because
as regards infinity there can be no difference of posi
tion. But this statement is by no means exact, and
even in an infinite universe of matter containing an
infinite number of stars, such as those we see, there
might well be such infinite diversities of distribution
and arrangement as would give to certain positions
all the advantages which I submit we actually possess.
Supposing, for example, that beyond the vast ring
of the Milky Way the stars rapidly decrease in
number in all directions for a distance of a hundred
or a thousand times the diameter of that ring, and
that then for an equal distance they slowly increase
again and become aggregated into systems or
135
136 MAN S PLACE IN THE UNIVERSE [CHAP.
universes totally distinct from ours in form and
structure, and so remote that they can influence us
in no way whatever. Then, I maintain, our position
within our own stellar universe might have exactly
the same importance, and be equally suggestive, as
if ours were the only material universe in existence
as if the apparent diminution in the number of stars
(which is an observed fact) indicated a continuous
diminution, leading at some unknown distance to
entire absence of luminous- -that is, of active, energy-
emitting aggregations of matter. 1 As to whether
there are such other material universes or not I
offer no opinion, and have no belief one way or the
other. I consider all speculations as to what may
or may not exist in infinite space to be utterly value
less. I have limited my inquiries strictly to the
evidence accumulated by modern astronomers, and
to direct inferences and logical deductions from that
evidence. Yet, to my great surprise, my chief critic
declares that Dr. Wallace s underlying error is,
indeed, that he has reasoned from the area which
we can embrace with our limited perceptions to the
infinite beyond our mental or intellectual grasp. I
have distinctly not done this, but many astronomers
have done so. The late Richard Proctor not only
continually discussed the question of infinite matter
as well as infinite space, but also argued, from the
supposed attributes of the Deity, for the necessity
of holding this material universe to be infinite, and
the last chapter of his Other Worlds than Ours is
mainly devoted to such speculations. In a later
1 In a letter to Knowledge. June 1903, Mr. W. H. T. Monck puts the
same point in a mathematical form.
VIL] ARE THE STARS INFINITE IN NUMBER? 137
work, Our Place among Infinities, he says that the
teachings of science bring us into the presence of
the unquestionable infinities of time and of space,
and the presumable infinities of matter and of opera
tion hence therefore into the presence of infinity
of energy. But science teaches us nothing about
these infinities as such. They remain none the less
inconceivable, however clearly we may be taught to
recognise their reality. All this is very reason
able, and the last sentence is particularly important.
Nevertheless, many writers allow their reasonings
from facts to be influenced by these ideas of in
finity. In Proctor s posthumous work, Old and New
Astronamy, the late Mr. Ranyard, who edited it,
writes : If we reject as abhorrent to our minds the
supposition that the universe is not infinite, we are
thrown back on one of two alternatives either the
ether which transmits the light of the stars to us
is not perfectly elastic, or a large proportion of the
light of the stars is obliterated by dark bodies/
Here we have a well-informed astronomer allowing
his abhorrence of the idea of a finite universe to
affect his reasoning on the actual phenomena we
can observe doing in fact exactly what my critic
erroneously accuses me of doing. But setting aside
all ideas and prepossessions of the kind here in
dicated, let us see what are the actual facts revealed
by the best instruments of modern astronomy, and
what are the natural and logical inferences from
those facts.
138 MAN S PLACE IN THE UNIVERSE [CHAP.
ARE THE STARS INFINITE IN NUMBER?
The views of those astronomers who have paid
attention to this subject are, on the whole, in favour
of the view that the stellar universe is limited in
extent and the stars therefore limited in number.
A few quotations will best exhibit their opinions on
this question, with some of the facts and observa
tions on which they are founded.
Miss A. M. Clerke, in her admirable volume, The
System of the Stars, says : The sidereal world pre
sents us, to all appearance, with a finite system. . . .
The probability amounts almost to certainty that
star-strewn space is of measurable dimensions. For
from innumerable stars a limitless sum-total of radia
tions should be derived, by which darkness would
be banished from our skies; and the " intense inane,"
glowing with the mingled beams of suns individually
indistinguishable, would bewilder our feeble senses
with its monotonous splendour. . . . Unless, that is
to say, light suffer some degree of enfeeblement in
space. . . . But there is not a particle of evidence
that any such toll is exacted ; contrary indications
are strong ; and the assertion that its payment is
inevitable depends upon analogies which may be
wholly visionary. We are then, for the present,
entitled to disregard the problematical effect of a
more than dubious cause/
Professor Simon Newcomb, one of the first of
American mathematicians and astronomers, arrives
at a similar conclusion in his most recent volume,
The Stars (1902). He says, in his conclusions at
the end of the work : That collection of stars which
VIL] ARE THE STARS INFINITE IN NUMBER? 139
we call the universe is limited in extent. The
smallest stars that we see with the most powerful
telescopes are not, for the most part, more distant
than those a grade brighter, but are mostly stars of
less luminosity situate in the same regions (p. 319).
And on page 229 of the same work he gives reasons
for this conclusion, as follows : * There is a law of
optics which throws some light on the question.
Suppose the stars to be scattered through infinite
space so that every great portion of space is, in the
general average, equally rich in stars. Then at some
great distance we describe a sphere having its centre
in our sun. Outside this sphere describe another one
of a greater radius, and beyond this other spheres
at equal distances apart indefinitely. Thus we shall
have an endless succession of spherical shells, each
of the same thickness. The volume of each of these
shells will be nearly proportional to the squares of
the diameters of the spheres which bound it. Hence
each of the regions will contain a number of stars
increasing as the square of the radius of the region.
Since the amount of light we receive from each star
is as the inverse square of its distance, it follows
that the sum total of the light received from each
of these spherical shells will be equal. Thus as we
add sphere after sphere we add equal amounts of
light without limit. The result would be that if the
system of stars extended out indefinitely the whole
heavens would be filled with a blaze of light as bright
as the sun.
But the whole light given us by the stars is vari
ously estimated at from one-fortieth to one-twentieth
or, as an extreme limit, to one-tenth of moonlight,
140 MAN S PLACE IN THE UNIVERSE [CHAP.
while the sun gives as much light as 300,000 full
moons, so that starlight is only equivalent at a
fair estimate to the six-millionth part of sunlight.
Keeping this in mind, the possible causes of the
extinction of almost the whole of the light of the
stars (if they are infinite in number and distributed,
on the average, as thickly beyond the Milky Way
as they are up to its outer boundary) are absurdly
inadequate. These causes are (i) the loss of light in
passing through the ether, and (2) the stoppage of
light by dark stars or diffused meteoritic dust. As
to the first, it is generally admitted that there is
not a particle of evidence of its existence. There
is, however, some distinct evidence that, if it exists,
it is so very small in amount that it would not produce
a perceptible effect for any distances less remote
than hundreds or perhaps thousands of times as far
as the farthest limits of the Milky Way are from us.
This is indicated by the fact that the brightest stars
are not always, or even generally, the nearest to us,
as is shown both by their small proper motions and
the absence of measurable parallax. Mr. Gore states
that out of twenty-five stars, with proper motions of
more than two seconds annually, only two are above
the third magnitude. Many first magnitude stars,
including Canopus, the second brightest star in the
heavens, are so remote that no parallax can be found,
notwithstanding repeated efforts. They must there
fore be much farther off than many small and tele
scopic stars, and perhaps as far as the Milky Way,
in which so many brilliant stars are found ; whereas
if any considerable amount of light were lost in
passing that distance we should find but few stars
VIL] ARE THE STARS INFINITE IN NUMBER? 141
of the first two or three magnitudes that were
very remote from us. Of the twenty-three stars of
the first magnitude, only ten have been found to have
parallaxes of more than one-twentieth of a second,
while five range from that small amount down to one
or two hundredths of a second, and there are two
with no ascertainable parallax. Again, there are 309
stars brighter than magnitude 3*5, yet only thirty-one
of these have proper motions of more than 100" a
century, and of these only eighteen have parallaxes
of more than one-twentieth of a second. These
figures are from tables given in Professor Newcomb s
book, and they have very great significance, since they
indicate that the brightest stars are not the nearest
to us. More than this, they show that out of the
seventy-two stars whose distance has been measured
with some approach to certainty, only twenty-three
(having a parallax of more than one-fiftieth of a
second) are of greater magnitudes than 3*5, while
no less than forty-nine are smaller stars down to the
eighth or ninth magnitude, and these are on the
average much nearer to us than the brighter stars !
Taking the whole of the stars whose parallaxes
are given by Professor Newcomb, we find that the
average parallax of the thirty-one bright stars (from
3*5 magnitude up to Sirius) is cru seconds; while
that of the forty-one stars below 3*5 magnitude down
to about 9*5, is 0*21 seconds, showing that they are,
on the average, only half as far from us as the
brighter stars. The same conclusion was reached
by Mr. Thomas Lewis of the Greenwich Observa
tory in 1895, namely, that the stars from 270 magni
tude down to about 8*40 magnitude have, on the
H2 MAN S PLACE IN THE UNIVERSE [CHAP.
average, double the parallaxes of the brighter stars.
This very curious and unexpected fact, however it
may be accounted for, is directly opposed to the idea
of there being any loss of light by the more distant
as compared with the nearer stars ; for if there should
be such a loss it would render the above phenomenon
still more difficult of explanation, because it would
tend to exaggerate it. The bright stars being on
the whole farther away from us than the less bright
down to the eighth and ninth magnitudes, it follows,
if there is any loss of light, that the bright stars are
really brighter than they appear to us, because,
owing to their enormous distance some of their light
has been lost before it reached us. Of course it may
be said that this does not demonstrate that no light
is lost in passing through space ; but, on the other
hand, it is exactly the opposite of what we should
expect if the more distant stars were perceptibly
dimmed by this cause, and it may be considered to
prove that if there is any loss it is exceedingly small,
and will not affect the question of the limits of our
stellar system, which is all that we are dealing
with.
This remarkable fact of the enormous remoteness
of the majority of the brighter stars is equally effec
tive as an argument against the loss of light by dark
stars or cosmic dust, because, if the light is not
appreciably diminished for stars which have less than
the fiftieth of a second of parallax, it cannot greatly
interfere with our estimates of the limits of our
universe.
Both Mr. E. W. Maunder of the Greenwich
Observatory and Professor W. W. Turner of Oxford
VIL] ARE THE STARS INFINITE IN NUMBER? 143
lay great stress on these dark bodies, and the former
quotes Sir Robert Ball as saying, the dark stars are
incomparably more numerous than those that we can
see . . . and to attempt to number the stars of our
universe by those whose transitory brightness we can
perceive would be like estimating the number of
horseshoes in England by those which are red-hot/
But the proportion of dark stars (or nebulae) to
bright ones cannot be determined a priori, since it
must depend upon the causes that heat the stars, and
how frequently those causes come into action as com
pared with the life of a bright star. We do know,
both from the stability of the light of the stars during
the historic period, and much more precisely by the
enormous epochs during which our sun has supported
life upon this earth- -yet which must have been
incomparably less than its whole existence as a
light-giver that the life of most stars must be
counted by hundreds or perhaps by thousands of
millions of years. But we have no knowledge what
ever of the rate at which true stars are born. The
so - called new stars which occasionally appear
evidently belong to a different category. They
blaze out suddenly and almost as suddenly fade away
into obscurity or total invisibility. But the true stars
probably go through their stages of origin, growth,
maturity, and decay, with extreme slowness, so that
it is not as yet possible for us to determine by
observation when they are born or when they die.
In this respect they correspond to species in the
organic world. They would probably first be known
to us as stars or minute nebulae at the extreme limit
of telescopic vision or of photographic sensitiveness,
144 MAN S PLACE IN THE UNIVERSE [CHAP.
and the growth of their luminosity might be so
gradual as to require hundreds, perhaps thousands of
years to be distinctly recognisable. Hence the
argument derived from the fact that we have never
witnessed the birth of a true permanent star, and
that, therefore, such occurrences are very rare, is
valueless. New stars may arise every year or every
day without our recognising them ; and if this is the
case, the reservoir of dark bodies, whether in the form
of large masses or of clouds of cosmic dust, so far
from being incomparably greater than the whole of
the visible stars and nebulae, may quite possibly be
only equal to it, or at most a few times greater ; and
in that case, considering the enormous distances that
separate the stars (or star-systems) from each other,
they would have no appreciable effect in shutting out
from our view any considerable proportion of the
luminous bodies constituting our stellar universe. It
follows, that Professor Newcomb s argument as to
the very small total light given by the stars has not
been even weakened by any of the facts or arguments
adduced against it.
Mr. W. H. S. Monck, in a letter to Knowledge
(May 1903), puts the case very strongly so as to
support my view. He says : The highest estimate
that I have seen of the total light of the full moon
is 300060 f that of the sun. Suppose that the dark
bodies were a hundred and fifty thousand times as
numerous as the bright ones. Then the whole
sky ought to be as bright as the illuminated
portion of the moon. Every one knows that
this is not so. But it is said that the stars,
though infinite, may only extend to infinity in par-
VIL] ARE THE STARS INFINITE IN NUMBER? 145
ticular directions, e.g. in that of the Galaxy. Be
it so. Where, in the very brightest portion of the
Galaxy, will we find a part equal in angular magni
tude to the moon which affords us the same quantity
of light? In the very brightest spot, the light
probably does not amount to one hundredth part that
of the full moon. It follows that, even if dark stars
were fifteen million times as numerous as the bright
ones, Professor Newcomb s argument would still
apply against an infinite universe of stars of the
same average density as the portion we see.
TELESCOPIC EVIDENCE AS TO THE LIMITS OF THE
STAR SYSTEM
Throughout the earlier portion of the nineteenth
century every increase of power and of light-giving
qualities of telescopes added so greatly to the number
of the stars which became visible, that it was generally
assumed that this increase would go on indefinitely,
and that the stars were really infinite in number and
could not be exhausted. But of late years it has
been found that the increase in the number of stars
visible in the larger telescopes was not so great as
might be expected, while in many parts of the
heavens a longer exposure of the photographic plate
adds comparatively little to the number of stars
obtained by a shorter exposure with the same
instrument.
Mr. J. E. Gore s testimony on this point is very
clear. He says : Those who do not give the sub
ject sufficient consideration, seem to think that the
K
146 MAN S PLACE IN THE UNIVERSE [CHAP.
number -of the stars is practically infinite, or at least,
that the number is so great that it cannot be
estimated. But this idea is totally incorrect, and
due to complete ignorance of telescopic revelations.
It is certainly true that, to a certain extent, the
larger the telescope used in the examination of the
heavens, the more the number of the stars seems to
increase ; but we now know that there is a limit to
this increase of telescopic vision. And the evidence
clearly shows that we are rapidly approaching this
limit. Although the number of stars visible in the
Pleiades rapidly increases at first with increase in the
size of the telescope used, and although photography
has still further increased the number of stars in this
remarkable cluster, it has recently been found that
an increased length of exposure beyond three
hours adds very few stars to the number visible on
the photograph taken at the Paris Observatory in
1885, on which over two thousand stars can be
counted. Even with this great number on so small
an area of the heavens, comparatively large vacant
places are visible between the stars, and a glance at
the original photograph is sufficient to show that
there would be ample room for many times the
number actually visible. I find that if the whole
heavens were as rich in stars as the Pleiades,
there would be only thirty-three millions in both
hemispheres/
Again, referring to the fact that Celoria, with a
telescope showing stars down to the eleventh magni
tude, could see almost exactly the same number of
stars near the north pole of the Galaxy as Sir William
Herschel found with his much larger and more
VIL] ARE THE STARS INFINITE IN NUMBER? 147
powerful telescope, he remarks : Their absence,
therefore, seems certain proof that very faint stars do
not exist in that direction, and that here, at least, the
sidereal universe is limited in extent.
Sir John Herschel notes the same phenomena,
stating that even in the Milky Way there are found
spaces absolutely dark and completely void of any
star, even of the smallest telescopic magnitude * ;
while in other parts * extremely minute stars, though
never altogether wanting, occur in numbers so
moderate as to lead us irresistibly to the conclusion
that in these regions we Stt. fairly through the starry
stratum, since it is impossible otherwise (supposing
their light not intercepted) that the numbers of the
smaller magnitudes should not go on continually
increasing ad infinitum. In such cases, moreover, the
ground of the heavens, as seen between the stars, is
for the most part perfectly dark, which again would
not be the case if innumerable multitudes of stars,
too minute to be individually discernible, existed
beyond. And again he sums up as follows :
Throughout by far the larger portion of the extent
of the Milky Way in both hemispheres, the general
blackness of the ground of the heavens on which
its stars are projected, and the absence of that
innumerable multitude and excessive crowding of
the smallest visible magnitudes, and of glare pro
duced by the aggregate light of multitudes too small
to affect the eye singly, which the contrary supposi
tion would appear to necessitate, must, we think, be
considered unequivocal indications that its dimen
sions in directions where these conditions obtain, are
not only not infinite, but that the space-penetrating
148 MAN S PLACE IN THE UNIVERSE [CHAP.
power of our telescopes suffices fairly to pierce
through and beyond it/ 1
This expression of opinion by the astronomer
who, probably beyond any now living, was the most
competent authority on this question, to which he
devoted a long life of observation and study extend
ing over the whole heavens, cannot be lightly set
aside by the opinions or conjectures of those who
seem to assume that we must believe in an infinity
of stars if the contrary cannot be absolutely proved.
But as not a particle of evidence can be adduced to
prove infinity, and as all the facts and indications
point, as here shown, in a directly opposite direction,
we must, if we are to trust to evidence at all in this
matter, arrive at the conclusion that the universe of
stars is limited in extent.
Dr. Isaac Roberts gives similar evidence as re
gards the use of photographic plates. He writes :-
Eleven years ago photographs of the Great Nebula
in Andromeda were taken with the 2O-inch reflector,
and exposures of the plates during intervals up to
four hours ; and upon some of them were depicted
stars to the faintness of I7th to i8th magnitude,
and nebulosity to an equal degree of faintness. The
films of the plates obtainable in those days were less
sensitive than those which have been available during
the past five years, and during this period photo
graphs of the nebula with exposures up to four
hours have been taken with the 2O-inch reflector.
No extensions of the nebulosity, however, nor
increase in the number of the stars can be seen on
1 Outlines of Astronomy (last edition), pp. 578-9. In the passages
quoted the italics are Sir John Herschel s.
vii.] ARE THE STARS INFINITE IN NUMBER? 149
the later rapid plates than were depicted upon the
earlier slower ones, though the star-images and the
nebulosity have greater density on the later plates. 7
Exactly similar facts are recorded in the cases of
the Great Nebula in Orion, and the group of the
Pleiades. In the case of the Milky Way in Cygnus
photographs have been taken with the same instru
ment, but with exposures varying from one hour to
two hours and a half, but no fainter stars could be
found on one than on the other ; and this fact has
been confirmed by similar photographs of other areas
in the sky.
THE LAW OF DIMINISHING NUMBERS OF STARS
We will now consider another kind of evidence
equally weighty with the two already adduced. This
is what may be termed the law of diminishing numbers
beyond a certain magnitude, as observed by larger
and larger telescopes.
For some years past star-magnitudes have been
determined very accurately by means of careful
photometric comparisons. Down to the sixth magni
tude stars are visible to the naked eye, and are hence
termed lucid stars. All fainter stars are telescopic,
and continuing the magnitudes in a series in which
the difference in luminosity between each successive
magnitude is equal, the seventeenth magnitude is
reached and indicates the range of visibility in the
largest telescopes now in existence. By the scale
now used a star of any magnitude gives nearly two
and a half times as much light as one of the next
lower magnitude, and for accurate comparison the
ISO MAN S PLACE IN THE UNIVERSE [CHAP.
apparent brightness of each star is given to the tenth
of a magnitude which can easily be observed. Of
course, owing to differences in the colour of stars,
these determinations cannot be made with perfect
accuracy, but no important error is due to this cause.
According to this scale a sixth magnitude star gives
about one-hundredth part of the light of an average
first magnitude star. Sirius is so exceptionally bright
that it gives nine times as much light as a standard
or average first magnitude star.
Now it is found that from the first to the sixth
magnitude the stars increase in number at the rate
of about three and a half times those of the preceding
magnitudes. The total number of stars down to the
sixth magnitude is given by Professor Newcornb as
7647. For higher magnitudes the numbers are so
great that precision and uniformity are more difficult
of attainment ; yet there is a wonderful continuance
of the same law of increase down to the tenth magni
tude, which is estimated to include 2,311,000 stars,
thus conforming very nearly with the ratio of 3*5 as
determined by the lucid stars.
But when we pass beyond the tenth magnitude to
those vast numbers of faint stars only to be seen in
the best or the largest telescopes, there appears to
be a sudden change in the ratio of increased numbers
per magnitude. The numbers of these stars are so
great that it is impossible to count the whole as with
the higher magnitude stars, but numerous counts
have been made by many astronomers in small
measured areas in different parts of the heavens, so
that a fair average has been obtained, and it is
possible to make a near approximation to the total
VIL] ARE THE STARS INFINITE IN NUMBER? 151
number visible down to the seventeenth magnitude.
The estimate of these by astronomers who have made
a special study of this subject is, that the total
number of visible stars does not exceed one hundred
millions. 1
But if we take the number of stars down to the
ninth magnitude, which are known with considerable
accuracy, and find the numbers in each succeeding
magnitude down to the seventeenth, according to the
same ratio of increase which has been found to
correspond very nearly in the case of the higher
magnitudes, Mr. J. E. Gore finds that the total
number should be about 1400 millions. Of course
neither of these estimates makes any pretence to
exact accuracy, but they are founded on all the facts
at present available, and are generally accepted by
astronomers as being the nearest approach that can
be made to the true numbers. The discrepancy is,
however, so enormous that probably no careful
observer of the heavens with very large telescopes
doubts that there is a very real and very rapid
diminution in the numbers of the fainter as compared
with the brighter stars.
There is, however, yet one more indication of the
decreasing numbers of the faint telescopic stars, which
is almost conclusive on this question, and, so far as
I am aware, has not yet been used in this relation.
I will therefore briefly state it.
Mr. J. E. Gore in Concise Knowledge Astronomy, pp. 541-2.
152 MAN S PLACE IN THE UNIVERSE [CHAP.
THE LIGHT RATIO AS INDICATING THE NUMBER
OF FAINT STARS
Professor Newcomb points out a remarkable result
depending on the fact that, while the average light
of successively lower magnitudes diminishes in a ratio
of 2 5, their numbers increase at nearly a ratio of
3*5. From this it follows that, so long as this law of
increase continues, the total of star-light goes on
increasing by about forty per cent, for each successive
magnitude, and he gives the following table to
illustrate it :
Mag. i . . . Total Light = i
,, 2 . . . ,, =14
?j 3 * ~ 2
4 )> == 2 o
jj 5 = 4
6 . . . =57
7 . . . =8 o
8 . . . =11-3
9 . . . =16-0
10 . . . =22-6
Total light to Mag. 10 = 74-8
Thus the total amount of the light given by all
stars down to the tenth magnitude is seventy-four
times as great as that from the few first magnitude
stars. We also see that the light given by the stars
of any magnitude is twice as much as that of the stars
two magnitudes higher in the scale, so that we can
easily calculate what additional light we ought to
receive from each additional magnitude if they con
tinue to increase in numbers below the tenth as they
do above that magnitude. Now it has been calculated
VII.] ARE THE STARS INFINITE IN NUMBER? 153
as the result of careful observations, that the total
light given by stars down to nine and a half magni
tude is one-eightieth of full moonlight, though some
make it much more. But if we continue the table
of light-ratios from this low starting-point down to
magnitude seventeen and a half, we shall find, if the
numbers of the stars go on increasing at the same
rate as before, that the light of all combined should
be at least seven times as great as moonlight ; whereas
the photometric measurements make it actually about
one-twentieth. And as the calculation from light-
ratios only includes stars just visible in the largest
telescopes, and does not include all those proved to
exist by photography, we have in this case a demonstra
tion that the numbers of the stars below the tenth and
down to the seventeenth magnitude diminish rapidly.
We must remember that the minuter telescopic
stars preponderate enormously in and near the
Milky Way. At a distance from it they diminish
rapidly, till near its poles they are almost entirely
absent. This is shown by the fact (already referred
to at p. 146) that Professor Celoria of Milan, with a
telescope of less than three inches aperture, counted
almost as many stars in that region as did Herschel
with his eighteen-inch reflector. But if the stellar
universe extends without limit we can hardly suppose
it to do so in one plane only ; hence the absence of
the minuter stars and of diffused milky light over the
larger part of the heavens is now held to prove
that the myriads of very minute stars in the Milky
Way really belong to it, and not to the depths of
space far beyond.
It seems to me that here we have a fairly direct
154 MAN S PLACE IN THE UNIVERSE [CHAP.
proof that the stars of our universe are really limited
in number.
There are thus four distinct lines of argument all
pointing with more or less force to the conclusion
that the stellar universe we see around us, so far
from being infinite, is strictly limited in extent and of
a definite form and constitution. They may be briefly
summarised as follows :
(1) Professor Newcomb shows that, if the stars
were infinite in number, and if those we see were
approximately a fair sample of the whole, and further,
if there were not sufficient dark bodies to shut out
almost the whole of their light, then we should receive
from them an amount of light theoretically greater
than that of sunlight. I have shown, at some length,
that neither of these causes of loss of light will account
for the enormous disproportion between the theoretical
and the actual light received from the stars ; and
therefore Professor Newcomb s argument must be
held to be a valid one against the infinite extent of
our universe. Of course, this does not imply that
there may not be any number of other universes in
space, but as we know absolutely nothing of them-
even whether they are material or non-material all
speculation as to their existence is worse than useless.
(2) The next argument depends on the fact that
all over the heavens, even in the Milky Way itself,
there are areas of considerable extent, besides rifts,
lanes, and circular patches, where stars are either
quite absent or very faint and few in number. In
many of these areas the largest telescopes show no
more stars than those of moderate size, while the few
stars seen are projected on an intensely dark back-
VIL] ARE THE STARS INFINITE IN NUMBER? 155
ground. Sir William Herschel, Humboldt, Sir John
Herschel, R. A. Proctor, and many living astronomers
hold that, in these dark areas, rifts, and patches, we
see completely through our stellar universe into the
starless depths of space beyond.
(3) Then we have the remarkable fact that the
steady increase in the number of stars, down to the
ninth or tenth magnitudes, following one constant
ratio either gradually or suddenly changes, so that
the total number from the tenth down to the seven
teenth magnitudes is only about one-tenth of what it
would have been had the same ratio of increase con
tinued. The conclusion to be drawn from this fact
clearly is, that these faint stars are becoming more
and more thinly scattered in space, while the dark
background on which they are usually seen shows
that, except in the region of the Milky Way, there
are not multitudes of still smaller invisible stars beyond
them.
(4) The last indication of a limited stellar universe
the estimate of numbers by the light-ratio of each
successive magnitude- -powerfully supports the three
preceding arguments.
The four distinct classes of evidence now adduced
must be held to constitute, as nearly as the circum
stances permit, a satisfactory proof that the stellar
universe, of which our solar system forms a part, has
definite limits ; and that a full knowledge of its form,
structure, and extent, is not beyond the possibility
of attainment by the astronomers of the future.
CHAPTER VIII
OUR RELATION TO THE MILKY WAY
WE now approach what may be termed the very
heart of the subject of our inquiry, the determination
of how we are actually situated within this vast but
finite universe, and how that position is likely to
affect our globe as being the theatre of the develop
ment of life up to its highest forms.
We begin with our relation to the Milky Way
(which we have fully described in our fourth chapter),
because it is by far the most important feature in the
whole heavens. Sir John Herschel termed it the
ground-plane of the sidereal system ; and the more
it is studied the more we become convinced that the
whole of the stellar universe stars, clusters of stars,
and nebulae are in some way connected with it, and
are probably dependent on it or controlled by it.
Not only does it contain a greater number of stars of
the higher magnitudes than any other part of the
heavens of equal extent, but it also comprises a great
preponderance of star-clusters, and a great extent of
diffused nebulous matter, besides the innumerable
myriads of minute stars which produce its character
istic cloud-like appearance. It is also the region of
those strange outbursts forming new stars ; while
gaseous stars of enormous bulk some probably a
156
viii.] OUR RELATION TO THE MILKY WAY 157
thousand or even ten thousand times that of our
sun, and of intense heat and brilliancy are more
abundant there than in any other part of the heavens.
It is now almost certain that these enormous stars
and the myriads of minute stars just visible with the
largest telescopes, are actually intermingled, and
together constitute its essential features ; in which
case the fainter stars are really small and cannot
be far apart, forming, as it were, the first aggrega
tions of the nebulous substratum, and perhaps
supplying the fuel which keeps up the intense
brilliancy of the giant suns. If this is so, then the
Galaxy must be the theatre of operation of vast
forces, and of continuous combinations of matter,
which escape our notice owing to its enormous
distance from us. Among its millions of minute
telescopic stars, hundreds or thousands may appear
or disappear yearly without being perceived by us,
till the photographic charts are completed and can
be minutely scrutinised at short intervals. As un
doubted changes have occurred in many of the
larger nebulae during the last fifty years, we may
anticipate that analogous changes will soon be noted
in the stars and the nebulous masses of the Milky
Way. Dr. Isaac Roberts has even observed changes
in nebulae after such a short interval as eight years.
THE MILKY WAY A GREAT CIRCLE
Notwithstanding all its irregularities, its divisions,
and its diverging branches, astronomers are generally
agreed that the Milky Way forms a great circle in
the heavens. Sir John Herschel, whose knowledge
158 MAN S PLACE IN THE UNIVERSE [CHAP.
of it was unrivalled, stated that its course conforms,
as nearly as the indefiniteness of its boundary will allow
it to be fixed, to that of a great circle ; and he gives
the Right Ascension and Declination of the points
where it crosses the equinoctial, in figures which de
fine those points as being exactly opposite each other.
He also defines its northern and southern poles by
other figures, so as to show that they are the poles of
a great circle. And after referring to Struve s view
that it was not a great circle, he says, * I retain my
own opinion. Professor Newcomb says that its
position * is nearly always near a great circle of the
sphere ; and again he says : that we are in the
galactic plane itself seems to be shown in two ways :
(i) the equality in the counts of stars on the two
sides of this plane all the way to its poles; and (2)
the fact that the central line of the Galaxy is a great
circle, which it would not be if we viewed it from one
side of its central plane (The Stars, p. 317). Miss
Clerke, in her History of Astronomy, speaks of ( our
situation in the galactic plane as one of the un
disputed facts of astronomy ; while Sir Norman
Lockyer, in a lecture delivered in 1899, said, the
middle line of the Milky Way is really not dis
tinguishable from a great circle/ and again in the
same lecture but the recent work, chiefly of Gould
in Argentina, has shown that it practically is a great
circle. 1
About this fact, then, there can be no dispute.
A great circle is a circle dividing the celestial sphere
into two equal portions, as seen from the earth, and
therefore the plane of this circle must pass through
1 Nature, October 26, 1899.
VIIL] OUR RELATION TO THE MILKY WAY 159
the earth. Of course the whole thing is on such a
vast scale, the Milky Way varying from ten to thirty
degrees wide, that the plane of its circular course
cannot be determined with minute accuracy. But
this is of little importance. When carefully laid
down on a chart, as in that of Mr. Sidney Waters
(see end of volume), we can see that its central line
does follow a very even circular course, conforming
as nearly as may be to a great circle. We are
therefore certainly well within the space that would
be enclosed if its northern and southern margins
o
were connected together across the vast interven
ing abyss, and in all probability not far from the
central plane of that enclosed space.
THE FORM OF THE MILKY WAY AND OUR
POSITION ON ITS PLANE
Although the Galaxy forms a great circle in the
heavens from our point of view, it by no means
follows that it is circular in plan. Being unequal
in width and irregular in outline, it might be elliptic
or even angular in shape without being at all
obviously so to us. If we were standing in an
open plain or field two or three miles in diameter,
and bounded in every direction by woods of very
irregular height and density and great diversity of
tint, we should find it difficult to judge of the shape
of the field, which might be either a true circle, an
oval, a hexagon, or quite irregular in outline, without
our being able to detect the exact shape unless some
parts were very much nearer to us than others.
Again, just as the woods bounding the field might
160 MAN S PLACE IN THE UNIVERSE [CHAP.
be either a narrow belt of nearly uniform width, or
might in some places be only a few yards wide and
in others stretch out for miles, so there have been
many opinions as to the width of the Milky Way in
the direction of its plane, that is, in the direction in
which we look towards it. Lately, however, as
the result of long-continued observation and study,
astronomers are fairly well agreed as to its general
form and extent, as will be seen by the following
statements of fact and reasoning
Miss Clerke, after giving the various views of
many astronomers and as the historian of modern
astronomy her opinion has much weight considers
that the most probable view of it is, that it is really
very much what it seems to us an immense ring
with streaming appendages extending from the main
body in all directions, producing the very complex
effect we see. The belief seems to be now spreading
that the whole universe of stars is spherical or
spheroidal, the Milky Way being its equator, and
therefore in all probability circular or nearly so in
plan ; and it is also held that it must be rotating-
perhaps very slowly as nothing else can be sup
posed to have led to the formation of such a vast
ring, or can preserve it when formed.
Professor Newcomb considers, from the numbers
of the stars in all directions towards the Milky Way
being approximately equal, that there cannot be much
difference in our distance from it in various directions.
It would follow that its plan is approximately circular
or broadly elliptic. The existence of ring-nebulae
may be held to render such a form probable.
Sir Norman Lockyer gives facts which tend in the
VIIL] OUR RELATION TO THE MILKY WAY 161
same direction. In an article in Nature of Novem
ber 8th, 1900, he says: We find that the gaseous
stars are not only confined to the Milky Way, but
they are the most remote in every direction, in
every galactic longitude ; all of them have the
smallest proper motion/ And again, referring to
the hottest stars being equally remote on all sides of
us, he says : It is because we are in the centre,
because the solar system is in the centre, that the
observed effect arises. He also considers that the
ring-nebula in Lyra nearly represents the form of our
whole system ; and he adds : We practically know
that in our system the centre is the region of least
disturbance, and therefore cooler conditions/
These various facts and conclusions of some of the
most eminent astronomers all point to one definite
inference, that our position, or that of the solar
system, is not very far from the centre of the vast
ring of stars constituting the Milky Way, while the
same facts imply a nearly circular form to this ring.
Here, more than as regards our position in the plane
of the Galaxy, there is no possibility of precise deter
mination ; but it is quite certain that if we were
situated very far away from the centre, say, for
instance, one-fourth of its diameter from one side of
it and three-fourths from the other, the appearances
would not be what they are, and we should easily
detect the excentricity of our position. Even if we
were one-third the diameter from one side and two-
thirds from the other, it will, I think, be admitted
that this also would have been ascertained by the
various methods of research now available. We
must, therefore, be somewhere between the actual
L
1 62 MAN S PLACE IN THE UNIVERSE [CHAP.
centre and a circle whose radius is one-third of the
distance to the Milky Way. But if we are about
midway between these two positions, we shall only be
one-sixth of the radius or one-twelfth of the diameter
of the Milky Way from its exact centre ; and if we
form part of a cluster or group of stars slowly re
volving around that centre, we should probably
obtain all the advantages, if any, that may arise
from a nearly central position in the entire star-
system.
This question of our situation within the great
circle of the Milky Way is of considerable import
ance from the point of view I am here suggesting, so
that every fact bearing upon it should be noted ; and
there is one which has not, I think, been given the
full weight due to it. It is generally admitted that
the greater brilliancy of some parts of the Milky
Way is no indication of nearness, because surfaces
possess equal brilliancy from whatever distance they
are seen. Thus each planet has its special brilliancy
or reflective power, technically termed its albedo,
and this remains the same at all distances if the other
conditions are similar. But notwithstanding this
well-known fact, Sir John Herschel s remark that
the greater brightness of the southern Milky Way
conveys strongly the impression of greater prox
imity/ and therefore, that we are excentrically placed
in its plane, has been adopted by many writers as if
it were the statement of a fact, or at least a clearly
expressed opinion, instead of being a mere * impres
sion, and really a misleading one. I therefore wish
to adduce a phenomenon which has a real bearing on
the question. It is evident that, if the Milky Way
viii.] OUR RELATION TO THE MILKY WAY 163
were actually of uniform width throughout, then differ
ences of apparent width would indicate differences of
distance. In the parts nearer to us it would appear
wider, where more remote, narrower ; but in these
opposite directions there would not necessarily be
any differences in brightness. We should, however,
expect that in the parts nearer to us the lucid stars,
as well as those within any definite limits of magni
tude, would be either more numerous or more wide
apart on the average. No such difference as this,
however, has been recorded ; but there is a peculiar
correspondence in the opposite portions of the Galaxy
which is very suggestive. In the beautiful charts of
the Nebulae and Star Clusters by the late Mr. Sidney
Waters, published by the Royal Astronomical Society
and here reproduced by their permission (see end of
volume), the Milky Way is delineated in its whole
extent with great detail and from the best authorities.
These charts show us that, in both hemispheres, it
reaches its maximum extension on the right and left
margins of the charts, where it is almost equal in
extent ; while in the centre of each chart, that is at
its nearest points to the north and south poles respec
tively, it is at its narrowest portion ; and, although
this part in the southern hemisphere is brightest and
most strongly defined, yet the actual extent, including
the fainter portions, is, again, not very unequal in the
opposite segments. Here we have a remarkable and
significant symmetry in the proportions of the Milky
Way, which, taken in connection with the nearly
symmetrical scattering of the stars in all parts of the
vast ring, is strongly suggestive of a nearly circular
form and of our nearly central position within its
1 64 MAN S PLACE IN THE UNIVERSE [CHAP.
plane. There is one other feature in this delineation
of the Milky Way which is worthy of notice. It has
been the universal practice to speak of it as being
double through a considerable portion of its extent,
and all the usual star-maps show the division greatly
exaggerated, especially in the northern hemisphere ;
and this division was considered so important as to
lead to the cloven-disc theory of its form, or that it
consisted of two separate irregular rings, the nearer
one partly hiding the more distant ; while various
spiral combinations were held by others to be the
best way of explaining its complex appearance. But
this newer map, reduced from a large one by Lord
Rosse s astronomer, Dr. Boeddicker, who devoted
five years to its delineation, shows us that there is no
actual division in any portion of it in the northern
hemisphere, but that everywhere, throughout its
whole width, it consists of numerous intermingled
streams and branches, varying greatly in luminosity,
and with many faint or barely distinguishable exten
sions along its margins, yet forming one unmistak
able nebulous belt ; and the same general character
applies to it in the southern hemisphere as delineated
by Dr. Gould.
Another feature, which is well shown to the eye by
these more accurate maps, is the regular curvature of
the central line of the Milky Way. We can judge of
this almost sufficiently by the eye ; but if, with a pair
of compasses, we find the proper radius and centre of
curvature, we shall see that the true circular curve is
always in the very centre of the nebulous mass, and
the same radius applied in the same manner to the
opposite hemisphere gives a similar result. It will
VIIL] OUR RELATION TO THE MILKY WAY 165
be noted that as the Milky Way is obliquely situated
on these charts, the centre of the curve will be about
in R.A. oh. 4om. in the map of the southern hemi
sphere, and in R.A. i2h. 4om. in that of the northern
hemisphere ; while the radius of curvature will be
about the length of the chord of eight hours of R.A.
as measured on the margin of the maps. This great
regularity of curve of the central line of the Galaxy
strongly suggests rotation as the only means by
which it could have originated and be maintained.
THE SOLAR CLUSTER
Astronomers are now generally agreed that there
is a cluster of stars of which our sun forms a part,
though its exact dimensions, form, and limits are still
under discussion. Sir William Herschel long ago
arrived at the conclusion that the Milky Way con
sists of stars very differently scattered from those im
mediately around us. Dr. Gould believed that there
were about five hundred bright stars much nearer to
us than the Milky Way, which he termed the solar
cluster. And Miss Clerke observes that the actual
existence of such a cluster is indicated by the fact
that an enumeration of the stars in photometric
order discloses a systematic excess of stars brighter
than the 4th magnitude, making it certain that there
is an actual condensation in the neighbourhood of the
sun that the average allowance of cubical space per
star is smaller within a sphere enclosing him with a
radius, say, of 140 light-years, than further away. 1
But the most interesting inquiry into this subject
1 The System of the Stars, p. 385.
1 66 MAN S PLACE IN THE UNIVERSE [CHAP.
is that by Professor Kapteyn of Groningen, one of
the most painstaking students of the distribution of
the stars. He founds his conclusions mainly on the
proper motions of the stars, this being the best
general indication of distance in the absence of actual
determination of parallax. He made use of the
proper motions and the spectra of more than two
thousand stars, and he finds that a considerable
body of stars having large proper motions, and also
presenting the solar type of spectra, surround our sun
in all directions, and show no increased density, as
the more distant stars do, towards the Milky Way.
He finds also that towards the centre of this cluster
stars are far closer together than near its outer limits
(he says there are ninety-eight times as many), that it
is roughly spherical in shape, and that the maximum
compression is, as nearly as can be ascertained, at the
centre of the circle of the Milky Way, while the sun
is at some distance away from this central point. 1
It is a very suggestive fact that most of the stars
belonging to this cluster have spectra of the solar type,
which indicates that they are of the same general
chemical constitution as our sun, and are also at about
the same stage of evolution ; and this may well have
arisen from their origin in a great nebulous mass
situated at or near the centre of the galactic plane,
and probably revolving round their common centre
of gravity.
As Kapteyn s result was based on materials which
were not so full or reliable as those now available,
Professor S. Newcomb has examined the question
1 This account of Professor Kapteyn s research is taken from an
article by Miss A. M. Clerke in Knowledge, April 1893.
VIIL] OUR RELATION TO THE MILKY WAY 167
himself, using two recent lists of stars, one limited
to those having proper motions of 10" a century, of
which there are 295, and the other of nearly 1500
stars with appreciable proper motions/ They are
situated in two zones, each about 5 in breadth and
cutting across the Milky Way in different parts of
its course. They afford, therefore, a good test of the
distribution of these nearer stars with regard to the
Galaxy. The result is, that on the average these
stars are not more numerous in or near the Milky
Way than elsewhere ; and Professor Newcomb ex
presses himself on this point as follows : The con
clusion is interesting and important. If we should blot
out from the sky all the stars having no proper motion
large enough to be detected, we should find remaining
stars of all magnitudes ; but they would be scattered
almost uniformly over the sky, and show little or
no tendency to crowd towards the Galaxy, unless,
perhaps, in the region near iqh. of Right Ascension. :
A little consideration will show that, as the stars
of all magnitudes which are, on the average, nearest
to us are spread over the sky in all directions and
1 The Stars, p. 256. The region here referred to is that where the
Milky Way has its greatest width (though nearly as wide in the part
exactly opposite), and where it may perhaps extend somewhat in our
direction.
Miss A. M. Clerke informs me that in April 1901 Kapteyn withdrew
the conclusions arrived at in 1893, as being founded on illegitimate
reasoning as to the relation of parallaxes to proper motions. But as
this relation is still accepted, under certain limitations, by Professor
Newcomb and other astronomers, who have arrived independently at
very similar results, it seems not improbable that, after all, Professor
Kapteyn s conclusions may not require very much modification. Pro
fessor Newcomb also tells us (The Stars, p. 214, footnote) that he has
seen the latest of Professor Kapteyn s papers, down to 1901 ; but he
does not therefore express any doubt as to his own conclusions as here
referred to.
1 68 MAN S PLACE IN THE UNIVERSE [CHAP.
1 almost uniformly, this necessarily implies that they
form a cluster or group, and that our sun is some
where not very far from the centre of this group.
Again, Professor Newcomb refers to * the remarkable
equality in the number of stars in opposite directions
from us. We do not detect any marked difference
between the numbers lying round the opposite poles
of the Galaxy, nor, so far as known, between the
star-density in different regions at equal distances
from the Milky Way (The Stars, p. 315). And
again he refers to the same question at p. 317,
where he says : * So far as we can judge from the
enumeration of the stars in all directions, and from
the aspect of the Milky Way, our system is near the
centre of the stellar universe.
It will, I think, now be clear to my readers that
the four main astronomical propositions stated in my
article which appeared in the New York Independent
and in the Fortnightly Review, and which were either
denied or declared to be unproved by my astrono
mical critics, have been shown to be supported by
so many converging lines of evidence, that it is no
longer possible to deny that they are, at least pro
visionally, fairly well established. These facts are,
(1) that the stellar universe is not of infinite extent;
(2) that our sun is situated in the central plane of
the Milky Way; (3) that it is also situated near to
the centre of that plane ; (4) that we are surrounded
by a group or cluster of stars of unknown extent,
which occupy a place not far removed from the
centre of the galactic plane, and therefore, near to
the centre of our universe of stars.
Not only are these four propositions each sup-
VIIL] OUR RELATION TO THE MILKY WAY 169
ported by converging lines of evidence, including
some which I believe have not before been adduced
in their support, but a number of astronomers, ad
mittedly of the first rank, have arrived at the same
conclusions as to the bearing of the evidence, and
have expressed their convictions in the clearest
manner, as quoted by me. It is their conclusions
which I appeal to and adopt ; yet my two chief
astronomical critics positively deny that there is any
valid evidence of the finiteness of the stellar universe,
which one of them terms a myth, and he even
accuses me of having started it. Both of them,
however, agree in stating very strongly one objection
to my main thesis that our central position (not
necessarily at the precise centre) in the stellar
universe has a meaning and a purpose, in connection
with the development of life and of man upon this
earth, and, so far as we know, here only. With this
one objection, the only one that in my opinion has
the slightest weight, I will now proceed to deal.
THE SUN S MOTION THROUGH SPACE
The two astronomers who did me the honour to
criticise my original article laid the greatest stress
on the fact, that even if I had proved that the
sun now occupied a nearly central position in the
great star-system, it was really of no import
ance whatever, because, at the rate the sun was
travelling, five million years ago we were deep in
the actual stream of the Milky Way ; five million
years hence we shall have completely crossed the
gulf which it encircles, and again be a member of
one of its constituent groups, but on the opposite
1 70 MAN S PLACE IN THE UNIVERSE [CHAP.
side. And ten million years are regarded by
geologists and biologists as but a trifle on account
to meet their demands upon the bank of Time/
Thus speaks one of my critics. The other is equally
crushing. He says : If there is a centre to the
visible universe, and if we occupy it to-day, we
certainly did not do so yesterday, and shall not do
so to-morrow. The Solar System is known to be
moving among the stars with a velocity which would
carry us to Sirius within 100,000 years, if we hap
pened to be travelling in his direction, as we are
not. In the 50 or 100 million years during which,
according to geologists, this earth has been a habit
able globe, we must have passed by thousands of
stars on the right hand and on the left. ... In his
eagerness to limit the universe in space, Dr. Wallace
has surely forgotten that it is equally important, for
his purpose, to limit it in time ; but incomparably more
difficult in the face of ascertained facts. . . . Indeed,
so far from our having tranquilly enjoyed a central
position in unbroken continuity for scores or perhaps
hundreds of millions of years, we should in that time
have traversed the universe from boundary to
boundary. 1
Now the average reader of these two criticisms,
taking account of the high official position of both
writers, would accept their statements of the case as
being demonstrated facts, requiring no qualification
whatever, and would conclude that my whole argu
ment had been thereby rendered worthless, and all
that I founded upon it a fantastic dream. But if, on
the other hand, I can show that their stated facts as
1 See Knowledge and The Fortnightly Review of April 1903.
vin.] OUR RELATION TO THE MILKY WAY 171
to the sun s motion are by no means demonstrated,
because founded upon assumptions which may be
quite erroneous ; and further, that if the facts should
turn out to be substantially correct, they have both
omitted to state well-known and admitted qualifica
tions which render the conclusions they derive from
the facts very doubtful, then the average reader will
learn the valuable lesson that official advocacy,
whether in medicine, law, or science, is never to be
accepted till the other side of the case has been
heard. Let us see, therefore, what the facts really
are.
Professor Simon Newcomb calculates that, if there
are one hundred million stars in the stellar universe
each five times the mass of our sun, and spread over
a space which light would require thirty thousand
years to cross, then any mass traversing such a system
with a velocity of more than twenty-five miles a
second, would fly off into infinite space never to
return. Now as there are many stars which have,
apparently, very much more than this velocity, it
would follow that the visible universe is unstable.
It also implies that these great velocities were not
acquired in the system itself, but that the bodies
which possess them must have entered it from with
out, thus requiring other universes as the feeders of
our universe.
For the accuracy of the above statement the
authority of Professor Newcomb is an ample guar
antee ; but there may be modifications required in
the data on which it is founded, and these may
greatly alter the result. If I do not mistake, the
estimate of a hundred million stars is founded on
MAN S PLACE IN THE UNIVERSE [CHAP.
actual counts or estimates of stars of successive
magnitudes in different parts of the heavens, and it
does not include either those of the denser star-
clusters nor the countless millions just beyond the
reach of telescopes in the Milky Way. Neither does
it make allowance for the dark stars supposed by
some astronomers to be many times more numerous
than the bright ones, nor for the vast number of the
nebulae, great and small, in calculating the total mass
of the stellar system. 1 In his latest work Professor
Newcomb says, The total number of stars is to be
counted by hundreds of millions ; and hence the
controlling power of the system on bodies within it
will be many times greater than that given above,
and might even be ample to retain within its bounds
such a rapidly moving star as Arcturus, which is be
lieved to be travelling at the rate of more than three
hundred miles a second. But there is another very
important limitation to the conclusions to be drawn
from Professor Newcomb s calculation. It assumes
the stars to be nearly uniformly distributed through
the whole of the space to which the system extends.
But the facts are very different. The existence of
clusters, some of which comprise many thousands of
stars, is one example of irregularity of distribution,
and any one of these larger clusters would probably
be able to change the course of even the swiftest
stars passing near it. The larger nebulae might
have the same effect, since the late Mr. Ranyard,
taking all his data so as to produce a minimum
1 Sir R. Ball in an article in Good Words (April 1903) says that
luminosity is an exceptional phenomenon in nature, and that luminous
stars are but the glow-worms and fire-flies of the universe, as compared
with the myriads of other animals.
VIIL] OUR RELATION TO THE MILKY WAY 173
result, calculated the probable mass of the Orion
nebula to be four and a half million times that of the
sun, and there may be many other nebulae equally
large. But far more important is the fact of the
vast ring of the Milky Way, which is now universally
held by astronomers to be, not only apparently but
really, more densely crowded with stars and also
with vast masses of nebulous matter than any other
part of the heavens, so that it may possibly comprise
within itself a very large proportion of the whole
of the matter of the visible universe. This is ren
dered more probable by the fact that the great
majority of star-clusters lie along its course, most of
the huge gaseous stars belong to it, while the oc
currence there only of * new stars is evidence of a
superabundance of matter in various forms leading
to frequent heat-producing collisions, just as the
frequent occurrence of meteoric showers on our earth
is evidence of the superabundance of meteoric matter
in the solar system.
It is recognised by mathematicians that within
any great system of bodies subject to the law of
gravitation there can be no such thing as motion
of any of them in a straight line ; neither can any
amount of motion arise within such a system through
the action of gravitation alone capable of carrying
any of its masses out of the system. The ultimate
tendency must be towards concentration rather than
towards dispersal.
It seems, therefore, only reasonable to consider
whatever motions and whatever velocities we find
among the stars, as having been produced by the
gravitative power of the larger aggregations, modified
174 MAN S PLACE IN THE UNIVERSE [CHAP.
perhaps by electrical repulsive forces, by collisions,
and by the results of those collisions ; and we may
look to the changes now visibly going on in some of
the nebulae and clusters as indications of the forces
that have probably brought about the actual condition
of the whole stellar universe.
If we examine the beautiful photographs of nebulae
by Dr. Roberts and other observers, we find that
they are of many forms. Some are extremely
irregular and almost like patches of cirrus clouds,
but a large number are either distinctly spiral in
form, or show indications of becoming spiral, and this
has been found to be the case even with some of
the large irregular nebulae. Then again we have
numerous ring-formed nebulae, usually with a star
involved in dense nebulosity in the centre, separated
by a dark space of various widths from the outer
ring. All these kinds of nebulae have stars involved
in them, and apparently forming part of their
structure, while others which do not differ in appear
ance from ordinary stars are believed by Dr. Roberts
to lie between us and the nebula. In the case of
many of the spiral nebulae, stars are often strung
along the coils of the spiral, while other curved lines
of stars are seen just outside the nebula, so that it
is impossible to avoid the conclusion that both are
really connected with it, the outer lines of stars
indicating a former greater extension of the nebula
whose material has been used up in the growth of
these stars. Some of these spiral nebulae show beauti
fully regular convolutions, and these usually have
a large central star-like mass, as in M. 100 Comae
and I. 84 Comae, in Vol. n. PI. 14 of Dr. Roberts s
VIII.] OUR RELATION TO THE MILKY WAY 175
photographs. The straight white streaks across the
nebula of the Pleiades and some others are believed
by Dr. Roberts to be indications of spiral nebulae
seen edgewise. In other cases, clusters of stars are
more or less nebulous, and the arrangement of the
stars seems to indicate their development from a
spiral nebula. It is to be noted that many of the
objects classed as planetary nebulae by Sir John
Herschel are shown by the best photographs to be
really of the ring-type, though often with a very
narrow division between the ring and the central
mass. This form may therefore be of frequent
occurrence.
But if this annular form with some kind of central
nucleus, often very large, is produced under certain
conditions by the action of the ordinary laws of
motion upon more or less extensive masses of dis
crete matter, why may not the same laws acting
upon similar matter once dispersed over the whole
extent of the existing stellar universe, or even beyond
what are now its farthest limits, have led to the
aggregation of the vast annular formation of the
Milky Way, with all the subordinate centres of con
centration or dispersal to be found within or around
it? And if this is a reasonable conception, may we
not hope that by a concentration of attention upon a
few of the best marked and most favourably situated
annular and spiral systems, sufficient knowledge of
their internal motions may be obtained which may
serve as a guide to the kind of motion we may
expect to find in the great galactic ring and its
subordinate stars? We may then perhaps discover
that the proper motions of the stars, and of our sun,
i;6 MAN S PLACE IN THE UNIVERSE [CHAP.
which now seem so erratic, are really all parts of a
series of orbital movements limited and controlled by
the forces of the great system to which they belong,
so that, if not mathematically stable, they may yet
be sufficiently so to endure for some thousand
millions of years.
It is a suggestive fact that the calculated position
of the solar apex -the point towards which our
sun appears to move is now found to be much
more nearly in the plane of the Milky Way than the
position first assigned to it, and Professor Newcomb
adopts, as most likely to be accurate, a point near
the bright star Vega in the constellation Lyra. Other
calculators have placed it still farther east, while
Rancken and Otto Stumpe assign it a position
actually in the Milky Way ; and Mr. G. C. Bompas
concludes that the sun s plane of motion nearly co
incides with that of the Galaxy. M. Rancken found
that 1 06 stars near the Milky Way showed, in their
very small proper motions, a drift along it in a
direction from Cassiopeia towards Orion, and this,
it is supposed, may be partly due to our sun s motion
in an opposite direction.
In many other parts of the heavens there are
groups of stars which have almost identical proper
motions a phenomenon which the late R. A. Proctor
termed star-drift ; and he especially pointed out
that five of the stars of the Great Bear were all
drifting in the same direction ; and although this
has been denied by later writers, Professor Newcomb,
in his recent book on The Stars, declares that Proctor
was right, and explains that the error of his critics
was due to not making allowance for the divergence
VIIL] OUR RELATION TO THE MILKY WAY 177
of the circles of right ascension. The Pleiades are
another group, the stars of which drift in the same
direction, and it is a most suggestive fact that photo
graphs now show this cluster to be embedded in
a vast nebula, which, therefore, has also a proper
motion ; but some of the smaller stars do not partake
of it. Three stars in Cassiopeia also move together,
and no doubt many other similarly connected groups
remain to be discovered.
These facts have a very important bearing on the
question of the motion of our sun in space. For
this motion has been determined by comparing the
motions of large numbers of stars which are assumed
to be wholly independent of each other, and to move,
as it were, at random. Miss A. M. Clerke, in her
System of the Stars, puts this point very clearly, as
follows : * For the assumption that the absolute
movements of the stars have no preference for one
direction over another, forms the basis of all in
vestigations hitherto conducted into the translatory
advance of the solar system. The little fabric of
laboriously acquired knowledge regarding it at once
crumbles if that basis has to be removed. In all
investigations of the sun s movement, the movements
of the stars have been regarded as casual irregulari
ties ; should they prove to be in any visible degree
systematic, the mode of treatment adopted (and there
is no other at present open to us) becomes invalid,
and its results null and void. The point is then of
singular interest, and the evidence bearing upon it
deserves our utmost attention.
Mr. W. H. S. Monck, a well-known astronomer,
takes the same view. He says: The proof of this
M
i;8 MAN S PLACE IN THE UNIVERSE [CHAP.
motion rests on the assumption that if we take a
sufficient number of stars, their real motions in all
directions will be equal, and that therefore the
apparent preponderances which we observe in par
ticular directions result from the real motion of the
sun. But there is no impossibility in a systematic
motion of the majority of the stars used in these
researches which might reconcile the observed facts
with a motionless sun. And, in the second place,
if the sun is not in the exact centre of gravity of
the universe, we might expect him to be moving
in an orbit around this centre of gravity, and our
observations on his actual motion are not sufficiently
numerous or accurate to enable us to affirm that he
is moving in a right line rather than such an orbit.
Now this systematic motion, which would render
all calculations as to the sun s motion inaccurate or
even altogether worthless, is by many astronomers
held to be an observed reality. The star-drift, first
pointed out by Proctor, has been shown to exist in
many other groups of stars, while the curious arrange
ments of stars all over the heavens in straight lines,
or regular curves, or spirals, strongly suggests a wide
extension of the same kind of relation. But even
more extensive systematic movements have been
observed or suggested by astronomers. Sir D. Gill,
by an extensive research, believes that he has found
indications of a rotation of the brighter fixed stars
as a whole in regard to the fainter fixed stars as a
whole. Mr. Maxwell Hall has also found indications
of a movement of a large group of stars, including
our sun, around a common centre, situated in a
direction towards Epsilon Andromedae, and at a
VIIL] OUR RELATION TO THE MILKY WAY 179
distance of about 490 years of light-travel. These
last two motions are not yet established ; but they
seem to prove two important facts (a) that eminent
astronomers believe that some systematic motions
must exist among the stars, or they would not
devote so much labour to the search for them ; and
(d) that extensive systematic motions of some kind
do exist, or even these results would not have been
obtained.
Mr. W. W. Campbell, of the Lick Observatory,
thus remarks on the uncertainty of determinations of
the sun s motions : The motion of the solar system
is a purely relative quantity. It refers to specified
groups of stars. The results for various groups may
differ widely, and all be correct. It would be easy
to select a group of stars with reference to which
the solar motion would be reversed 180 from the
values assigned above (Astrophysical Journal, vol.
xiii. p. 87. 1901).
It must be remembered that, within a uniform
cluster of stars, each moving round the common
centre of gravity of the whole cluster, Kepler s laws
do not prevail, the law being that the angular velo
cities are all identical, so that the more distant stars
move faster than those nearer the centre, subject to
modifications, however, due to the varying density
of the cluster. But if the cluster is nearly globular,
there must be stars moving round the centre in every
plane, and this would lead to apparent motions in
many directions as viewed by us, although those
which were moving in the same plane as ourselves
would, when compared with remote stars outside
the cluster, appear to be all moving in the same
iSo MAN S PLACE IN THE UNIVERSE [CHAP.
direction and at the same rate, forming, in fact, one
of those drifting systems of stars already referred
to. Again, if in the process of formation of
our cluster, smaller aggregations already having a
rotatory motion were drawn into it, this might lead
to their revolving in an opposite direction to those
which were formed from the original nebula, thus
increasing the diversities of apparent motion.
The evidence now briefly set forth fully justifies,
I submit, the remarks as to the statements of my
astronomical critics at the beginning of this section.
They have both given the accepted views as to
direction and rate of movement of our sun without
any qualification whatever, as if they were astro
nomical facts of the same certainty and the same
degree of accuracy as the sun s distance from the
earth ; and they will assuredly have been so under
stood by the great body of non-mathematical readers.
It appears, however, if the authorities I have quoted
are right, that the whole calculation rests upon
certain assumptions, which are certainly to some
extent, and may be to a very large extent, erroneous.
This is my reply to one part of their criticism.
In the next place, they both assert, or imply, not
only that the sun s motion is now in a straight line,
but that it has been in a straight line from some
enormously remote period when it first entered the
stellar system on one side, and will so continue to
move till it reaches the utmost bounds of that system
on the other side. And this is stated by them both,
not as a possibility, but as a certainty. They use
such terms as must and will be, leaving no room
for any doubt whatever. But such a result implies
viii.] OUR RELATION TO THE MILKY WAY 181
the abrogation of the law of gravitation, since under
its action motion in a straight line in the midst of
thousands or millions of suns of various sizes is an
absolute impossibility ; while it also implies that the
sun must have been started on its course from some
other system outside the Milky Way, with such a
precise determination of direction as not to collide
with, or even make a near approach to, any one
of the suns or clusters of suns, or vast nebulous
masses, during its passage through the very midst
of the stellar universe.
This is my reply to the main point of their criticism,
and I think I am justified in saying that nothing in
my whole article is so demonstrably baseless as the
statements I have now examined.
Considering then the whole bearing of the evidence,
I refuse to accept the unsupported dicta of those who
would have us believe that our admitted position
not far from the centre of the stellar universe is a
mere temporary coincidence of no significance what
ever ; or that our sun and hosts of other similar
orbs near to us have come together by an accident,
and are being dispersed into surrounding space, never
to meet again. Until this is proved by indisputable
evidence, it seems to me far more probable that we
are moving in an orbit of some kind around the
centre of gravity of a vast cluster, as determined
by the investigations of Kapteyn, Newcomb, and
other astronomers ; and, consequently, that the nearly
central position we now occupy may be a permanent
one. For even if our sun s orbit should have a
diameter a thousand times that of Neptune, it would
182 MAN S PLACE IN THE UNIVERSE [CHAP.VIII.
be but a small fraction of the diameter of the Milky
Way ; while so vast is the scale of our universe, that
it might be even a hundred thousand times as great
and still leave us deeply immersed in the solar cluster,
and very much nearer to the dense central portion
than to its more diffused outer regions.
Here the subject may be left for the present.
After having studied the evidence afforded by the
essential conditions of life-development on the earth,
and the numerous indications that these conditions
do not exist on any of the other planets of the solar
system, it may be again touched upon in a general
review of the conclusions arrived at.
CHAPTER IX
THE UNIFORMITY OF MATTER AND ITS LAWS THROUGH
OUT THE STELLAR UNIVERSE
I HAVE shown in the second chapter of this work that
none of the previous writers on the question of the
habitability of the other planets have really dealt
with the subject in any adequate manner, since not
only do they appear to be quite unaware of the
delicate balance of conditions which alone renders
organic life possible on any planet, but they have
altogether omitted any reference to the fact that not
only must the conditions be such as to render life
possible now, but these conditions must have persisted
during the long geological epochs needed for the
slow development of life from its most rudimentary
forms. It will therefore be necessary to enter into
some details both as to the physical and chemical
essentials for a continuous development of organic
life, and also into the combination of mechanical and
physical conditions which are required on any planet
to render such life possible.
THE UNIFORMITY OF MATTER
One of the most important and far-reaching of the
discoveries due to the spectroscope is that of the
183
1 84 MAN S PLACE IN THE UNIVERSE [CHAP.
wonderful identity of the elements and material com
pounds in earth and sun, stars and nebulae, and also
of the identity of the physical and chemical laws that
determine the states and forms assumed by matter.
More than half the total number of the known
elements have been already detected in the sun,
including all those which compose the bulk of the
earth s solid material, with the one exception of
oxygen. This is a very large proportion when we
consider the very peculiar conditions which enable
us to detect them. For we can only recognise an
element in the sun when it exists at its surface in an
incandescent state, and also above its surface in the
form of a somewhat cooler gas. Many of the elements
may rarely or never be brought to the surface of so
vast a body, or if they do sometimes appear there, it
may not be in sufficient quantity or in sufficient purity
to produce any bands in the spectroscope, while the
cooler gas or vapour may either not be present, or
be so dispersed as not to produce sufficient absorption
to render its spectral lines visible. Again, it is
believed that many elements are dissociated by the
intense heat of the sun, and may not be recognisable
by us, or they may only exist at its surface in a
compound form unknown on the earth ; and in some
such way those lines of the solar spectrum which
remain still unrecognised may have been produced.
One of these unknown lines was that of Helium, a
gas found soon afterwards in the rare mineral
Cleveite, and since detected frequently in many stars.
Some of the stars have spectra very closely resem
bling that of the sun. The dark lines are almost as
numerous, and most of them correspond accurately
ix.] THE UNIFORMITY OF MATTER, ETC. 185
with solar lines, so that we cannot doubt their having
almost exactly the same chemical constitution, and
being also in the same condition as regards heat and
stage of development. Other stars, as we have
already stated, exhibit mainly lines of hydrogen,
sometimes combined with fine metallic lines. Of the
spectra of the nebulae comparatively little is known,
but many are decidedly gaseous, while others show
a continuous spectrum indicating a more complex
constitution.
But we also obtain considerable knowledge of the
matter of non-terrestrial bodies by the analysis of
the numerous meteorites which fall upon the earth.
Most of these belong to some of the many meteoric
streams which circulate round the sun, and which
may be supposed to give us samples of planetary
matter. But as it is now believed that many of them
are produced by the debris of comets, and the orbits
of some of these indicate that they have come from
stellar space and have been drawn into our system
by the attractive power of the larger planets, it is
almost certain that the meteoric stones not infrequently
bring us matter from the remoter regions of space,
and probably afford us samples of the solid con
stituents of nebulae or the cooler stars. It is, there
fore, a most suggestive fact that none of these
meteorites have been found to contain a single non-
o
terrestrial element, although no less than twenty-four
elements have been found in them, and it will be of
interest to give the list of these, as follows : Oxygen,
Hydrogen, Chlorine, Sulphur, Phosphorus, Carbon,
Silicon, Iron, Nickel, Cobalt, Magnesium, Chromium,
Manganese, Copper, Tin, Antimony, Aluminium,
i86 MAN S PLACE IN THE UNIVERSE [CHAP.
Calcium, Potassium, Sodium, Lithium, Titanium,
Arsenic, and Vanadium. Seven of the above, printed
in italics, have not yet been found in the sun, such
as Oxygen, Chlorine, Sulphur, and Phosphorus, which
form the constituents of many widespread minerals,
and they supply important gaps in the series of solar
and stellar elements. It may be noted that although
meteorites have supplied no new elements, they have
furnished examples of some new combinations of
these elements forming minerals distinct from any
found in our rocks.
The fact of the occurrence in meteorites not only
of minerals which are peculiar to them or are found
on the earth, but also of structures resembling our
breccias, veins, and even slicken-side surfaces, has
been held to be opposed to the meteoritic theory of
the origin of suns and planets, because meteorites
seem to be thus proved to be the fragments of suns
or worlds, not their primary constituents. But these
cases are exceptional, and Mr. Sorby, who made a
special study of meteorites, concluded that their
materials have usually been in a state of fusion or
even of vapour, as they now exist in the sun, and
that they became condensed into minute globular
particles, which afterwards collected into larger
masses, and may have been broken up by mutual
impact, and again and again become aggregated
together thus presenting features which are com
pletely in accordance with the meteoritic theory.
But, quite recently, Mr. T. C. Chamberlin has
applied the theory of tidal distortion to showing how
solid bodies in space, without ever coming into actual
contact, must sometimes be torn apart or disrupted
ix.] THE UNIFORMITY OF MATTER, ETC. 187
into numerous fragments by passing near to each
other. Especially when a small body passes near a
much larger one, there is a certain distance of approach
(termed the Roche limit) when the increasing differ
ential force of gravity will be sufficient to tear asunder
the smaller body and cause the fragments either to
circulate around it or to be dispersed in space. 1 In
this way, therefore, those larger meteorites which
exhibit planetary structure may have been produced.
Of course they would rarely have been true planets
attached to a sun, but more frequently some of the
smaller dark suns, which may possess many of the
physical characteristics of planets, and of which there
may be myriads in the stellar spaces.
On the whole, then, we have positive knowledge
of the existence, in the sun, stars, and planetary and
stellar spaces, of such a large proportion of the
elements of our globe, and so few indications of any
not forming part of it, that we are justified in the
statement, that the whole stellar universe is, broadly
speaking, constructed of the same series of elementary
substances as those we can study upon our earth,
and of which the whole realm of nature, animal,
vegetable, and mineral, is composed. The evidence
of this identity of substance is really far more com
plete than we could expect, considering the very
limited means of inquiry that we possess ; and we
shall, therefore, not be justified in assuming that any
important difference exists.
When we pass from the elements of matter to the
laws which govern it, we also find the clearest proofs
of identity. That the fundamental law of gravitation
The Aslrophysical Journal, vol. xiv., July 1901, p. 17.
MAN S PLACE IN THE UNIVERSE [CHAP.
extends to the whole physical universe is rendered
almost certain by the fact that double stars move
round their common centre of gravity in elliptical
orbits which correspond well with both observation
and calculation. That the laws of light are the same
both here and in inter-planetary space is indicated by
the fact that the actual measurement of the velocity
of light on the earth s surface gives a result so com
pletely identical with that prevailing to the limits of
the solar system, that the measurement of the sun s
distance, by means of the eclipses of Jupiter s satel
lites combined with the measured velocity of light,
agrees almost exactly with that obtained by means
of the transits of Venus, or through our nearest
approach to the planets Mars or Eros.
Again, the more recondite laws of light are found
to be identical in sun and stars with those observed
within the narrow bounds of laboratory experiments.
The minute change of position of spectral lines caused
by the source of light moving towards or away from
us enables us to determine this kind of motion in
the most distant stars, in the planets, or in the moon,
and these results can be tested by the motion of the
earth either in its orbit or in its rotation ; and these
latter tests agree with the theoretical determination
of what must occur, dependent on the wave-lengths
of the different dark lines of the solar spectrum deter
mined by measurements in the laboratory.
In like manner, minute changes in the widening
or narrowing of spectral lines, their splitting up, their
increase or decrease in number, and their arrangement
so as to form flutings, can all be interpreted by
experiments in the laboratory, showing that such
ix.] THE UNIFORMITY OF MATTER, ETC. 189
phenomena are due to alterations of temperature, of
pressure, or of the magnetic field, thus proving that
the very same physical and chemical laws act in the
same way here and in the remotest depths of space.
These various discoveries give us the certain con
viction that the whole material universe is essentially
one, both as regards the action of physical and
chemical laws, and also in its mechanical relations of
form and structure. It consists throughout of the
very same elements with which we are so familiar
on our earth ; the same ether whose vibrations bring
us light and heat, electricity and magnetism, and a
whole host of other mysterious and as yet imperfectly
known forces ; gravitation acts throughout its vast
extent ; and in whatever direction and by whatever
means we obtain a knowledge of the stellar universe,
we find the same mechanical, physical, and chemical
laws prevailing as upon our earth, so that we have
in some cases been actually enabled to reproduce in
our laboratories phenomena with which we had first
become acquainted in the sun or among the stars.
We may therefore feel it to be an almost certain
conclusion that the elements being the same, the
laws which act upon, and combine, and modify those
elements being the same organised living beings
wherever they may exist in this universe must be,
fundamentally, and in essential nature, the same also.
The outward forms of life, if they exist elsewhere,
may vary almost infinitely, as they do vary on the
earth ; but, throughout all this variety of form from
fungus or moss to rose-bush, palm or oak ; from
mollusc, worm, or butterfly to humming-bird, elephant,
or man the biologist recognises a fundamental unity
190 MAN S PLACE IN THE UNIVERSE [CHAP. ix.
of substance and of structure, dependent on the
absolute requirements of the growing, moving, devel
oping, living organism, built up of the same elements,
combined in the same proportions, and subject to the
same laws. We do not say that organic life could
not exist under altogether diverse conditions from
those which we know or can conceive, conditions
which may prevail in other universes constructed
quite differently from ours, where other substances
replace the matter and ether of our universe, and
where other laws prevail. But, within the universe
we know, there is not the slightest reason to suppose
organic life to be possible, except under the same
general conditions and laws which prevail here. We
will, therefore, now proceed to describe, very gener
ally, what are the conditions essential to the existence
and the continuous development of vegetable and
animal life.
CHAPTER X
THE ESSENTIAL CHARACTERS OF THE LIVING ORGANISM
BEFORE trying to comprehend the physical conditions
on any planet which are essential for the develop
ment and maintenance of a varied and complex
system of organic life comparable to that of our
earth, we must obtain some knowledge of what life
is, and of the fundamental nature and properties of
the living organism.
Physiologists and philosophers have made many
attempts to define * life, but in most cases in aiming
at absolute generality they have been vague and un-
instructive. Thus De Blainville defined it as * The
twofold internal movement of composition and de
composition, at once general and continuous ; while
Herbert Spencer s latest definition was Life is the
continuous adjustment of internal relations to external
relations. But neither of these is sufficiently pre
cise, explanatory, or distinctive, and they might
almost be applied to the changes occurring in a
sun or planet, or to the elevation and gradual forma
tion of a continent. One of the oldest definitions,
that of Aristotle, seems to come nearer the mark :
Life is the assemblage of the operations of nutrition,
growth, and destruction. But these definitions of
life are unsatisfactory, because they apply to an
191
192 MAN S PLACE IN THE UNIVERSE [CHAP.
abstract idea rather than to the actual living
organism. The marvel and mystery of life, as we
know it, resides in the body which manifests it, and
this living body the definitions ignore.
The essential points in the living body, as seen in
its higher developments, are, first, that it consists
throughout of highly complex but very unstable
forms of matter, every particle of which is in a
continual state of growth or decay ; that it absorbs
or appropriates dead matter from without ; takes this
matter into the interior of its body ; acts upon it
mechanically and chemically, rejecting what is useless
or hurtful ; and so transforming the remainder as to
renew every atom of its own structure internal and
external, at the same time throwing off, particle by
particle, all the worn-out or dead portions of its own
substance. Secondly, in order to be able to do all
this, its whole body is permeated throughout by
branching vessels or porous tissues, by which liquids
and gases can reach every part and carry on the various
processes of nutrition and excretion above referred
to. As Professor Burdon Sanderson well puts it :
The most distinctive peculiarity of living matter as
compared with non-living is, that it is ever changing
while ever the. same. And these changes are the
more remarkable because they are accompanied, and
even produced, by a very large amount of mechanical
work in animals by means of their normal activities
in search of food, in assimilating that food, in
continually renewing and building up their whole
organism, and in many other ways ; in plants by
building up their structure, which often involves
raising tons of material high into the air, as in forest
x.] CHARACTERS OF LIVING ORGANISM 193
trees. As a recent writer puts it : The most pro
minent, and perhaps the most fundamental, pheno
menon of life is what may be described as the
EnergyT ra ffi c or tne function of trading in energy.
The chief physical function of living matter seems to
consist in absorbing energy, storing it in a higher
potential state, and afterwards partially expending
it in the kinetic or active form.
Thirdly and perhaps most marvellous of all- -all
living organisms have the power of reproduction or
increase, in the lowest forms by a process of self-
division or fission, as it is termed, in the higher by
means of reproductive cells, which, though in their
earliest stage quite indistinguishable physically or
chemically in very different species, yet possess the
mysterious power of developing a perfect organism,
identical with its parents in all its parts, shapes,
and organs, and so wonderfully resembling them,
that the minutest distinctive details of size, form,
and colour, in hair or feathers, in teeth or claws, in
scales, spines, or crests, are reproduced with very
close accuracy, though often involving metamorphic
changes during growth of so strange a nature that,
.f they were not familiar to us but were narrated as
occurring only in some distant and almost inacces
sible region, would be treated as travellers tales,
ncredible and impossible as those of Sindbad the
bailor.
In order that the substance of living bodies should
>e able to undergo these constant changes while
(reserving the same form and structure in minute
.etails that they should be, as it were, in a constant
i Professor F. ]. Allen : What is Life f
N
194 MAN S PLACE IN THE UNIVERSE [CHAP.
state of flux while remaining sensibly unchanged, it
is necessary that the molecules of which they are
built up should be so combined as to be easily
separated and as easily united be, as it is termed,
labile or flowing ; and this is brought about by their
chemical composition, which, while consisting of few
elements, is yet highly complex in structure, a large
number of chemical atoms being combined in an
endless variety of ways.
The physical basis of life, as Huxley termed it,
is protoplasm, a substance which consists essentially
of only four common elements, the three gases,
nitrogen, hydrogen, and oxygen, with the non-
metallic solid, carbon ; hence all the special products
of plants and animals are termed carbon-compounds,
and their study constitutes one of the most extensive
and intricate branches of modern chemistry. Their
complexity is indicated by the fact that the molecule
of sugar contains 45, and that of stearine no less than
173, constituent atoms. The chemical compounds of
carbon are far more numerous than those of all the
other chemical elements combined ; and it is this
wonderful variety and the complexity of its possible
combinations which explain the fact, that all the
various animal tissues skin, horn, hair, nails, teeth,
muscle, nerve, etc., consist of the same four elements
(with occasionally minute quantities of sulphur, phos
phorus, lime, or silica, in some of them), as proved
by the marvellous fact that these tissues are all pro
duced as well by the grass-eating sheep or ox as by
the fish- or flesh-eating seal or tiger. And the marvel
is still further increased when we consider that the
innumerable diverse substances produced by plants
X.] CHARACTERS OF LIVING ORGANISM 195
and animals are all formed out of the same three
or four elements. Such are the endless variety
of organic acids, from prussic acid to those of the
various fruits ; the many kinds of sugars, gums,
and starches ; the number of different kinds of oil,
wax, etc. ; the variety of essential oils which are
mostly forms of turpentines, with such substances
as camphor, resins, caoutchouc, and gutta-percha ;
and the extensive series of vegetable alkaloids, such
as nicotine from tobacco, morphine from opium,
strychnine, curarine, and other poisons ; quinine,
belladonna, and similar medicinal alkaloids ; together
with the essential principles of our refreshing drinks,
tea, coffee, and cocoa, and others too numerous to
be named here all alike consisting solely of the four
common elements from which almost our whole
organism is built up. If this were not indisputably
proved, it would scarcely be credited.
Professor F. J. Allen considers that the most
important element in protoplasm, and that which
confers upon it its most essential properties in the
living organism its extreme mobility and transposi-
bility is nitrogen. This element, though inert in
itself, readily enters into compounds when energy is
supplied to it, the most striking illustration of which
is the formation of ammonia, a compound of nitrogen
and hydrogen, produced by electric discharges through
the atmosphere. Ammonia, and certain oxides of
nitrogen produced in the atmosphere in the same
way, are the chief sources of the nitrogen assimilated
by plants, and through them by animals ; for although
plants are continually in contact with the free nitrogen
of the atmosphere, they are unable to absorb it. By
196 MAN S PLACE IN THE UNIVERSE [CHAP.
their leaves they absorb oxygen and carbon-dioxide
to build up their woody tissues, while by their roots
they absorb water in which ammonia and oxides of
nitrogen are dissolved, and from these they produce
the protoplasm which builds up the whole substance
of the animal world. The energy required to pro
duce these nitrogen-compounds is given up by them
when undergoing further changes, and thus the pro
duction of ammonia by electricity in the atmosphere,
and its being carried by rain into the soil, constitute
the first steps in that long series of operations which
culminates in the production of the higher forms
of life.
But the remarkable transformations and combina
tions continually going on in every living body, which
are, in fact, the essential conditions of its life, are
themselves dependent on certain physical conditions
which must be always present. Professor Allen re
marks : The sensitiveness of nitrogen, its proneness
to change its state of combination and energy, appear
to depend on certain conditions of temperature,
pressure, etc., which exist at the surface of this earth.
Most vital phenomena occur between the tempera
ture of freezing water and 104 F. If the general
temperature of the earth s surface rose or fell 72 F.
(a small amount relatively), the whole course of life
would be changed, even perchance to extinction.
Another important, and even more essential fact, in
connection with life, is the existence in the atmo
sphere of a small but nearly constant proportion of
carbonic acid gas, this being the source from which
the whole of the carbon in the vegetable and animal
kingdoms is primarily derived. The leaves of plants
X.] CHARACTERS OF LIVING ORGANISM 197
absorb carbonic acid gas from the atmosphere, and
the peculiar substance, chlorophyll, from which they
derive their green colour, has the power, under the
influence of sunlight, to decompose it, using the
carbon to build up its own structure and giving out
the oxygen. In the laboratory the carbon can only
be separated from the oxygen by the application of
heat, under which certain metals burn by combining
with the oxygen, thus setting free the carbon.
Chlorophyll has a highly complex chemical structure
very imperfectly known, but it is said to be only
produced when there is iron in the soil.
The leaves of plants, so often looked upon as
mere ornamental appendages, are among the most
marvellous structures in living organisms, since in
decomposing carbonic acid at ordinary temperatures
they do what no other agency in nature can perform.
In doing this they utilise a special group of ether-
waves which alone appear to have this power. The
complexity of the processes going on in leaves is
well indicated in the following quotation :
We have seen how green leaves are supplied
with gases, water, and dissolved salts, and how they
can trap special ether-waves. The active energy of
these waves is used to transmute the simple inorganic
compounds into complex organic ones, which in the
process of respiration are reduced to simpler sub
stances again, and the potential energy transformed
into kinetic. These metabolic changes take place
in living cells full of intense activities. Currents
course through the protoplasm and cell-sap in every
direction, and between the cells which are also united
by strands of protoplasm. The gases used and
i 9 3 MAN S PLACE IN THE UNIVERSE [CHAP.
given off in respiration and assimilation are floated
in and out, and each protoplasm particle burned or
unburned is the centre of an area of disturbance.
Pure protoplasm is influenced equally by all rays :
that associated with chlorophyll is affected by certain
red and violet rays in particular. These, especially
the red ones, bring about the dissociation of the
elements of the carbonic acid, the assimilation of the
carbon, and the excretion of the oxygen.
It is this vigorous life-activity ever at work in the
leaves, the roots, and the sap-cells, that builds up
the plant, in all its wondrous beauty of bud and
foliage, flower and fruit ; and at the same time pro
duces, either as useful or waste-products, all that
wealth of odours and flavours, of colours and textures,
of fibres and varied woods, of roots and tubers, of
gums and oils and resins innumerable, that, taken
altogether, render the world of vegetable life
perhaps more varied, more beautiful, more enjoy
able, more indispensable to our higher nature than
even that of animals. But there is really no com
parison between them. We could have plants with
out animals ; we could not have animals without
plants. And all this marvel and mystery of veget
able life, a mystery which we rarely ponder over
because its effects are so familiar, is usually held to
be sufficiently explained by the statement that it is
all due to the special properties of protoplasm. Well
might Huxley say, that protoplasm is not only a
substance but a structure or mechanism, a mechanism
kept at work by solar heat and light, and capable
of producing a thousand times more varied and
1 Art. * Vegetable Physiology in Chambers s Encyclopedia.
x.] CHARACTERS OF LIVING ORGANISM 199
marvellous results than all the human mechanism
ever invented.
But besides absorbing carbonic acid from the at-
o
mosphere, separating and utilising the carbon and
giving out the oxygen, plants as well as animals
continually absorb oxygen from the atmosphere,
and this is so universally the case that oxygen is
said to be the food of protoplasm, without which it
cannot continue to live ; and it is the peculiar but
quite invisible structure of the protoplasm which
enables it to do this, and also in plants to absorb
an enormous amount of water as well.
But although protoplasm is so complex chemically
as to defy exact analysis, being an elaborate structure
of atoms built up into a molecule in which each atom
must occupy its true place (like every carved stone
in a Gothic cathedral), yet it is, as it were, only the
starting-point or material out of which the infinitely
varied structures of living bodies are formed. The
extreme mobility and changeability of the structure
of these molecules enables the protoplasm to be
continually modified both in constitution and form,
and, by the substitution or addition of other elements,
to serve special purposes. Thus, when sulphur in
small quantities is absorbed and built into the mole
cular structure, proteids are formed. These are most
abundant in animal structures, and give the nourish-
o
ing properties to meat, cheese, eggs, and other animal
foods ; but they are also found in the vegetable
kingdom, especially in nuts and seeds such as grain,
peas, etc. These are generally known as nitro
genous foods, and are very nutritious, but not so
easily digestible as meat. Proteids exist in very
200 MAN S PLACE IN THE UNIVERSE [CHAP.
varied forms and often contain phosphorus as well
as sulphur, but their main characteristic is the large
proportion of nitrogen they contain, while many other
animal and vegetable products, as most roots, tubers,
and grains, and even fats and oils, are mainly com
posed of starch and sugar. In its chemical and
physiological aspects protein is thus described by
Professor W. D. Haliburton : Proteids are pro
duced only in the living laboratory of animals and
plants ; proteid matter is the all-important material
present in protoplasm. This molecule is the most
complex that is known ; it always contains five and
often six or even seven elements. The task of
thoroughly understanding its composition is neces
sarily vast, and advance slow. But, little by little,
the puzzle is being solved, and this final conquest
of organic chemistry, when it does arrive, will furnish
physiologists with new light on many of the dark
places of physiological science.
What makes protoplasm and its modifications still
more marvellous is the power it possesses of absorb
ing and moulding a number of other elements in
various parts of living organisms for special uses.
Such are silica in the stems of the grass family, lime
and magnesia in the bones of animals, iron in blood,
and many others. Besides the four elements con
stituting protoplasm, most animals and plants contain
also in some parts of their structure sulphur, phos
phorus, chlorine, silicon, sodium, potassium, calcium,
magnesium, and iron ; while, less frequently, fluorine,
iodine, bromine, lithium, copper, manganese, and
aluminium are also found in special organs or
1 Address to the British Association, 1902, Section Physiology.
X.] CHARACTERS OF LIVING ORGANISM 201
structures ; and the molecules of all these are carried
by the protoplasmic fluids to the places where they
are required and built into the living structure, with
the same precision and for similar ends as brick and
stone, iron, slate, wood, and glass are each utilised
in their proper places in any large building. 1 The
organism, however, is not built, but grows. Every
organ, every fibre, cell, or tissue is formed from
diverse materials, which are first decomposed into
their elementary molecules, transformed by the proto
plasm or by special solvents formed from it, carried
to the places where they are needed by the vital
fluids, and there built up atom by atom or molecule
by molecule into the special structures of which they
are to form a part.
But even this marvel of growth and repair of
every individual organism is far surpassed by the
greater marvel of reproduction. Every living thing
of the higher orders arises from a single microscopic
cell, when fertilised, as it is termed, by the absorp
tion of another microscopic cell derived from a
different individual. These cells are often, even
under the highest powers of the microscope, hardly
distinguishable from other cells which occur in all
animals and plants and of which their structure is
built up ; yet these special cells begin to grow in a
totally different manner, and instead of forming one
particular part of the organism, develop inevitably
into a complete living thing with all the organs,
powers, and peculiarities of its parents, so as to be
This enumeration of the elements that enter into the structure of
plants and animals is taken from Professor F. J. Allen s paper already
referred to.
202 MAN S PLACE IN THE UNIVERSE [CHAP.
recognisably of the same species. If the simple
growth of the fully formed organism is a mystery,
what of this growth of thousands of complex
organisms each with all its special peculiarities, yet
all arising from minute germs or cells the diverse
natures of which are wholly indistinguishable by the
highest powers of the microscope? This, too, is said
to be the work of protoplasm under the influence of
heat and moisture, and modern physiologists hope
some day to learn * how it is done. It may be well here
to give the views of a modern writer on this point.
Referring to a difficulty which had been stated by
Clerk- Max well twenty-five years ago, that there was
not room in the reproductive cell for the millions of
molecules needed to serve as the units of growth for
all the different structures in the body of the higher
animals, Professor M Kendrick says: But to-day,
it is reasonable from existing data to suppose that
the germinal vesicle might contain a million of
millions of organic molecules. Complex arrange
ments of these molecules suited for the development
of all the parts of a highly complicated organism,
might satisfy all the demands of the theory of
heredity. Doubtless the germ was a material system
through and through. The conception of the
physicist was, that molecules were in various states
of movement ; and the thinkers were striving toward
a kinetic theory of molecules and of atoms of solid
matter, which might be as fruitful as the kinetic
theory of gases. There were motions atomic and
molecular. It was conceivable that the peculiarities
of vital action might be determined by the kind of
motion that took place in the molecules of what we
x.] CHARACTERS OF LIVING ORGANISM 203
call living matter. It might be different in kind from
some of the motions dealt with by physicists. Life is
continually being created from non-living material
such, at least, is the existing view of growth by the
assimilation of food. The creation of living matter
out of non-living may be the transmission to the dead
matter of molecular motions which are sut generis in
form. This is the modern physiological view of
how it may be done/ and it seems hardly more
intelligible than the very old theory of the origin of
stone axes, given by Adrianus Tollius in 1649, and
quoted by Mr. E. B. Tylor, who says: He gives
drawings of some ordinary stone axes and hammers,
and tells how naturalists say that they are generated
in the sky by a fulgureous exhalation conglobed in
a cloud by the circumfixed humour, and are, as it
were, baked hard by intense heat, and the weapon
becomes pointed by the damp mixed with it flying
from the dry part, and leaving the other end denser,
but the exhalations press it so hard that it breaks
through the cloud and makes thunder and lightning.
But he says if this is really the way in which they
are generated, it is odd they are not round, and that
they have holes through them. It is hardly to
be believed, he thinks. 1 And so, when the physi
ologists, determined to avoid the assumption of any
thing beyond matter and motion in the germ, impute
the whole development and growth of the elephant
or of man from minute cells internally alike, by means
of kinds of motion and the transmission of motions
which are sui generis in form/ many of us will be
1 Early History of Mankind, 2nd ed. p. 227.
204 MAN S PLACE IN THE UNIVERSE [CHAP.
inclined to say with the old author It is hardly to
be believed, I think.
This brief statement of the conclusions arrived at
by chemists and physiologists as to the composition
and structure of organised living things has been
thought advisable, because the non-scientific reader
has often no conception of the incomparable marvel
and mystery of the life-processes he has always seen
going on, silently and almost unnoticed, in the world
around him. And this is still more the case now that
two-thirds of our population are crowded into cities
where, removed from all the occupations, the charms,
and the interests of country life, they are driven to
seek occupation and excitement in the theatre, the
music-hall, or the tavern. How little do these know
what they lose by being thus shut out from all quiet
intercourse with nature ; its soothing sights and
sounds ; its exquisite beauties of form and colour ;
its endless mysteries of birth, and life, and death.
Most people give scientific men credit for much
greater knowledge than they possess in these matters;
and many educated readers will, I feel sure, be sur
prised to find that even such apparently simple
phenomena as the rise of the sap in trees are not yet
completely explained. As to the deeper problems of
life, and growth, and reproduction, though our physi
ologists have learned an infinite amount of curious
or instructive facts, they can give us no intelligible
explanation of them.
The endless complexities and confusing amount of
detail in all treatises on the physiology of animals
and plants are such, that the average reader is over
whelmed with the mass of knowledge presented to
X.] CHARACTERS OF LIVING ORGANISM 205
him, and concludes that after such elaborate re
searches everything must be known, and that the
almost universal protests against the need of any
causes but the mechanical, physical, and chemical
laws and forces are well founded. I have, there
fore, thought it advisable to present a kind of bird s-
eye view of the subject, and to show, in the words of
the greatest living authorities on these matters, both
how complex are the phenomena and how far our
teachers are from being able to give us any adequate
explanation of them.
I venture to hope that the very brief sketch of the
subject I have been able to give will enable my
readers to form some faint general conception of the
infinite complexity of life and the various problems
connected with it ; and that they will thus be the
better enabled to appreciate the extreme delicacy of
those adjustments, those forces, and those complex
conditions of the environment, that alone render
life, and above all the grand age-long panorama
of the development of life, in any way possible. It
is to these conditions, as they prevail in the world
around us, that we will now direct our attention.
CHAPTER XI
THE PHYSICAL CONDITIONS ESSENTIAL FOR
ORGANIC LIFE
THE physical conditions on the surface of our earth
which appear to be necessary for the development
and maintenance of living organisms may be dealt
with under the following headings :
1. Regularity of heat-supply, resulting in a limited
range of temperature.
2. A sufficient amount of solar light and heat.
3. Water in great abundance, and universally
distributed.
4. An atmosphere of sufficient density, and con
sisting of the gases which are essential for vegetable
and animal life. These are Oxygen, Carbonic-acid
gas, Aqueous vapour, Nitrogen, and Ammonia.
These must all be present in suitable proportions.
5. Alternations of day and night.
SMALL RANGE OF TEMPERATURE REQUIRED FOR
GROWTH AND DEVELOPMENT
Vital phenomena for the most part occur between
the temperatures of freezing water and 104 Fahr.,
and this is supposed to be due mainly to the
properties of nitrogen and its compounds, which
206
CHAP. XL] ESSENTIAL LIFE-CONDITIONS 207
between these temperatures only can maintain those
peculiarities which are essential to life- -extreme
sensitiveness and lability ; facility of change as
regards chemical combination and energy ; and other
properties which alone render nutrition, growth, and
continual repair possible. A very small increase or
decrease of temperature beyond these limits, if con
tinued for any considerable time, would certainly
destroy most existing forms of life, and would not
improbably render any further development of life
impossible except in some of its lowest forms.
As one example of the direct effects of increased
temperature, we may adduce the coagulation of
albumen. This substance is one of the proteids, and
plays an important part in the vital phenomena of
both plants and animals, and its fluidity and power
of easy combination and change of form are lost by
any degree of coagulation which takes place at about
1 60 Fahr.
The extreme importance to all the higher organ
isms of a moderate temperature is strikingly shown
by the complex and successful arrangements for
maintaining a uniform degree of heat in the interior
of the body. The normal blood-heat in a man is
98 Fahr., and this is constantly maintained within
one or two degrees though the external temperature
may be more than fifty degrees below the freezing-
point. High temperatures upon the earth s surface
do not range so far from the mean as do the low. In
the greater part of the tropics the air-temperature
seldom reaches 96 Fahr., though in arid districts and
deserts, which occur chiefly along the margins of the
northern and southern tropics, it not unfrequently
208 MAN S PLACE IN THE UNIVERSE [CHAP.
surpasses 110 Fahr., and even occasionally rises to
115 or 120 in Australia and Central India. Yet
with suitable food and moderate care the blood-
temperature of a healthy man would not rise or fall
more than one or at most two degrees. The great
importance of this uniformity of temperature in all
the vital organs is distinctly shown by the fact that
when, during fevers, the temperature of the patient
rises six degrees above the normal amount, his con
dition is critical, while an increase of seven or eight
degrees is an almost certain indication of a fatal
result. Even in the vegetable kingdom seeds will
not germinate under a temperature of four or five
degrees above the freezing-point.
Now this extreme sensibility to variations of in
ternal temperature is quite intelligible when we con
sider the complexity and instability of protoplasm,
and of all the proteids in the living organism, and
how important it is that the processes of nutrition
and growth, involving constant motion of fluids and
incessant molecular decompositions and recombina
tions, should be effected with the greatest regularity.
And though a few of the higher animals, including
man, are so perfectly organised that they can adapt
or protect themselves so as to be able to live under
very extreme conditions as regards temperature,
yet this is not the case with the great majority,
or with the lower types, as evidenced by the
almost complete absence of reptiles from the arctic
regions.
It must also be remembered that extreme cold and
extreme heat are nowhere perpetual. There is always
some diversity of seasons, and there is no land
XL] ESSENTIAL LIFE-CONDITIONS 209
animal which passes its whole life where the tem
perature never rises above the freezing point.
THE NECESSITY OF SOLAR LIGHT
Whether the higher animals and man could have
been developed upon the earth without solar light,
even if all the other essential conditions were pre
sent, is doubtful. That, however, is not the point
I am at present considering, but one that is much
more fundamental. Without plant life land animals
at all events could never have come into existence,
because they have not the pow r er of making proto
plasm out of inorganic matter. The plant alone
can take the carbon out of the small proportion of
carbonic acid in the atmosphere, and with it, and the
other necessary elements, as already described, build
up those wonderful carbon compounds which are
the very foundation of animal life. But it does this
solely by the agency of solar light, and even uses a
special portion of that light. Not only, therefore, is
a sun needed to give light and heat, but it is quite
possible that any sun would not answer the purpose.
A sun is required whose light possesses those special
rays which are effective for this operation, and as we
know that the stars differ greatly in their spectra, and
therefore in the nature of their light, all might not be
able to effect this great transformation, which is one
of the very first steps in rendering animal life possible
on our earth, and therefore probably on all earths.
o
210 MAN S PLACE IN THE UNIVERSE [CHAP.
WATER A FIRST ESSENTIAL OF ORGANIC LIFE
It is hardly necessary to point out the absolute
necessity of water, since it actually constitutes a
very large proportion of the material of every living
organism, and about three-fourths of our own bodies.
Water, therefore, must be present everywhere, in one
form or another, on any globe where life is possible.
Neither animal nor plant can exist without it. It
must also be present in such quantity and so distri
buted as to be constantly available on every part of
a globe where life is to be maintained ; and it is
equally necessary that it should have persisted in
equal profusion throughout those enormous geo
logical epochs during which life has been developing.
We shall see later on how very special are the condi
tions that have secured this continuous distribution
of water on our earth, and we shall also learn that
this large amount of water, its wide distribution, and
its arrangement with regard to the land-surface, is an
essential factor in producing that limited range of
temperature which, as we have seen, is a primary
condition for the development and maintenance of
life.
THE ATMOSPHERE MUST BE OF SUFFICIENT DENSITY
AND COMPOSED OF SUITABLE GASES
The atmosphere of any planet on which life can
be developed must have several qualities which are
unconnected with each other, and the coincidence of
which may be a rare phenomenon in the universe.
The first of these is a sufficient density, which is
XL] ESSENTIAL LIFE-CONDITIONS 211
required for two purposes as a storer of heat, and
in order to supply the oxygen, carbonic acid, and
aqueous vapour in sufficient quantities for the
requirements of vegetable and animal life.
As a reservoir of heat and a regulator of tempera
ture, a rather dense atmosphere is a first necessity, in
co-operation with the large quantity and wide distri
bution of water referred to in the last section. The
very different character of our south-west from our
north-east winds is a good illustration of its power
of distributing heat and moisture. This it does
owing to the peculiar property it possesses of
allowing the sun s rays to pass freely through it to
the earth which it warms, but acting like a blanket
in preventing the rapid escape of the non-luminous
heat so produced. But the heat stored up during
the day is given out at night, and thus secures a
uniformity of temperature which would not otherwise
exist. This effect is strikingly seen at high altitudes,
where the temperature becomes lower and lower, till
at a not very great elevation, even in the tropics,
snow lies on the ground all the year round. This is
almost wholly due to the rarity of the air, which, on
that account, has not so much capacity for heat. It
also allows the heat it acquires to radiate more freely
than denser air, so that the nights are much colder.
At about 18,000 feet high our atmosphere is exactly
half its density at the sea-level. This is consider
ably higher than the usual snow-line, even under the
equator, whence it follows that if our atmosphere
was only half its present density it would render
the earth unsuitable for the higher forms of animal
life. It is not easy to say exactly what would be the
212 MAN S PLACE IN THE UNIVERSE [CHAP.
result as regards climate ; but it seems likely that,
except perhaps in limited areas in the tropics, where
conditions were very favourable, the whole land-
surface would become buried in snow and ice. This
appears inevitable, because evaporation from the
oceans by direct sun-heat would be more rapid than
now ; but as the vapour rose in the rare atmosphere
it would rapidly become frozen, and snow would fall
almost perpetually, although it might not lie per
manently on the ground in the equatorial lowlands.
It appears certain, therefore, that with half our
present bulk of atmosphere life would be hardly
possible on the earth on account of lowered tem
perature alone. And as there would certainly be an
added difficulty in the needful supply of oxygen to
animals and carbonic acid to plants, it seems highly
probable that a reduction of density of even one-
fourth might be sufficient to render a large portion
of the globe a snow- and ice-clad waste, and the
remainder liable to such extremes of climate that
only low forms of life could have arisen and been
permanently maintained.
THE GASES OF THE ATMOSPHERE
Coming now to consider the constituent gases of
the atmosphere, there is reason to believe that they
form a mixture as nicely balanced in regard to
animal and vegetable life as are the density and the
temperature. At a first view of the subject we
might conclude that oxygen is the one great essen
tial for animal life, and that all else is of little im
portance. But further consideration shows us that
XL] ESSENTIAL LIFE-CONDITIONS 213
nitrogen, although merely a diluent of the oxygen
as regards the respiration of animals, is of the first
importance to plants, which obtain it from the
ammonia formed in the atmosphere and carried
down into the soil by the rain. Although there is
only one part of ammonia to a million of air, yet
upon this minute proportion the very existence of
the animal world depends, because neither animals
nor plants can assimilate the free nitrogen of the air
into their tissues.
Another fundamentally important gas in the
atmosphere is carbonic acid, which forms about
four parts in ten thousand parts of air, and, as
already stated, is the source from which plants build
up the great bulk of their tissues, as well as those
protoplasms and proteids so absolutely necessary as
food for animals. An important fact to notice here
is, that carbonic acid, so essential to plants, and to
animals through plants, is yet a poison to animals.
When present in much more than the normal
quantity, as it often is in cities and in badly
ventilated buildings, it becomes highly prejudicial
to health ; but this is believed to be partly due to
the various corporeal emanations and other impuri
ties associated with it. Pure carbonic acid gas to
the amount of even one per cent, in otherwise pure
air can, it is said, be breathed for a time without bad
effects, but anything more than this proportion will
soon produce suffocation. It is probable, therefore,
that a very much smaller proportion than one per
cent., if constantly present, would be dangerous to
life ; though no doubt, if this had always been the
proportion, life might have been developed in
214 MAN S PLACE IN THE UNIVERSE [CHAP.
adaptation to it. Considering, however, that this
poisonous gas is largely given out by the higher
animals as a product of respiration, it would
evidently be dangerous to the permanence of life
if the quantity forming a constant constituent of the
atmosphere were much greater than it is.
AQUEOUS VAPOUR IN THE ATMOSPHERE
This water-gas, although it occurs in the
atmosphere in largely varying quantities, is yet, in
two distinct ways, essential to organic life. It
prevents the too rapid loss of moisture from the
leaves of plants when exposed to the sun, and it is
also absorbed by the upper surface of the leaf and
by the young shoots, which thus obtain both water
and minute quantities of ammonia when the supply
by the roots is insufficient. But it is of even more
vital importance in supplying the hydrogen which,
when united with the nitrogen of the atmosphere by
electrical discharges, produces the ammonia, which is
the main source of all the proteids of the plant, which
proteids are the very foundation of animal life.
From this brief statement of the purposes served
by the various gases forming our atmosphere, we see
that they are to some extent antagonistic, and that
any considerable increase of one or the other would
lead to results that might be injurious either directly
or in their ultimate results. And as the elements
which constitute the bulk of all living matter possess
properties which render them alone suitable for the
9
purpose, we may conclude that the proportions in
which they exist in our atmosphere cannot be very
XL] ESSENTIAL LIFE-CONDITIONS 215
widely departed from wherever organic forms are
developed.
THE ALTERNATION OF DAY AND NIGHT
Although it is difficult to decide positively whether
alternations of light and darkness at short intervals
are absolutely essential for the development of the
various higher forms of life, or whether a world in
which light was constant might do as well, yet on
the whole it seems probable that day and night
are really important factors. All nature is full
of rhythmic movements of infinitely varied kinds,
degrees, and durations. All the motions and func
tions of living things are periodic ; growth and
repair, assimilation and waste, go on alternately. All
our organs are subject to fatigue and require rest.
All kinds of stimulus must be of short duration or
injurious results follow. Hence the advantage of
darkness, when the stimuli of light and heat are
partially removed, and we welcome * tired nature s
sweet restorer, balmy sleep giving rest to all the
senses and faculties of body and mind, and endowing
us with renewed vigour for another period of activity
and enjoyment of life.
Plants as well as animals are invigorated by this
nightly repose ; and all alike benefit by these longer
periods of greater and less amounts of work caused
by summer and winter, dry and wet seasons. It is a
suggestive fact, that where the influence of heat and
light is greatest- -within the tropics the days and
nights are of equal length, giving equal periods of
activity and rest. But in cold and Arctic regions
216 MAN S PLACE IN THE UNIVERSE [CHAP.
where, during the short summer, light is nearly
perpetual, and all the functions of life, in vegetation
especially, go on with extreme rapidity, this is
followed by the long rest of winter, with its short
days and greatly lengthened periods of darkness.
Of course, all this is rather suggestion than proof.
It is possible that in a world of perpetual day or in
one of perpetual night, life might have been developed.
But on the other hand, considering the great variety
of physical conditions which are seen to be necessary
for the development and preservation of life in its
endless varieties, any prejudicial influences, however
slight, might turn the scale, and prevent that har
monious and continuous evolution which we know
must have occurred.
So far I have only considered the question of day
and night as regards the presence or absence of
light. But it is probably far more important in its
heat aspect ; and here its period becomes of great,
perhaps vital, importance. With its present duration
of twelve hours day and twelve night on the average,
there is not time, even between the tropics, for the
earth to become so excessively heated as to be inimical
to life ; while a considerable portion of the heat, stored
up in the soil, the water, and the atmosphere, is given
out at night, and thus prevents a too sudden and
injurious contrast of heat and cold. If the day and
night were each very much longer say 50 or TOO
hours- -it is quite certain that, during a day of that
duration, the heat would become so great as to be
inimical, perhaps prohibitive, to most forms of life;
while the absence of all sun-heat for an equally long
period would result in a temperature far below the
XL] ESSENTIAL LIFE-CONDITIONS 217
freezing point of water. It is doubtful whether any
hieh forms of animal life could have arisen under
o
such great and continual contrasts of temperature.
We will now proceed to point out the special
features which, in our earth, have combined to bring
about and to maintain the various and complex
conditions we have seen to be essential for life as it
exists around us.
CHAPTER XII
THE EARTH IN ITS RELATION TO THE DEVELOPMENT
AND MAINTENANCE OF LIFE
THE first circumstance to be considered in relation
to the habitability of a planet is its distance from the
sun. We know that the heating power of the sun
upon our earth is ample for the development of life in
an almost infinite variety of forms ; and we have a
large amount of evidence to show that, were it not
for the equalising power of air and water, distributed
as they are with us, the heat received from the sun
would be sometimes too great and sometimes too
little. In some parts of Africa, Australia, and India,
the sandy soil becomes so hot that an egg can be
cooked by placing it just below the surface. On the
other hand, at an elevation of about 12,000 feet in
lat. 40 it freezes every night, and throughout the
day in all places sheltered from the sun. Now, both
these temperatures are adverse to life, and if either
of them persisted over a considerable portion of the
earth, the development of life would have been impos
sible. But the heat derived from the sun is inversely
as the square of the distance, so that at half the
distance we should have four times as much heat,
and at twice the distance only one-fourth of the heat.
Even at two-thirds of the distance we should receive
218
en. xii.] THE EARTH IN RELATION TO LIFE 219
more than twice as much heat ; and, considering the
facts as to the extreme sensitiveness of protoplasm
and the coagulation of albumen, it seems certain that
we are situated in what has been termed the temperate
zone of the solar system, and that we could not be
removed far from our present position without
endangering a considerable portion of the life now
existing upon the earth, and in all probability render
ing the actual development of life, through all its
phases and gradations, impossible.
THE OBLIQUITY OF THE ECLIPTIC
The effect of the obliquity of the earth s equator
to its path round the sun, upon which depend our
varying seasons and the inequality of day arid night
throughout all the temperate zones, is very generally
known. But it is not usually considered that this
obliquity is of any great importance as regards the
suitability of the earth for the development and main
tenance of life ; and it seems to have been passed
over as an accident hardly worth notice, because
almost any other obliquity or none at all would have
been equally advantageous. But if we consider what
the direction of the earth s axis might possibly have
been, we shall find that it is really a matter of great
importance from our present point of view.
Let us suppose, first, that the earth s axis was,
like that of Uranus, almost exactly in the plane of its
orbit or directed towards the sun. There can be
little doubt that such a position would have rendered
our world unfitted for the development of life. For
the result would be the most tremendous contrasts
220 MAN S PLACE IN THE UNIVERSE [CHAP.
of the seasons ; at mid-winter, on one half the globe,
arctic night and more than arctic cold would prevail ;
while on the other half there would be a midsummer
of continuous day with a vertical sun and such an
amount of heat as nowhere exists with us. At the
two equinoxes the whole globe would enjoy equal
day and night, all our present tropics and part of the
sub-tropical zone having the sun at noon so near to
the zenith as to have the essential of a tropical climate.
But the change to about a month of constant sunshine
or a month of continuous night would be so rapid,
that it seems almost impossible that either vegetable
or animal life would ever have developed under such
terrible conditions.
The other extreme direction of the earth s axis,
exactly at right angles to the plane of the orbit, would
be much more favourable, but would still have its
disadvantages. The whole surface from equator to
poles would enjoy equal day and night, and every
part would receive the same amount of sun-heat all
the year round, so that there would be no change of
seasons ; but the heat received would vary with the
latitude. In our latitude the sun s altitude at noon
all the year would be less than 40, the same as now
occurs at the equinoxes, and we might therefore have
a perpetual spring as regards temperature. But the
constancy of the heat in the equatorial and tropical
regions and of cold towards the poles would lead to a
more constant and more rapid circulation of air, and
we should probably experience such continuous north
westerly winds as to render our climate always cold
and probably very damp. Near the poles the sun
would always be on, or close to, the horizon, and
XII.] THE EARTH IN RELATION TO LIFE 221
would give so little heat that the sea might be per
petually frozen and the land deeply snow-buried ; and
these conditions would probably extend into the
temperate zone, and possibly so far south as to render
life impossible in our latitudes, since whatever results
arose would be due to permanent causes, and we
know how powerful are snow and ice to extend their
sway over adjacent areas if not counteracted by
summer heat or warm moist winds. On the whole,
therefore, it seems probable that this position of the
earth s axis would result in a much smaller portion
of its surface being capable of supporting a luxuriant
and varied vegetable and animal life than is now
the case ; while the extreme uniformity of conditions
everywhere present might be so antagonistic to the
great law of rhythm that seems to pervade the
universe, and be in other ways so unfavourable, that
life-development would probably have taken quite a
different course from that which it has taken.
It appears almost certain, therefore, that some in
termediate position of the axis would be the most
favourable ; and that which actually exists seems to
combine the advantage of change of seasons with
good climatical conditions over the largest possible
area. We know that during the greater part of the
epoch of life-development this area was much greater
than at present, since a luxuriant vegetation of
deciduous and evergreen trees and shrubs extended
up to and within the Arctic Circle, leading to the
formation of coal-beds both in palaeozoic and tertiary
times ; the extremely favourable conditions for organic
life which then prevailed over so large a portion of
the globe s surface, and which persisted down to a
222 MAN S PLACE IN THE UNIVERSE [CHAP.
comparatively recent epoch, lead to the conclusion
that no more favourable degree of obliquity was
possible than that which we actually possess. A
short account of the evidence on this interesting
subject will now be given.
PERSISTENCE OF MILD CLIMATES THROUGH
GEOLOGICAL TIME
The whole of the geological evidence goes to show
that in remote ages the climate of the earth was
generally more uniform, though perhaps not warmer,
than it is now, and this can be best explained by a
slightly different distribution of sea and land, which
allowed the warm waters of the tropical oceans to
penetrate into various parts of the continents (which
were then more broken up than they are now), and
also to extend more freely into the Arctic regions.
So soon as we go back into the tertiary period, we
find indications of a warmer climate in the north
temperate zone ; and when we reach the middle of
that period, we find abundant indications, both in
plant and animal remains, of mild climates near to
the Arctic Circle, or actually within it.
On the west coast of Greenland, in 70 N. lat.,
there are found abundance of fossil plants very
beautifully preserved, among which are many different
species of oaks, beeches, poplars, plane-trees, vines,
walnuts, plums, chestnuts, sequoias, and numerous
shrubs- -i 37 species in all, indicating a vegetation
such as now grows in the north temperate parts
of America and Eastern Asia. And even further
XIL] THE EARTH IN RELATION TO LIFE 223
north, in Spitzbergen, in N. lat. 78 and 79, a some
what similar flora is found, not quite so varied, but
with oaks, poplars, birches, planes, limes, hazels,
pines, and many aquatic plants such as may now be
found in West Norway and in Alaska, nearly twenty
degrees further south.
Still more remote, in the Cretaceous period, fossil
plants have been found in Greenland, consisting of
ferns, cycads, conifers, and such trees and shrubs as
poplars, sassafras, andromedas, magnolias, myrtles,
and many others, similar in character and often
identical in species with fossils of the same period
found in Central Europe and the United States,
indicating a widespread uniformity of climate, such
as would be brought about by the great ocean-
currents carrying the warm waters of the tropics
into the Arctic seas.
Still further back, in the Jurassic period, we have
proofs of a mild climate in East Siberia and at Ando
in Norway just within the Arctic Circle, in numerous
plant remains, and also remains of great reptiles
allied to those found in the same strata in all parts
of the world. Similar phenomena occur in the still
earlier Triassic period ; but we will pass on to the
much more remote Carboniferous period, during
which most of the great coal-beds of the world were
formed from a luxuriant vegetation, consisting mostly
of ferns, giant horse-tails, and primitive conifers.
The luxuriance of these plants, which are often found
beautifully preserved and in immense quantities, is
supposed to indicate an atmosphere in which car
bonic acid gas was much more abundant than now ;
and this is rendered probable by the small number
224 MAN S PLACE IN THE UNIVERSE [CHAP.
and low type of terrestrial animals, consisting of a
few insects and amphibia.
But the interesting point is, that true coal-beds,
with similar fossils to those of our own coal-measures,
are found at Spitzbergen and at Bear Island in East
Siberia, both far within the Arctic Circle, again in
dicating a great uniformity of climate, and probably
a denser and more vapour-laden atmosphere, which
would act as a blanket over the earth and preserve
the heat brought to the Arctic seas by the ocean
currents from the warmer regions.
The still earlier silurian rocks are also found
abundantly in the Arctic regions, but their fossils
are entirely of marine animals. Yet they show
the same phenomena as regards climate, since the
corals and cephalopodous mollusca found in the
Arctic beds closely resemble those of all other parts
of the earth. 1
Many other facts indicate that throughout the
enormous periods required for the development of
the varied forms of life upon the earth, the great
phenomena of nature were but little different from
those that prevail in our own times. The slow and
gentle processes by which the various vegetable
and animal remains were preserved are shown by
the perfect state in which many of the fossils exist.
Often trunks of trees, cycads, and tree-ferns are
found standing erect, with their roots still imbedded
in the soil they grew in. Large leaves of poplars,
maples, oaks, and other trees are often preserved in
1 For a fuller account of this Arctic fauna and flora see the works of
Sir C. Lyell, Sir A. Geikie, and other geologists. A full summary of it
is also given in the author s Island Life.
XII.] THE EARTH IN RELATION TO LIFE 225
as perfect a state as if gathered by a botanist and
dried between paper for his herbarium, and the same
is especially the case with the beautiful ferns of the
Permian and Carboniferous periods. Throughout
these and most other formations well-preserved ripple-
marks are found in the solidified mud or sand of old
seashores, differing in no respect from similar marks
to be found on almost every coast to-day. Equally
interesting are the marks of rain-drops preserved in
the rocks of almost all ages. Sir Charles Lyell
has given illustrations of recent impressions of rain
drops on the extensive mud-flats of Nova Scotia,
and also an illustration of rain-drops on a slab of
shale from the carboniferous formation of the same
country ; and the two are as much alike as the
prints of two different showers a few days apart.
The general size and form of the drops are almost
identical, and imply a great similarity in the general
atmospheric conditions.
We must not forget that this presence of rain
throughout geological time implies, as we have seen
in our last chapter, a constant and universal distribu
tion of atmospheric dust. The two chief sources of
this dust the total quantity of which in the atmo
sphere must be enormous are volcanoes and deserts,
and we are therefore sure that these two great natural
phenomena have always been present. Of volcanoes
we have ample independent evidence in the presence
of lavas and volcanic ashes, as well as actual stumps
or cores of old volcanoes, through all geological for
mations ; and we can have little doubt that deserts
also were present, though perhaps not always so
extensive as they are now. It is a very suggestive
p
226 MAN S PLACE IN THE UNIVERSE [CHAP.
fact that these two phenomena, usually held to be
blots on the fair face of nature, and even to be
opposed to belief in a beneficent Creator, should
now be proved to be really essential to the earth s
habitability.
Notwithstanding this prevalence of warm and
uniform conditions, there is also evidence of con
siderable changes of climate ; and at two periods-
in the Eocene and in the remote Permian there
are even indications of ice-action, so that some
geologists believe that there were then actual glacial
epochs. But it seems more probable that they imply
only local glaciation, owing to there having been high
land and other suitable conditions for the production
of glaciers in certain areas.
The whole bearing of the geological evidence
indicates the wonderful continuity of conditions
favourable for life, and for the most part of climatal
conditions more favourable than those now prevail
ing, since a larger extent of land towards the North
Pole was available for an abundant vegetation, and
in all probability for an equally abundant animal
life. We know, too, that there was never any
total break in life-development ; no epoch of such
lowering or raising of temperature as to destroy all
life ; no such general subsidence as to submerge the
whole land-surface. Although the geological record
is in parts very imperfect, yet it is, on the whole,
wonderfully complete ; and it presents to our view a
continuous progress, from simple to complex, from
lower to higher. Type after type becomes highly
specialised in adaptation to local or climatal condi
tions, and then dies out, giving room for some other
XII.] THE EARTH IN RELATION TO LIFE 227
type to arise and be specialised in harmony with
the changed conditions. The general character of
<_> o
the inorganic change appears to have been from
more insular to more continental conditions, accom
panied by a change from more uniform to less
uniform climates, from an almost sub-tropical warmth
and moisture, extending up to the Arctic Circle, to
that diversity of tropical, temperate, and cold areas,
capable of supporting the greatest possible variety
in the forms of life, and which seems especially
adapted to stimulate mankind to civilisation and
social development by means of the necessary
struggle against, and utilisation of, the various forces
of nature.
WATER, ITS AMOUNT AND DISTRIBUTION
ON THE EARTH
Although it is generally known that the oceans
occupy more than two-thirds of the whole surface of
the globe, the enormous bulk of the water in proportion
to the land that rises above its surface is hardly ever
appreciated. But as this is a matter of the greatest
importance, both as regards the geological history of
the globe and the special subject we are here dis
cussing, it will be necessary to enter into some
details in regard to it.
According to the best recent estimates, the land
area of the globe is 0*28 of the whole surface, and
the water area 072. But the mean height of the
land above the sea-level is found to be 2250 feet,
while the mean depth of the seas and oceans is
13,860 feet; so that though the water area is two
and a half times that of the land, the mean depth of
228 MAN S PLACE IN THE UNIVERSE [CHAP.
the water is more than six times the mean height of
the land. This is, of course, due to the fact that
lowlands occupy most of the land-area, the plateaus
and high mountains a comparatively small portion of
it ; while, though the greatest depths of the oceans
about equal the greatest heights of the mountains,
yet over enormous areas the oceans are deep enough
to submerge all the mountains of Europe and tem
perate North America, except the extreme summits
of one or two of them. Hence it follows that the
bulk of the oceans, even omitting all the shallow seas,
is more than thirteen times that of the land above
sea-level ; and if all the land-surface and ocean-floors
were reduced to one level, that is, if the solid mass
of the globe were a true oblate spheroid, the whole
would be covered with water about two miles deep.
The diagram here given will render this more intel
ligible and will serve to illustrate what follows.
Diagram of proportionate mean height of Land and depth of Oceans,
Land
Area. -28 of area
ofGlobe,
Ocean
Area. -72 of area of Globe.
In this diagram the lengths of the sections repre
senting land and ocean are proportionate to their
areas, while the thickness of each is proportionate
to their mean height and mean depth respectively.
Hence the two sections are in correct proportion to
their cubic contents.
A mere inspection of this diagram is sufficient to
XIL] THE EARTH IN RELATION TO LIFE 229
disprove the old idea, still held by a few geologists
and by many biologists, that oceans and continents
have repeatedly changed places during geological
times, or that the great oceans have again and again
been bridged over to facilitate the distribution of
beetles or birds, reptiles or mammals. We must
remember that although the diagram shows the con
tinents and oceans as a whole, yet it also shows, with
quite sufficient accuracy, the proportions of each of
the great continents to the oceans which are adjacent
to them. It must also be borne in mind that there
can be no elevation on a large scale without a corre
sponding subsidence elsewhere ; because if there were
not, a vast unsupported hollow would be left beneath
the rising land or in some part adjacent to it.
Now, looking at the diagram and at a chart or
globe, try to imagine the ocean-bottom rising grad
ually, to form a continent joining Africa with South
America or with Australia (both of which are de
manded by many biologists) : it is clear that, while
such an elevation was going on, either some con
tinental land or some other part of the ocean-bed
must sink to a corresponding amount. We shall
then see, that if such changes of elevation on a con
tinental scale have taken place again and again at
different periods, it would have been almost impos
sible, on every occasion, to avoid a whole continent
being submerged (or even all the continents) in order
to equalise subsidence with elevation while new
continents were being raised up from the abyssal
depths of the ocean. We conclude, therefore, that
with the exception of a comparatively narrow belt
around the continents, which may be roughly indi-
230 MAN S PLACE IN THE UNIVERSE [CHAP.
cated by the thousand fathom line of soundings, the
great ocean depths are permanent features of the
earth s surface. It is this stability of the general
distribution of land and water that has secured the
continuity of life upon the earth. Had the great
oceanic basins, on the other hand, been unstable,
changing places with the land at various periods of
geological time, they would, almost certainly, again
and again have swallowed up the land in their vast
abysses, and have thus destroyed all the organic life
of the world.
There are many confirmatory proofs of this view
(which is now widely accepted by geologists and
physicists), and a few of them may be briefly stated.
1. None of the continents present us with marine
deposits of any one geological age and occupying a
large part of the surface of each, as must have been
the case had they ever been sunk deep beneath the
ocean and again elevated ; neither do any of them
contain extensive formations corresponding to the
deep oceanic clays and oozes, which again they
must have done had they been at any time raised
up from the ocean depths.
2. All the continents present an almost complete
and continuous series of rocks of all geological ages,
and in each of the great geological periods there are
found fresh water and estuarine deposits, and even
old land-surfaces, demonstrating continuity of con
tinental or insular conditions.
3. All the great oceans possess, scattered over
them, a few or many islands termed oceanic, and
characterised by a volcanic or coralline structure,
with no ancient stratified rocks in any one of them ;
XII.] THE EARTH IN RELATION TO LIFE 231
and in none of these is there found a single indi
genous land mammal or amphibian. It is incredible
that, if these oceans had ever contained extensive
continents, and if these oceanic islands are as even
now they are often alleged to be- -parts of these
now submerged continents, not one fragment of any
of the old stratified rocks, which characterise all
existing continents, should remain to show their
origin. In the Atlantic we find the Azores, Madeira,
and St. Helena ; in the Indian Ocean, Mauritius,
Bourbon, and Kerguelen Island ; in the Pacific, the
Fiji, Samoan, Society, Sandwich, and Galapagos
Islands, all without exception telling us the same
tale, that they have been built up from the ocean
depths by submarine volcanoes and coralline growths,
but have never formed part of continental areas.
4. The contours of the floors of all the great oceans,
now fairly well known through the soundings of ex
ploring vessels and for submarine telegraph lines,
also give confirmatory evidence that they have never
been continental land. For if any part of them were
a sunken continent, that part must have retained
some impress of its origin. Some of the numerous
mountain ranges which characterise every continent
would have remained. We should find slopes of
from 20 to 50 not uncommon, while valleys bordered
by rocky precipices, as in Lake Lucerne and a hundred
others, or isolated rock- walled mountains like Roraima,
or ranges of precipices as in the Ghats of India or
the Fiords of Norway, would frequently be met with.
But not a single feature of this kind has ever been
found in the ocean abysses. Instead of these we
have vast plains which, if the water were removed,
232 MAN S PLACE IN THE UNIVERSE [CHAP.
would appear almost exactly level, with no abrupt
slopes anywhere. When we consider that deposits
from the land never reach these remote ocean depths,
and that there is no wave-action below a few hundred
feet, these continental features once submerged would
be indestructible ; and their total absence is, there
fore, itself a demonstration that none of the great
oceans are on the sites of submerged continents.
How OCEAN DEPTHS WERE PRODUCED
It is a very difficult problem to determine how the
vast basins which are filled by the great oceans,
especially that of the Pacific, were first produced.
When the earth s surface was still in a molten state,
it would necessarily take the form of a true oblate
spheroid, with a compression at the poles due to its
speed of rotation, which is supposed to have been
very great. The crust formed by the gradual cooling
of such a globe would be of the same general form,
and, being thin, would easily be fractured or bent so
as to accommodate itself to any unequal stresses
from the interior. As the crust thickened and the
whole mass slowly cooled and contracted, fissures and
crumpling would occur, the former serving as outlets
for volcanic activities whose results are found through
out all geological ages ; the latter producing mountain
chains in which the rocks are almost always curved,
folded, or even thrust over each other, indicating the
mighty forces due to the adjustments of a solid crust
upon a shrinking fluid or semi-fluid interior.
But during this whole process there seem to be no
forces at work that could lead to the production of
XIL] THE EARTH IN RELATION TO LIFE 233
such a feature as the Pacific, a vast depression
covering nearly one-third of the whole surface of
the globe. The Atlantic Ocean, being smaller and
nearly opposite to the Pacific, but approximately of
equal depth, may be looked upon as a complementary
phenomenon which will be probably explained as a
result of the same causes as the vaster cavity.
So far as I am aware, there is only one suggested
cause of the formation of these great oceans that
seems adequate ; and as that cause is to some
extent supported by quite independent astronomical
evidence, and also directly bears upon the main
subject of the present volume, it must be briefly
considered.
A few years ago, Professor George Darwin, of
Cambridge, arrived at a certain conclusion as to the
origin of the moon, which is now comparatively
well known by Sir Robert Ball s popular account of
it in his small volume, Time and Tide. Briefly
stated, it is as follows. The tides produce friction on
the earth and very slowly increase the length of our
day, and also cause the moon to recede further from
us. The day is lengthened only by a small fraction
of a second in a thousand years, and the moon is
receding at an equally imperceptible rate. But as
these forces are constant, and have always acted on
the earth and moon, as we go back and back into the
almost infinite past we come to a time when the
rotation of the earth was so rapid that gravity at the
equator could hardly retain its outer portion, which
was spread out so that the form of the whole mass
was something like a cheese with rounded edges.
And about the same epoch the distance of the moon
234 MAN S PLACE IN THE UNIVERSE [CHAP.
is found to have been so small that it was actually
touching the earth. All this is the result of mathe
matical calculation from the known laws of gravita
tion and tidal effects ; and as it is difficult to see how
so large a body as the moon could have originated in
any other way, it is supposed that at a still earlier
period the moon and earth were one, and that the
moon separated from the parent mass owing to centri
fugal force generated by the earth s rapid rotation.
Whether the earth was liquid or solid at this epoch,
and exactly how the separation occurred, is not ex
plained either by Professor Darwin or Sir Robert
Ball ; but it is a very suggestive fact that, quite
recently, it has been shown, by means of the spectro
scope, that double stars of short period do originate
in this way from a single star, as already described
in our sixth chapter; but in these cases it seems
probable that the parent star is in a gaseous state.
These investigations of Professor G. Darwin have
been made use of by the Rev. Osmond Fisher (in
his very interesting and important work, Physics
of the Earths Crust] to account for the basins of the
great oceans, the Pacific being the chasm left when
the larger portion of the mass of the moon parted
from the earth.
Adopting, as I do, the theory of the origin of the
earth by meteoric accretion of solid matter, we must
consider our planet as having been produced from
one of those vast rings of meteorites which in great
numbers still circulate round the sun, but which at
the much earlier period now contemplated were both
more numerous and much more extensive,
to irregularities of distribution in such a ring and
XIL] THE EARTH IN RELATION TO LIFE 235
through disturbance by other bodies, aggregations
of various sizes would inevitably occur, and the
largest of these would in time draw in to itself all
the rest, and thus form a planet. During the early
stages of this process the particles would be so small,
and would come together so gradually, that little heat
would be produced, and there would result merely a
loose aggregation of cold matter. But as the process
went on and the mass of the incipient planet became
considerable- -perhaps half that of the earth- -the
rest of the ring would fall in with greater and greater
velocity ; and this, added to the gravitative com
pression of the growing mass might, w r hen nearly its
present size, have produced sufficient heat to liquefy
the outer layers, while the central portion remained
solid and to some extent incoherent, with probably
large quantities of heavy gases in the interstices.
When the amount of the meteoric accretions became
so reduced as to be insufficient to keep up the heat
to the melting-point, a crust would form, and might
have reached about half or three-fourths of its present
thickness when the moon became separated.
Let us now try to picture to ourselves what
happened. We should have a globe somewhat
larger than our earth is now, both because it then
contained the material of the moon and also because
it was hotter, revolving so rapidly as to be very
greatly flattened at the poles ; while the equatorial
belt bulged out enormously, and would probably have
separated in the form of a ring with a very slight
increase of the time of rotation, which is supposed to
have been about four hours. This globe would have
a comparatively thin crust, beneath which there was
236 MAN S PLACE IN THE UNIVERSE [CHAP.
molten rock to an unknown depth, perhaps a few
hundreds, perhaps more than a thousand miles. At
this time the attraction of the sun acting on the
molten interior produced tides in it, causing the thin
crust to rise and fall every two hours, but to so small
an extent only about a foot or so as not necessarily
to fracture it ; but it is calculated that this slight
rhythmic undulation coincided with the normal period
of undulation due to such a large mass of heavy
liquid, and so tended to increase the instability due
to rapid rotation.
The bulk of the moon is about one-fiftieth part
that of the earth, and an easy calculation shows us
that, taking the area of the Pacific, Atlantic, and
Indian Oceans combined as about two-thirds that of
the globe, it would require a thickness (or depth) of
about forty miles to furnish the material for the
moon. We must, of course, assume that there were
some inequalities in the thickness of the crust and in
its comparative rigidity, so that when the critical
moment came and the earth could no longer retain its
equatorial protuberance against the centrifugal force
due to rotation combined with the tidal undulations
caused by the sun, instead of a continuous ring slowly
detaching itself, the crust gave way in two or more
great masses where it was weakest, and as the tidal
wave passed under it and a quantity of the liquid
substratum rose with it, the whole would break up
and collect into a sub-globular mass a short distance
from the earth, and continue revolving with it for
some time at about the same rate as the surface had
rotated. But as tidal action is always equal on oppo
site sides of a globe, there would be a similar disrup-
XII.] THE EARTH IN RELATION TO LIFE 237
tion there, forming, it may be supposed, the Atlantic
basin, which, as may be seen on a small globe, is
almost exactly opposite a part of the Central Pacific.
So soon as these two great masses had separated
from the earth, the latter would gradually settle down
into a state of equilibrium, and the molten matter of
the interior, which would now fill the great oceanic
basins up to a level of a few miles below the general
surface, would soon cool enough to form a thin crust.
The larger portion of the nascent moon would gradu
ally attract to itself the one or more smaller portions
and form our satellite ; and from that time tidal fric
tion by both moon and sun would begin to operate
and would gradually lengthen our day and, more
rapidly, our month in the way explained in Sir Robert
Ball s volume.
A very interesting point may now be referred to,
because it seems confirmatory of this origin of the
o-reat ocean basins. In Mr. Osmond Fisher s work
o
it is explained how the variations in the force of
gravity, at numerous points all over the world, have
been determined by observations with the pendulum,
and also how these variations afford a measure of the
thickness of the solid crust, which is of less specific
gravity than the molten interior on which it rests.
By this means a very interesting result was obtained.
The observations on numerous oceanic islands proved
that the sub-oceanic crust was considerably more
dense than the crust under the continents, but also
thinner, the result being to bring the average mass
of the sub-oceanic crust and oceans to an equality
with that of the continental crust, and this causes
the whirling earth to be in a state of balance, or
238 MAN S PLACE IN THE UNIVERSE [CHAP.
equilibrium. Now, both the thinness and the in
creased density of the crust seem to be well explained
by this theory of the origin of the oceanic basins.
The new crust would necessarily for a long time be
thinner than the older portion, because formed so
much later, but it would very soon become cool
enough to allow the aqueous vapour of the atmo
sphere and that given off through fissures from the
molten interior to collect in the ocean basins, which
would thenceforth be cooled more rapidly and kept
at a uniform temperature and also under a uniform
pressure, and these conditions would lead to the
steady and continuous increase of thickness, with a
greater compactness of structure than in the conti
nental areas. It is no doubt to this uniformity
of conditions, with a lowering of the bottom tem
perature throughout the greater part of geological
time, till it has become only a few degrees above the
freezing-point, that we owe the remarkable persistence
of the vast and deep ocean basins on which, as we
have seen, the continuity of life on the earth has
largely depended.
There is one other fact which lends some support
to this theory of the origin of the ocean basins-
their almost complete symmetry with regard to the
equator. Both the Atlantic and Pacific basins ex
tend to an equal distance north and south of the
equator, an equality which could hardly have been
produced by any cause not directly connected with
the earth s rotation. The polar seas which are co
terminous with the two great oceans are very much
shallower, and cannot, therefore, be considered as
forming part of the true oceanic basins.
XIL] THE EARTH IN RELATION TO LIFE 239
WATER AS AN EQUALISER OF TEMPERATURE
The importance of water in regulating the tem
perature of the earth is so great that, even if we
had enough water on the land for all the wants of
plants and animals, but had no great oceans, it is
almost certain that the earth could not have produced
and sustained the various forms of life which it now
possesses.
The effect of the oceans is twofold. Owing to
the great specific heat of water, that is, its property
of absorbing heat slowly but to a large amount, and
giving it out with equal slowness, the surface-waters
of the oceans and seas are heated by the sun so that
by the evening of a bright day they have become
quite warm to a depth of several feet. But air has
much less specific heat than water, a pound of water
in cooling one degree being capable of warming four
pounds of air one degree ; but as air is 770 times as
light as water, it follows that the heat from one cubic
foot of water will warm more than 3000 cubic feet
of air as much as it cools itself. Hence the enormous
surface of the seas and oceans, the larger part of
which is within the tropics, warms the whole of the
lower and denser portions of the air, especially dur
ing the night, and this warmth is carried to all parts
of the earth by the winds, and thus ameliorates the
climate. Another quite distinct effect is due to
the great ocean currents, like the Gulf Stream and
the Japan Current, which carry the warm water of
the tropics to temperate and arctic regions, and thus
render many countries habitable which would other-
240 MAN S PLACE IN THE UNIVERSE [CHAP.
wise suffer the rigour of an almost arctic winter.
These currents are, however, directly due to the
winds, and properly belong to the section on the
atmosphere.
The other equalising action, due primarily to the
great area of the seas and oceans, is a result of
the vast evaporating surface from which the land
derives almost all its water in the form of rain and
rivers ; and it is quite evident that if there were not
sufficient water-surface to produce an ample supply
of vapour for this purpose, arid districts would occupy
more and more of the earth s surface. How much
water-surface is necessary for life we do not know;
but if the proportions of water- and land-surfaces were
reversed, it seems probable that the larger proportion
of the earth might be uninhabitable. The vapour
thus produced has also a very great effect in equal
ising temperature ; but this also is a point which
will come better under our next chapter on the
atmosphere.
There are, however, some matters connected with
the water-supply of the earth, and its relation to the
development of life, that call for a few remarks here.
What has determined the total quantity of water on
the earth or on other planets does not appear to be
known ; but presumably it would depend, partially or
wholly, on the mass of the planet being sufficient to
enable it to retain by its gravitative force the oxygen
and hydrogen of which water is composed. As the
two gases are so easily combined to form water, but
can only be separated under special conditions, its
quantity would be dependent on the supply of
XII.] THE EARTH IN RELATION TO LIFE 241
hydrogen, which is but rarely found on the earth in
a free state. The important fact, however, is, that
we do possess so great a quantity of water, that
if the whole surface of the globe was as regularly
contoured as are the continents, and merely wrinkled
with mountain chains, then the existing water would
cover the whole globe nearly two miles deep, leaving
only the tops of high mountains above its surface as
rows of small islands, with a few larger islands formed
by what are now the high plateaus of Tibet and the
Southern Andes.
Now there seems no reason why this distribution
of the water should not have occurred in fact it
seems probable that it would have occurred, had it
not been for the fortunate coincidence of the forma
tion of enormously deep ocean basins. So far as I
am aware, no sufficient explanation of the formation
of these basins has been given but that of Mr.
Osmond Fisher, as here described, and that depends
upon three unique circumstances : (i) the formation
of a satellite at a very late period of the planet s
development when there was already a rather thick
crust ; (2) the satellite being- far larger in proportion
to its primary than any other in the solar system ;
and (3) its having been produced by fission from its
primary on account of extremely rapid rotation, com
bined with solar tides in its molten interior, and a
rate of oscillation of that molten interior coinciding
with the tidal period. 1
Whether this very remarkable theory of the origin
of our moon is the true one, and if so, whether the
Professor G. H. Darwin states that it is nearly certain that no other
satellite nor any of the planets originated in the same way as the moon.
Q
242 MAN S PLACE IN THE UNIVERSE [CHAP. xn.
explanation it seems to afford of the great oceanic
basins is correct, I am not mathematician enough to
judge. The tidal theory of the origin of the moon,
as worked out mathematically by Professor G. H.
Darwin, has been supported by Sir Robert Ball and
accepted by many other astronomers ; while the
researches of the Rev. Osmond Fisher into the
Physics of the Earths Crust, together with his
mathematical abilities and his practical work as a
geologist, entitle his opinion on the question of the
mode of origin of the ocean basins to the highest
respect. And, as we have seen, the existence of
these vast and deep ocean basins, produced by the
agency of a series of events so remarkable as to be
quite unique in the solar system, played an important
part in rendering the earth fit for the development of
the higher forms of animal life, while without them
it seems not improbable that the conditions would
have been such as to render any varied forms of
terrestrial life hardly possible.
CHAPTER XIII
THE EARTH IN RELATION TO LIFE I ATMOSPHERIC
CONDITIONS
WE have seen in our tenth chapter that the physical
basis of life- -protoplasm consists of the four ele
ments, oxygen, nitrogen, hydrogen, and carbon, and
that both plants and animals depend largely upon
the free oxygen in the air to carry on their vital pro
cesses ; while the carbonic acid and ammonia in the
atmosphere seem to be absolutely essential to plants.
Whether life could have arisen and have been highly
developed with an atmosphere composed of different
elements from ours it is, of course, impossible to say ;
but there are certain physical conditions which seem
absolutely essential whatever may be the elements
which compose it.
The first of these essentials is an atmosphere
which shall be of such density at the surface of the
planet, and of so great a bulk, as to be not too rare
to fulfil its various functions at all altitudes where
there is a considerable area of land. What deter
mines the total quantity of gaseous matter on the
surface of a planet will be, mainly, its mass, together
with the average temperature of its surface.
The molecules of gases are in a state of rapid
motion in all directions, and the lighter gases have
243
244 MAN S PLACE IN THE UNIVERSE [CHAP.
the most rapid motions. The average speed of the
motion of the molecules has been roughly determined
under varying conditions of pressure and temperature,
and also the probable maximum and minimum rates,
and from these data, and certain known facts as to
planetary atmospheres, Mr. G. Johnstone Stoney,
F.R.S., has calculated what gases will escape from
the atmospheres of the earth and the other planets.
He finds that all the gases which are constituents of
air have such comparatively low molecular rates of
motion that the force of gravity at the upper limits
of the earth s atmosphere is amply sufficient to retain
them ; hence the stability in its composition. But
there are two other gases, hydrogen and helium,
which are both known to enter the atmosphere, but
never accumulate so as to form any measurable
portion of it, and these are found to have sufficient
molecular motion to escape from it. With regard to
hydrogen, if the earth were much larger and more
massive than it is, so as to retain the hydrogen,
disastrous consequences might ensue, because, when
ever a sufficient quantity of this gas accumulated, it
would form an explosive mixture with the oxygen of
the atmosphere, and a flash of lightning or even the
smallest flame would lead to explosions so violent
and destructive as perhaps to render such a planet
unsuited for the development of life. We appear,
therefore, to be just at the major limit of mass to
secure habitability, except in such planets as may
have no continuous supply of free hydrogen.
Perhaps the most important mechanical functions
of the atmosphere dependent on its density are: (i)
xili.] THE AIR IN RELATION TO LIFE 245
the production of winds, which in many ways bring
about an equalisation of temperature, and which also
produce surface-currents on the ocean; and (2) the
distribution of moisture over the earth by means of
clouds which also have other important functions.
Winds depend primarily on the local distribution
of heat in the air, especially on the great amount of
heat constantly present in the equatorial zone, due to
the sun being always nearly vertical at noon, and to
its being similarly vertical at each tropic once a year,
with a longer day, leading to even higher tempera
tures than at the equator, and producing also that
continuous belt of arid lands or deserts which almost
encircle the globe in the region of the tropics.
Heated air being lighter, the colder air from the
temperate zones continually flows towards it, lifting
it up and causing it to flow over, as it were, to the
north and south. But as the inflow comes from an
area of less rapid to one of more rapid rotation, the
course of the air is diverted, and produces the north
east and south-east trades ; while the overflow from
the equator going to an area of less rapid rotation,
turns westward and produces the south-west winds
so prevalent over the north Atlantic and the north
temperate zone generally, and the north-west in the
southern hemisphere.
It is outside the zone of the equable trade- winds,
and in a region a few degrees on each side of the
tropics, that destructive hurricanes and typhoons
prevail. These are really enormous whirlwinds due
to the intensely heated atmosphere over the arid
regions already mentioned, causing an inrush of cool
air from various directions, thus setting up a rotatory
246 MAN S PLACE IN THE UNIVERSE [CHAP.
motion which increases in rapidity till equilibrium is
restored. The hurricanes of the West Indies and
Mauritius, and the typhoons of the Eastern seas, are
thus caused. Some of these storms are so violent
that no human structures can resist them, while the
largest and most vigorous trees are torn to pieces or
overturned by them. But if our atmosphere were
much denser than it is, its increased weight would
give it still greater destructive force ; and if to this
were added a somewhat greater amount of sun-heat
which might be due either to our greater proximity
to the sun or to the sun s greater size or greater heat-
intensity, these tempests might be so increased in
frequency and violence as to render considerable por
tions of the earth uninhabitable.
The constant and equable trade-winds have a very
important function in initiating those far-reaching
ocean-currents which are of the greatest importance
in equalising temperature. The well known Gulf
Stream is to us the most important of these currents,
because it plays the chief part in giving us the mild
climate we enjoy in common with the whole of
Western Europe, a mildness which is felt to a con
siderable distance within the Arctic Circle ; and, in
conjunction with the Japan current, which does the
same for the whole of the temperate regions of the
North Pacific, renders a large portion of the globe
better adapted for life than it would be without these
beneficial influences.
These equalising currents, however, are almost
entirely due to the form and position of the con
tinents, and especially to the fact that they are so
situated as to leave vast expanses of ocean along the
XIII.] THE AIR IN RELATION TO LIFE 247
equatorial zone, and extending north and south to
the arctic and antarctic regions. If with the same
amount of land the continents had been so grouped
as to occupy a considerable portion of the equatorial
oceans such as would have been the case had Africa
been turned so as to join South America, and Asia
been brought to the south-east so as to take the place
of part of the equatorial Pacific, then the great
ocean-currents would have been but feeble or have
hardly existed. Without these currents much of the
north and south temperate lands would have been
buried in ice, while the largest portion of the con
tinents would have been so intensely heated as
perhaps to be unsuited for the development of the
higher forms of animal life, since we have shown
(in chapters X. and XL) how delicate is the balance
and how narrow the limits of temperature which are
required.
There seems to be no reason whatever why some
such distribution of the sea and land should not have
existed, had it not been for the admittedly excep
tional conditions which led to the production of our
satellite, thus necessarily forming vast chasms along
the region of the equator where centrifugal force as
well as the internal solar tides were most powerful,
and where the thin crust was thus compelled to give
way. And as the highest authorities declare that
there are no indications of such an origin of satel
lites in the case of any other planet, the whole series
of conditions favourable to life on the earth become
all the more remarkable.
248 MAN S PLACE IN THE UNIVERSE [CHAP.
CLOUDS, THEIR IMPORTANCE AND THEIR CAUSES
Few persons have any adequate conception of the
real nature of clouds and of the important part they
take in rendering our world a habitable and an
enjoyable one.
On the average, the rainfall over the oceans is
much less than over the land, the whole region of the
trade-winds having usually a cloudless sky and very
little rain ; but in the intervening belt of calms, near
to the equator, a cloudy sky and heavy rains are
frequent. This arises from the fact that the warm,
moist air over the ocean is raised upwards, by the
cold and heavy air from north and south, into a
cooler region where it cannot hold so much aqueous
vapour, which is there condensed and falls as rain.
Generally, wherever the winds blow over extensive
areas of water on to the land, especially if there are
mountains or elevated plateaus which cause the
moisture-laden air to rise to heights where the
temperature is lower, clouds are formed and more or
less rain falls. But if the land is of an arid nature and
much heated by the sun, the air becomes capable of
holding still more aqueous vapour, and even dense
rain-clouds disperse without producing any rain-fall.
From these simple causes, with the large area of sea
as compared with the land upon our earth, by far the
larger portion of the surface is well supplied with
rain, which, falling most abundantly in the elevated
and therefore cooler regions, percolates the soil, and
gives rise to those innumerable springs and rivulets
which moisten and beautifv the earth, and which,
XIIL] THE AIR IN RELATION TO LIFE 249
uniting together, form streams and rivers, which
return to the seas and oceans whence they were
originally derived.
CLOUDS AND RAIN DEPEND UPON ATMOSPHERIC DUST
The beautiful system of aqueous circulation by
means of the atmosphere as sketched above was long
thought to explain the whole process, and to require
no further elucidation ; but about a quarter of a
century back a curious experiment was made which
indicated that there was another factor in the process
which had been entirely overlooked. If a small jet
of steam is sent into two large glass receivers, one
filled with ordinary air, the other with air which has
been filtered by passing through a thick layer of
cotton wool so as to keep back all particles of solid
matter, the first vessel will be instantly filled with
condensed cloudy-looking vapour, while in the other
vessel the air and vapour will remain perfectly
transparent and invisible. Another experiment was
then made to imitate more nearly what occurs in
nature. The two vessels were prepared as before,
but a small quantity of water was placed in each
vessel and allowed to evaporate till the air was
nearly saturated with vapour, which remained in
visible in both. Both vessels were then slightly
cooled, when instantly a dense cloud was formed in
that filled with unfiltered air, while the other remained
quite clear. These experiments proved that the
mere cooling of air below the dew point will not
cause the aqueous vapour in it to condense into drops
so as to form mist, fog, or cloud, unless small
250 MAN S PLACE IN THE UNIVERSE [CHAP,
particles of solid or liquid matter are present to act
as nuclei upon which condensation begins. The
density of a cloud will therefore depend not only on
the quantity of vapour in the air, but on the presence
of an abundance of minute dust-particles on which
condensation can begin.
That such dust exists everywhere in the air, even
up to great heights, is not a supposition but a
proved fact. By exposing glass plates covered with
glycerine in different places and at different altitudes
the number of these particles in each cubic foot of
air has been determined ; and it is found that not
only are they present everywhere at low levels, but
that there are a considerable number even at the
tops of the highest mountains. These solid particles
also act in another way. By radiation in the higher
atmosphere they become very cold, and thus con
dense the vapour by contact, just as the points of
grass-blades condense it to form dew.
When steam is escaping from an engine we see a
mass of dense white vapour, a miniature cloud ; and
if we are near it in cold, damp weather, we feel
little drops of rain produced from it. But on a fine,
warm day it rises quickly and soon melts away, and
entirely disappears. Exactly the same thing happens
on a larger scale in nature. In fine weather we
may have abundant clouds continually passing high
overhead, but they never produce rain, because as
the minute globules of water slowly fall towards the
earth, the warm dry air again turns them into in
visible vapour. Again, in fine weather, we often
see a small cloud on a mountain top which remains
there a considerable time, even though a brisk wind
XIIL] THE AIR IN RELATION TO LIFE 251
is blowing. The mountain top is colder than the
surrounding air, and the invisible vapour becomes
condensed into cloud by passing over it, but the
moment these cloud particles are carried past the
summit into the warmer and drier air they are again
evaporated and disappear. On Table Mountain,
near Cape Town, this phenomenon occurs on a large
scale, and is termed the table-cloth, the mass of white
fleecy cloud seeming to hang over the flat moun
tain top to some distance down, where it remains
for several months, while all around there is bright
sunshine.
Another phenomenon that indicates the universal
presence of dust to enormous heights in the atmo
sphere is the blue colour of the sky. This is caused
by the presence of such excessively minute particles
of dust through an enormous thickness of the higher
atmosphere- -probably up to a height of twenty or
thirty miles, or more that they reflect only the light of
short wave-length from the blue end of the spectrum.
This also has been proved by experiment. If a
glass cylinder, several feet long, is filled with pure
air from which all solid particles have been removed
by filtering and passing over red-hot platinum wires,
and a ray of electric light is passed through it, the
interior, when viewed laterally, appears quite dark,
the light passing through in a straight line and not
illuminating the air. But if a little more air is
passed through the filter so rapidly as to allow only
the minutest particles of dust to enter with it, the
vessel becomes gradually filled with a blue haze,
which gradually deepens into a beautiful blue, com
parable with that of the sky. If now some of the
252 MAN S PLACE IN THE UNIVERSE [CHAP.
unfiltered air is admitted, the blue fades away into
the ordinary tint of daylight.
Since it has been known that liquid oxygen is
blue, many people have concluded that this explains
the blue colour of the sky. But it has really nothing
to do with the point at issue. The blue of the
liquid oxygen becomes so excessively faint in the
gas, further attenuated as it is by the colourless
nitrogen, that it would have no perceptible colour
in the whole thickness of our atmosphere. Again,
if it had a perceptible blue tint we could not see it
against the blackness of space behind it ; but white
objects seen through it, such as the moon and clouds,
should all appear blue, which they do not do. The
blue we see is from the whole sky, and is therefore
reflected light ; and as pure air is quite transparent,
there must be solid or liquid particles so minute as
to reflect blue light only. In the lower atmosphere
the rain-producing particles are larger, and reflect
all the rays, thus diluting the blue colour near the
horizon, and, by refraction and reflection combined,
producing the various beautiful hues of sunrise and
sunset.
This production of exquisite colours by the dust in
the atmosphere, though adding greatly to the enjoy
ment of life, cannot be considered essential to it ;
but there is another circumstance connected with
atmospheric dust which, though little appreciated,
might have effects which can hardly be calculated.
If there were no dust in the atmosphere, the sky
would appear black even at noon, except in the
actual direction of the sun ; and the stars would be
visible in the day as well as at night. This would
XIIL] THE AIR IN RELATION TO LIFE 253
follow because air does not reflect light, and is not
visible. We should therefore receive no light from
the sky itself as we do now, and the north side of
every hill, house, and other solid objects, would be
totally dark, unless there were any surfaces in that
direction to reflect the light. The surface of the
ground at a little distance would be in sunshine, and
this would be the only source of light wherever
direct sunlight was cut off. To get a good amount
of pleasant light in houses it would be necessary to
have them built on nearly level ground, or on ground
rising to the north, and with walls of glass all round
and down to the floor line, to receive as much as
possible of the reflected light from the ground. What
effect this kind of light would have on vegetation it is
difficult to say, but trees and shrubs would probably
grow laterally towards the south, east, and west, so as
to get as much direct sunshine as possible.
A more important result would be that, as sunshine
would be perpetual during the day, so much evapora
tion would take place that the soil would become
arid and almost bare in places that are now covered
with vegetation, and plants like the cactuses of Arizona
and the euphorbias of South Africa would occupy a
large portion of the surface.
Returning now from this collateral subject of light
and colour to the more important aspect of the ques
tion- -the absence of cloud and rain we have to
consider what would happen, and in what way the
enormous quantity of water which would be evapor
ated under continual sunshine would be returned to
the earth.
The first and most obvious means would be by
254 MAN S PLACE IN THE UNIVERSE [CHAP.
abnormally abundant dews, which would be deposited
almost every night on every form of leafy vegetation.
Not only would all grass and herbage, but all the
outer leaves of shrubs and trees, condense so much
moisture as to take the place of rain so far as the
needs of such vegetation were concerned. But with
out arrangements for irrigation cultivation would
be almost impossible, because the bare soil would
become intensely heated during the day, and would
retain so much of its heat through the night so as
to prevent any dew forming upon it.
Some more effective mode, therefore, of return
ing the aqueous vapour of the atmosphere to the
earth and ocean, would be required, and this, I
believe, would be done by means of hills and
mountains of sufficient height to become decidedly
colder than the lowlands. The air from over the
oceans would be constantly loaded with moisture,
and whenever the winds blew on to the land the
air would be carried up the slopes of the hills into
the colder regions, and there be rapidly condensed
upon the vegetation, and also on the bare earth and
rocks of northern slopes, and wherever they cooled
sufficiently during the afternoon or night to be
below the temperature of the air. The quantity of
vapour thus condensed would reduce the atmospheric
pressure, which would lead to an inrush of air from
below, bringing with it more vapour, and this might
give rise to perpetual torrents, especially on northern
and eastern slopes. But as the evaporation would
be much greater than at the present time, owing to
perpetual sunshine, so the water returned to the
earth would be greater, and as it would not be so
XIIL] THE AIR IN RELATION TO LIFE 255
uniformly distributed over the land as it is now,
the result would perhaps be that extensive mountain
sides would become devastated by violent torrents,
rendering permanent vegetation almost impossible ;
while other and more extensive areas, in the absence
of rain, would become arid wastes that would support
only the few peculiar types of vegetation that are
characteristic of such regions.
Whether such conditions as here supposed would
prevent the development of the higher forms of life
it is impossible to say, but it is certain that they
would be very unfavourable, and might have much
more disastrous consequences than any we have here
suggested. We can hardly suppose that, with winds
and rock-formations at all like what they are now,
any world could be wholly free from atmospheric
dust. If, however, the atmosphere itself were much
less dense than it is, say one-half, which might very
easily have been the case, then the winds would have
less carrying power, and at the elevations at which
clouds are usually formed there would not be enough
dust-particles to assist in their formation. Hence fogs
close to the earth s surface would largely take the
place of clouds floating far above it, and these would
certainly be less favourable to human life and to
that of many of the higher animals than existing
conditions.
The world-wide distribution of atmospheric dust is
a remarkable phenomenon. As the blue colour of the
sky is universal, the whole of the higher atmosphere
must be pervaded by myriads of ultra-microscopical
particles, which, by reflecting the blue rays only, give
us not only the azure vault of heaven, but in com-
256 MAN S PLACE IN THE UNIVERSE [CHAP.
bination with the coarser dust of lower altitudes,
diffused daylight, the grand forms and motions of
the fleecy clouds, and the gentle rain from heaven
to refresh the parched earth and make it beautiful
with foliage and flowers. Over every part of the
vast Pacific Ocean, whose islands must produce a
minimum of dust, the sky is always blue, and its
thousand isles do not suffer for want of rain. Over
the great forest-plain of the Amazon valley, where
the production of dust must be very small, there is
yet abundance of rain-clouds and of rain. This is
due primarily to the two great natural sources of
dust the active volcanoes, together with the deserts
and more arid regions of the world ; and, in the
second place, to the density and wonderful mobility
of the atmosphere, which not only carries the finest
dust-particles to an enormous height, but distributes
them through its whole extent with such wonderful
uniformity.
Every dust-particle is of course much heavier than
air, and in a comparatively short time, if the atmo
sphere were still, would fall to the ground. Tyndall
found that the air of a cellar under the Royal Institu
tion in Albemarle Street, which had not been opened
for several months, was so pure that the path of a
beam of electric light sent through it was quite
invisible. But careful experiments show that not
only is the air in continual motion, but the motion is
excessively irregular, being hardly ever quite hori
zontal, but upwards and downwards and in every
intermediate direction, as well as in countless whirls
and eddies ; and this complexity of motion must
extend to a vast height, probably to fifty miles or
xili.] THE AIR IN RELATION TO LIFE 257
more, in order to provide a sufficient thickness of
those minutest particles which produce the blue of
the sky.
All this complexity of motion is due to the action
of the sun in heating the surface of the earth, and the
extreme irregularity of that surface both as regards
contour and its capacity for heat-absorption. In one
area we have sand or rock or bare clay, which, when
exposed to bright sunshine, become scorching hot ;
in another area we have dense vegetation, which,
owing to evaporation caused by the sunshine, remains
comparatively cool, and also the still cooler surfaces
of rivers and Alpine lakes. But if the air were much
less dense than it is, these movements would be less
energetic, while all the dust that was raised to any
considerable height would, by its own weight, fall
back again to the earth much more rapidly than it
does now. There would thus be much less dust
permanently in the atmosphere, and this would
inevitably lead to diminished rainfall and, partially,
to the other injurious effects already described.
ATMOSPHERIC ELECTRICITY
We have already seen that vegetable organisms
obtain the chief part of the nitrogen in their tissues
from ammonia produced in the atmosphere and carried
into the earth by rain. This substance can only be
thus produced by the agency of electrical discharges,
or lightning, which cause the combination of the
hydrogen in the aqueous valour with the free
nitrogen of the air. But clouds are important agents
R
258 MAN S PLACE IN THE UNIVERSE [CHAP.
in the accumulation of electricity in sufficient amount
to produce the violent discharges we know as light
ning, and it is doubtful whether without them there
would be any discharges through the atmosphere
capable of decomposing the aqueous vapour in it.
Not only are clouds beneficial in the production of
rain, and also in moderating the intensity of contin
uous sun-heat, but they are also requisite for the
formation of chemical compounds in vegetables which
are of the highest importance to the whole animal
kingdom. So far as we know, animal life could not
exist on the earth s surface without this source of
nitrogen, and therefore without clouds and lightning ;
and these, we have just seen, depend primarily on a
due proportion of dust in the atmosphere.
But this due proportion of dust is mainly supplied
by volcanoes and deserts, and its distribution and
constant presence in the air depends upon the density
of the atmosphere. This again depends on two other
factors : the force of gravity due to the mass of the
planet, and the absolute quantity of the free gases
constituting the atmosphere.
We thus find that the vast, invisible ocean of
air in which we live, and which is so important to us
that deprivation of it for a few minutes is destructive
of life, produces also many other beneficial effects of
which we usually take little account, except at times
when storm or tempest, or excessive heat or cold,
remind us how delicate is the balance of conditions
on which our comfort, and even our lives, depend.
But the sketch I have here attempted to give of
its varied functions shows us that it is really a most
complex structure, a wonderful piece of machinery, as
XIIL] THE AIR IN RELATION TO LIFE 259
it were, which in its various component gases, its
actions and reactions upon the water and the land,
its production of electrical discharges, and its furnish
ing the elements from which the whole fabric of
o
organic life is composed and perpetually renewed,
may be truly considered to be the very source and
foundation of life itself. This is seen, not only in
the fact of our absolute dependence upon it every
minute of our lives, but in the terrible effects pro
duced by even a slight degree of impurity in this
vital element. Yet it is among those nations that
claim to be the most civilised, those that profess to
be guided by a knowledge of the laws of nature, those
that most glory in the advance of science, that we
find the greatest apathy, the greatest recklessness, in
continually rendering impure this all-important neces
sary of life, to such a degree that the health of the
larger portion of their populations is injured and
their vitality lowered, by conditions which compel
them to breathe more or less foul and impure air
for the greater part of their lives. The huge and
ever-increasing cities, the vast manufacturing towns
belching forth smoke and poisonous gases, with the
crowded dwellings, where millions are forced to live
under the most terrible insanitary conditions, are the
witnesses to this criminal apathy, this incredible
recklessness and inhumanity.
For the last fifty years and more the inevitable
results of such conditions have been fully known ; yet
to this day nothing of importance has been done,
nothing is being done. In this beautiful land there
is ample space and a superabundance of pure air for
every individual. Yet our wealthy and our learned
260 MAN S PLACE IN THE UNIVERSE [CHAP.
classes, our rulers and law -makers, our religious
teachers and our men of science, all alike devote
their lives and energies to anything or everything
but this. Yet this is the one great and primary
essential of a people s health and well-being, to which
everything should, for the time, be subordinate. Till
this is done, and done thoroughly and completely,
our civilisation is naught, our science is naught, our
religion is naught, and our politics are less than
naught are utterly despicable ; are below contempt.
It has been the consideration of our wonderful
atmosphere in its various relations to human life, and
to all life, which has compelled me to this cry for the
children and for outraged humanity. Will no body
of humane men and women band themselves together,
o
and take no rest till this crying evil is abolished,
and with it nine-tenths of all the other evils that now
afflict us ? Let everything give way to this. As in
a war of conquest or aggression nothing is allowed
to stand in the way of victory, and all private rights
are subordinated to the alleged public weal, so, in
this war against filth, disease, and misery let nothing
stand in the way neither private interests nor vested
rights and we shall certainly conquer. This is the
gospel that should be preached, in season and out of
season, till the nation listens and is convinced. Let
this be our claim : Pure air and pure water for every
inhabitant of the British Isles. Vote for no one who
says * It can t be done/ Vote only for those who
declare It shall be done. It may take five or ten
or twenty years, but all petty ameliorations, all piece
meal reforms, must wait till this fundamental reform
is effected. Then, when we have enabled our people
xiii.] THE AIR IN RELATION TO LIFE 261
to breathe pure air, and drink pure water, and live
upon simple food, and work and play and rest under
healthy conditions, they will be in a position to
decide (for the first time) what other reforms are
really needed.
Remember! We claim to be a people of high
civilisation, of advanced science, of great humanity,
of enormous wealth ! For very shame do not let us
say We cannot arrange matters so that our people
may all breathe unpolluted, unpoisoned air !
CHAPTER XIV
THE EARTH IS THE ONLY HABITABLE PLANET IN
THE SOLAR SYSTEM
HAVING shown in the last three chapters how
numerous and how complex are the conditions which
alone render life possible on our earth, how nicely
balanced are opposing forces, and how curious and
delicate are the means by which the essential com
binations of the elements are brought about, it will
be a comparatively easy task to show how totally
unfitted are all the other planets either to develop
or to preserve the higher forms of life, and, in most
cases, any forms above the lowest and most rudi
mentary. In order to make this clear we will take
the most important of the conditions in order, and
see how the various planets fulfil them.
MASS OF A PLANET AND ITS ATMOSPHERE
The height and density of the atmosphere of a
planet is important as regards life in several ways.
On its density depends its power of carrying
moisture ; of holding a sufficient supply of dust-
particles for the formation of clouds ; of carrying
ultra-microscopic particles to such a height and in
such quantity as to diffuse the light of the sun by
262
CHAP, xiv.] THE ONLY HABITABLE PLANET 263
reflection from the whole sky ; of raising waves in
the ocean and thus aerating its waters, and of pro
ducing the ocean currents which so greatly equalise
temperature. Now this density depends on two
factors : the mass of the planet and the quantity of
the atmospheric gases. But there is good reason to
think that the latter depends directly upon the
former, because it is only when a certain mass is
attained that any of the lighter permanent gases can
be held on the surface of a planet. Thus, according
to Dr. G. Johnstone Stoney, who has specially
studied this subject, the moon cannot retain even
such a heavy gas as carbonic acid, or the still heavier
carbon disulphide ; while no particle of oxygen,
nitrogen, or water-vapour can possibly remain on it,
owing to the fact of its mass being only about one-
eightieth that of the earth. It is believed that there
are considerable quantities of gases in the stellar
spaces, and probably also within the solar system,
but perhaps in the liquid or solid form. In that state
they might be attracted by any small mass such as
the moon, but the heat of its surface when exposed
to the solar rays would quickly restore them to the
gaseous condition, when they would at once escape.
It is only when a planet attains a mass at least a
quarter that of the earth that it is capable of retain
ing water-vapour, one of the most essential of the
gases ; but with so small a mass as this, its whole
atmosphere would probably be so limited in amount
and so rare at the planet s surface that it would be
quite unable to fulfil the various purposes for which
an atmosphere is required in order to support life.
For their adequate fulfilment the mass of a planet
264 MAN S PLACE IN THE UNIVERSE [CHAP.
cannot be much less than that of the earth. Here
we come to one of those nice adjustments of which
so many have been already pointed out. Dr. John-
stone Stoney arrives at the conclusion that hydrogen
escapes from the earth. It is continually produced
in small quantities by submarine volcanoes, by
fissures in volcanic regions, from decaying vegeta
tion, and from some other sources ; yet, though some
times found in minute quantities, it forms no regular
constituent of our atmosphere. 1
The quantity of hydrogen combined with oxygen
to form the mass of water in our vast and deep
oceans is enormous. Yet if it had been only one-
tenth more than it actually is, the present land-surface
would have been almost all submerged. How the
adjustments occurred so that there was exactly enough
hydrogen to fill the vast ocean basins with water to
such a depth as to leave enough land-surface for the
ample development of vegetable and animal life,
and yet not so much as to be injurious to climate, it
is difficult to imagine. Yet the adjustment stares us
in the face. First, we have a satellite unique in size
as compared with its primary, and apparently in
lateness of origin ; then we have a mode of origin
for that satellite said to be certainly unique in the
solar system ; as a consequence of this origin, it is
believed, we have enormously deep ocean basins
symmetrically placed with regard to the equator an
arrangement which is very important for ocean
circulation ; then we must have had the right
1 Transactions of Royal Dublin Society* vol. vi. (ser. ii.), part xiii.
Of Atmospheres upon Planets and Satellites. 3 By G. Johnstone
Stoney, F.R.S., etc. etc.
xiv.] THE ONLY HABITABLE PLANET 265
quantity of hydrogen, obtained in some unknown
way, which formed water enough to fill these chasms,
so as to leave an ample area of dry land, but which
one-tenth more water would have ingulfed ; and,
lastly, we have oxygen enough left to form an
atmosphere of sufficient density for all the require
ments of life. It could not be that the surplus
hydrogen escaped when the water had been pro
duced, because it escapes very slowly, and it
combines so easily with free oxygen by means of
even a spark, as to make it certain that all the
available hydrogen was used up in the oceanic
waters, and that the supply from the earth s interior
has been since comparatively small in amount.
There is yet one more adjustment to be noticed.
All the facts now referred to show that the earth s
mass is sufficient to bring about the conditions
favourable for life. But if our globe had been a
little larger, and proportionately denser, in all
probability no life would have been possible.
Between a planet of 8000 and one of 9500
miles diameter is not a large difference, when com
pared with the enormous range of size of the other
planets. Yet this slight increase in diameter would
give two-thirds increase in bulk, and, with a corre
sponding increase of density due to the greater
gravitative force, the mass would be about double
what it is. But with double the mass the quantity
of gases of all sorts attracted and retained by gravity
would probably have been double ; and in that case
there would have been double the quantity of water
produced, as no hydrogen could then escape. But
the surface of the globe would only be one half
266 MAN S PLACE IN THE UNIVERSE [CHAP.
greater than at present, in which case the water
would have sufficed to cover the whole surface
several miles deep.
HABITABILITY OF OTHER PLANETS
When we look to the other planets of our system
we see everywhere illustrations of the relation of
size and mass to habitability. The smaller planets,
Mercury and Mars, have not sufficient mass to re
tain water-vapour, and, without it, they cannot be
habitable. All the larger planets can have very little
solid matter, as indicated by their very low density
notwithstanding their enormous mass. There is,
therefore, very good reason for the belief that the
adaptability of a planet for a full development of
life is primarily dependent, within very narrow
limits, on its size and, more directly, on its mass.
But if the earth owes its specially constituted
atmosphere and its nicely adjusted quantity of water
to such general causes as here indicated, and the
same causes apply to the other planets of the solar
system, then the only planet on which life can be
possible is Venus. As, however, it may be urged
that exceptional causes may have given other planets
an equal advantage in the matter of air and water,
we will briefly consider some of the other conditions
which we have found to be essential in the case of
the earth, but which it is almost impossible to con
ceive as existing, to the required extent, on any of
the other planets of the solar system.
xiv.] THE ONLY HABITABLE PLANET 267
A SMALL AND DEFINITE RANGE OF TEMPERATURE
We have already seen within what narrow limits
the temperature on a planet s surface must be main
tained in order to develop and support life. We
have also seen how numerous and how delicate are
the conditions, such as density of atmosphere, extent
and permanence of oceans, and distribution of sea
and land, which are requisite, even with us, in order
to render possible the continuous preservation of a
sufficiently uniform temperature. Slight alterations
one way or another might render the earth almost
uninhabitable, through its being liable to alternations
of too great heat or excessive cold. How then can
we suppose that any other of the planets, which have
either very much more or very much less sun-heat
than we receive, could, by any possible modification
of conditions, be rendered capable of producing and
supporting a full and varied life-development ?
Mars receives less than half the amount of sun-
heat per unit of surface that we do. And as it is
almost certain that it contains no water (its polar
snows being caused by carbonic acid or some other
heavy gas) it follows that, although it may produce
vegetable life of some low kinds, it must be quite
unsuited for that of the higher animals. Its small
size and mass, the latter only one-ninth that of the
earth, may probably allow it to possess a very rare
atmosphere of oxygen and nitrogen, if those gases
exist there, and this lack of density would render it
unable to retain during the night the very moderate
amount of heat it might absorb during the day.
This conclusion is supported by its low reflecting
268 MAN S PLACE IN THE UNIVERSE [CHAP.
power, showing that it has hardly any clouds in its
scanty atmosphere. During the greater part of the
twenty-four hours, therefore, its surface-temperature
would probably be much below the freezing point of
water ; and this, taken in conjunction with the total
absence of aqueous vapour or liquid water, would
add still further to its unsuitability for animal life.
In Venus the conditions are equally adverse in
the other direction. It receives from the sun almost
double the amount of heat that we receive, and this
alone would render necessary some extraordinary
combination of modifying agencies in order to reduce
and render uniform the excessively high temperature.
But it is now known that Venus has one peculiarity
which is in itself almost prohibitive of animal life, and
probably of even the lowest forms of vegetable life.
This peculiarity is, that through tidal action caused
by the sun, its day has been made to coincide with
its year, or, more properly, that it rotates on its
axis in the same time that it revolves round the sun.
Hence it always presents the same face to the sun ;
and while one half has a perpetual day, the other half
has perpetual night, with perpetual twilight through
refraction in a narrow belt adjoining the illuminated
half. But the side that never receives the direct
rays of the sun must be intensely cold, approximat
ing, in the central portions, to the zero of temperature,
while the half exposed to perpetual sunshine of double
intensity to ours must almost certainly rise to a
temperature far too great for the existence of proto
plasm, and probably, therefore, of any form of animal
life.
Venus appears to have a dense atmosphere, and
xiv.] THE ONLY HABITABLE PLANET 269
its brilliancy suggests that we see the upper surface
of a cloud-canopy, and this would no doubt greatly
reduce the excessive solar heat. Its mass, being a
little more than three-fourths that of the earth, would
enable it to retain the same gases as we possess.
But under the extraordinary conditions that prevail
on the surface of this planet, it is hardly possible that
the temperature of the illuminated side can be pre
served in a sufficient state of uniformity for the
development of life in any of its higher forms.
Mercury possesses the same peculiarity of keeping
one face always towards the sun, and as it is so much
smaller and so much nearer the sun its contrasts of
heat and cold must be still more excessive, and we
need hardly discuss the possibility of this planet being
habitable. Its mass being only one-thirtieth that
of the earth, water- vapour will certainly escape from
it, and, most probably, nitrogen and oxygen also, so
that it can possess very little atmosphere ; and this
is indicated by its low reflecting power, no less than
83 per cent, of the sun s light being absorbed, and
only 17 per cent, reflected, whereas clouds reflect 72
per cent. This planet is therefore intensely heated
on one side and frozen on the other ; it has no water
and hardly any atmosphere, and is therefore, from
every point of view, totally unfitted for supporting
living organisms.
Even if it is supposed that, in the case of Venus,
its perpetual cloud-canopy may keep down the surface
temperature within the limits necessary for animal
life, the extraordinary turmoil in its atmosphere
caused by the excessively contrasted temperatures
of its dark and light hemispheres must be extremely
2/0 MAN S PLACE IN THE UNIVERSE [CHAP.
inimical to life, if not absolutely prohibitive of it.
For on the greater part of the hemisphere that never
receives a ray of light or heat from the sun all the
water and aqueous vapour must be turned into ice
or snow, and it seems almost impossible that the air
itself can escape congelation. It could only do so
by a very rapid circulation of the whole atmosphere,
and this would certainly be produced by the enormous
and permanent difference of temperature between
the two hemispheres. Indications of refraction by
a dense atmosphere are visible during the planet s
transit over the sun s disc, and also when it is in
conjunction with the sun, and the refraction is so great
that Venus is believed to have an atmosphere much
higher than ours. But during the rapid circulation
of such an atmosphere, heated on one half the planet
and cooled on the other, most of the aqueous vapour
must be taken out of it on the dark side as fast as it
is produced on the heated side, though sufficient may
remain to produce a canopy of very lofty clouds
analogous to our cirri. The occasional visibility of
the dark side of Venus may be caused by an electrical
glow due to the friction of the perpetually overflowing
and inflowing atmosphere, this being increased by
reflection from a vast surface of perpetual snow.
If we consider all the exceptional features of this
planet, it appears certain that the conditions as regards
climate cannot now be such as to maintain a tem
perature within the narrow limits essential for life, while
there is little probability that at any earlier period
it can have possessed and maintained the necessary
stability during the long epochs which are requisite
for its development.
xiv.] THE ONLY HABITABLE PLANET 271
Before considering the condition of the larger
planets, it will be well to refer to an argument which
has been supposed to minimise the difficulties already
stated as to those planets which approach nearest to
the earth in size and distance from the sun.
THE ARGUMENT FROM EXTREME CONDITIONS
ON THE EARTH
In reply to the evidence showing how nice are the
adaptations required for life-development, it is often
objected that life does now exist under very extreme
conditions under tropic heat and arctic snows ; in
the burnt-up desert as well as in the moist tropical
forest ; in the air as well as in the water ; on lofty
mountains as well as on the level lowlands. This is
no doubt true, but it does not prove that life could
have been developed in a world where any of these
extremes of climate characterised the whole surface.
The deserts are inhabited because there are oases
where water is attainable, as well as in the surround
ing fertile areas. The arctic regions are inhabited
because there is a summer, and during that summer
there is vegetation. If the surface of the ground
were always frozen, there would be no vegetation
and no animal life.
The late Mr. R. A. Proctor put this argument of
the diversity of conditions under which life actually
does exist on the earth as well probably as it can be
put. He says : When we consider the various
conditions under which life is found to prevail, that
no difference of climatic relations, or of elevation, of
land, or of air, or of water, of soil in land, of freshness
272 MAN S PLACE IN THE UNIVERSE [CHAP.
or saltness in water, of density in air, appears (so far
as our researches have extended) to render life im
possible, we are compelled to infer that the power
of supporting life is a quality which has an exceed
ingly wide range in nature.
This is true, but with certain reservations. The
only species of animal which does really exist under
the most varied conditions of climate is man, and he
does so because his intellect renders him to some
extent the ruler of nature. None of the lower
animals have such a wide range, and the diversity of
conditions is not really so great as it appears to be.
The strict limits are nowhere permanently overpassed,
and there is always the change from winter to
summer, and the possibility of migration to less
inhospitable areas.
THE GREAT PLANETS ALL UNINHABITABLE
Having already shown that the condition of Mars,
both as regards water, atmosphere, and temperature,
is quite unfitted to maintain life, a view in which both
general principles and telescopic examination per
fectly agree, we may pass on to the outer planets,
which, however, have long been given up as adapted
for life even by the most ardent advocates for life
in other worlds.* Their remoteness from the sun-
even Jupiter being five times as far as the earth, and
therefore receiving only one twenty-fifth of the light
and heat that we receive per unit of surface renders
it almost impossible, even if other conditions were
favourable, that they should possess surface -tem
peratures adequate to the necessities of organic life.
xiv.] THE ONLY HABITABLE PLANET 273
But their very low densities, combined with very
large size, renders it certain that they none of them
have a solidified surface, or even the elements from
which such a surface could be formed.
It is supposed that Jupiter and Saturn, as well as
Uranus and Neptune, retain a considerable amount
of internal heat, but certainly not sufficient to keep
the metallic and other elements of which the sun and
earth consist in a state of vapour, for if so they would
be planetary stars and would shine by their own
light. And if any considerable portion of their bulk
consisted of these elements, whether in a solid or a
liquid state, their densities would necessarily be much
greater than that of the earth instead of very much
less- -Jupiter is under one-fourth the density of the
earth, Saturn under an eighth, while Uranus and
Neptune are of intermediate densities, though much
less in bulk even than Saturn.
It thus appears that the solar system consists of
two groups of planets which differ widely from each
other. The outer group of four very large planets
are almost wholly gaseous, and probably consist of
the permanent gases those which can only be lique
fied or solidified at a very low temperature. In no
other way can their small density combined with
enormous bulk be accounted for.
The inner group also of four planets are totally
unlike the preceding. They are all of small size, the
earth being the largest. They are all of a density
roughly proportionate to their bulk. The earth is
both the largest and the densest of the group ; not
only is it situated at that distance from the sun which,
through solar heat alone, allows water to remain in
s
274 MAN S PLACE IN THE UNIVERSE [CHAP.
the liquid state over almost the whole of its surface,
but it possesses numerous characteristics which secure
a very equable temperature, and which have secured
to it very nearly the same temperature during those
enormous geological periods in which terrestrial life
has existed. We have already shown that no other
planet possesses these characteristics now, and it is
almost equally certain that they never have possessed
them in the past, and never will possess them in the
future.
A LAST ARGUMENT FOR HABITABILITY OF
THE PLANETS
Although it has been admitted by the late Mr.
Proctor and some other astronomers that most of the
planets are not now habitable, yet, it is often urged,
they may have been so in the past or may become so
in the future. Some are now too hot, others are now
too cold ; some have now no water, others have too
much ; but all go through their appointed series of
stages, and during some of these stages life may be
or may have been possible. This argument, although
vague, will appeal to some readers, and it may, there
fore, be necessary to reply to it. This is the more
necessary as it is still made use of by astronomers.
In a criticism of my article in The Fortnightly Review,
M. Camille Flammarion, of the Paris Observatory,
dramatically remarks : Yes, life is universal, and
eternal, for time is one of its factors. Yesterday the
moon, to-day the earth, to-morrow Jupiter. In space
there are both cradles and tombs.
1 Knowledge^ June 1903.
XIV.] THE ONLY HABITABLE PLANET 275
It is thus suggested that the moon was once in
habited, and that Jupiter will be inhabited in some
remote future ; but no attempt is made to deal with
the essential physical conditions of these very diverse
objects, rendering them not only now, but always,
unfitted to develop and to maintain terrestrial or
aerial life. This vague supposition- -it can hardly
be termed an argument as regards past or future
adaptability for life, of all the planets and some of
the satellites in the solar system, is, however, rendered
invalid by an equally general objection to which its
upholders appear never to have given a moment s
consideration ; and as it is an objection which still
further enforces the view as to the unique position of
the earth in the solar system, it will be well to submit
it to the judgment of our readers.
LIMITATION OF THE SUN S HEAT
It is well known that there is, and has been for
nearly half a century, a profound difference of opinion
between geologists and physicists as to the actual or
possible duration in years of life upon the earth. The
geologists, being greatly impressed with the vast
results produced by the slow processes of the wearing
away of the rocks and the deposit of the material in
seas or lakes, to be again upheaved to form dry land,
and to be again carved out by rain and wind, by heat
and cold, by snow and ice, into hills and valleys and
grand mountain ranges ; and further, by the fact that
the highest mountains in every part of the globe
very often exhibit on their loftiest summits stratified
rocks which contain marine organisms, and were
276 MAN S PLACE IN THE UNIVERSE [CHAP.
therefore originally laid down beneath the sea ; and,
yet again, by the fact that the loftiest mountains are
often the most recent, and that these grand features
o
of the earth s surface are but the latest examples of
the action of forces that have been at work through
out all geological time studying all their lives the
detailed evidences of all these changes, have come to
the conclusion that they imply enormous periods only
to be measured by scores or hundreds of millions of
years.
And the collateral study of fossil remains in the
long series of rock-formations enforces this view. In
the whole epoch of human history, and far back into
prehistoric times during which man existed on the
earth, although several animals have become extinct,
yet there is no proof that any new one has been
developed. But this human era, so far as yet known,
going back certainly to the glacial epoch and almost
certainly to pre-glacial times, cannot be estimated at
less than a million, some think even several million
years ; and as there have certainly been some con
siderable alterations of level, excavation of valleys,
deposits of great beds of gravel, and other superficial
changes during this period, some kind of a scale of
measurement of geological time has been obtained,
by comparison with the very minute changes that
have occurred during the historical period. This
scale is admittedly a very imperfect one, but it is
better than none at all ; and it is by comparing these
small changes with the far greater ones which have
occurred during every successive step backward in
geological history that these estimates of geological
time have been arrived at. They are also supported
xiv.] THE ONLY HABITABLE PLANET 277
by the palaeontologists, to whom the vast panorama
of successive forms of life is an ever-present reality.
Directly they pass into the latest stage of the Tertiary
period the Pliocene of Sir Charles Lyell--all over
the world new forms of life appear which are evidently
the forerunners of many of our still existing species ;
and as they go a little further back, into the Miocene,
there are indications of a warmer climate in Europe,
and large numbers of mammals resembling those which
now inhabit the tropics, but of quite distinct species
and often of distinct genera and families. And here,
though we have only reached to about the middle of
the Tertiary period, the changes in the forms of life,
in the climate, and in the land-surfaces are so great
when compared with the very minute changes during
the human epoch, as to require us to multiply the
time elapsed many times over. Yet the whole of the
Tertiary period, during which all the great groups of
the higher animals were developed from a compara
tively few generalised ancestral forms, is yet the
shortest by far of the three great geological periods
the Mesozoic or Secondary, having been much longer,
with still vaster changes both in the earth s crust and
in the forms of life ; while the Palaeozoic or Primary,
which carries us back to the earliest forms of life as
represented by fossilised remains, is always estimated
by geologists to be at least as long as the other two
combined, and probably very much longer.
From these various considerations most geologists
who have made any estimates of geological time from
the period of the earliest fossiliferous rocks, have
arrived at the conclusion that about 200 millions of
years are required. But from the variety of the
278 MAN S PLACE IN THE UNIVERSE [CHAP.
forms of life at this early period it is concluded that a
very much greater duration is needed for the whole
epoch of life. Speaking of the varied marine fauna
of the Cambrian period, the late Professor Ramsay
says : In this earliest known varied life we find no
evidence of its having lived near the beginning of the
zoological series. In a broad sense, compared with
what must have gone before, both biologically and
physically, all the phenomena connected with this
old period seem, to my mind, to be of quite a recent
description ; and the climates of seas and lands were
of the very same kind as those the world enjoys at
the present day. And Professor Huxley held very
similar views when he declared : If the very small
differences which are observable between the croco
diles of the older Secondary formations and those of
the present day furnish any sort of an approximation
towards an estimate of the average rate of change
among reptiles, it is almost appalling to reflect how
far back in Palaeozoic times we must go before we can
hope to arrive at that common stock from which the
crocodiles, lizards, Ornithoscelida, and Plesiosauria,
which had attained so great a development in the
Triassic epoch, must have been derived.
Now, in opposition to these demands of the geolo
gists, in which they are almost unanimous, the most
celebrated physicists, after full consideration of all
possible sources of the heat of the sun, and knowing
the rate at which it is now expending heat, declare,
with complete conviction, that our sun cannot have
existed as a heat-giving body for so long a period,
and they would therefore reduce the time during
which life can possibly have existed on the earth to
xiv.] THE ONLY HABITABLE PLANET 279
about one-fourth of that demanded by geologists. In
one of his latest articles, Lord Kelvin says : * Now
we have irrefragable dynamics proving that the whole
life of our sun as a luminary is a very moderate
number of million years, probably less than 50 million,
possibly between 50 and 100 (PkiL Mag., vol. ii.,
Sixth Sen, p. 175, Aug. 1901). In my Island Life
(chap. X.) I have myself given reasons for thinking
that both the stratigraphical and biological changes
may have gone on more quickly than has been sup
posed, and that geological time (meaning thereby
the time during which the development of life upon
the earth has been going on) may be reduced so as
possibly to be brought within the maximum period
allowed by physicists ; but there will certainly be no
time to spare, and any planets dependent on our
sun whose period of habitability is either past or
to come, cannot possibly have, or have had, sufficient
time for the necessarily slow evolution of the higher
life-forms. Again, all physicists hold that the sun
is now cooling, and that its future life will be much
less than its past. In a lecture at the Royal Institu
tion (published in Nature Series, in 1889), Lord
Kelvin says : * It would, I think, be exceedingly
rash to assume as probable anything more than
twenty million years of the sun s light in the past
history of the earth, or to reckon more than five or
six million years of sunlight for time to come.
These extracts serve to show that, unless either
geologists or physicists are very far from any ap
proach to accuracy in their estimates of past or
future age of the sun, there is very great difficulty in
bringing them into harmony or in accounting for the
280 MAN S PLACE IN THE UNIVERSE [CHAP.
actual facts of the geological history of the earth and
of the whole course of life-development upon it. We
are, therefore, again brought to the conclusion that
there has been, and is, no time to spare ; that the
whole of the available past life-period of the sun has
been utilised for life-development on the earth, and
that the future will be not much more than may be
needed for the completion of the grand drama of
human history, and the development of the full possi
bilities of the mental and moral nature of man.
We have here, then, a very powerful argument,
from a different point of view than any previously
considered, for the conclusion that man s place in the
solar system is altogether unique, and that no other
planet either has developed or can develop such a
full and complete life-series as that which the earth
has actually developed. Even if the conditions had
been more favourable than they are seen to be
on other planets, Mercury, Venus, and Mars could
not possibly have preserved equability of conditions
long enough for life-development, since for unknown
ages they must have been passing slowly towards
their present wholly unsuitable conditions ; while
Jupiter and the planets beyond him, whose epoch of
life-development is supposed to be in the remote
future when they shall have slowly cooled down to
habitability, will then be still more faintly illuminated
and scantily warmed by a rapidly cooling sun, and
may thus become, at the best, globes of solid ice.
This is the teaching of science of the best science
of the twentieth century. Yet we find even astrono
mers who, more than any other exponents of science,
should give heed to the teachings of the sister-
xiv.] THE ONLY HABITABLE PLANET 281
sciences to which they owe so much, indulging in
such rhapsodies as the following : In our solar
system, this little earth has not obtained any special
privileges from Nature, and it is strange to wish to
confine life within the circle of terrestrial chemistry.
And again : Infinity encompasses us on all sides, life
asserts itself, universal and eternal, our existence is
but a fleeting moment, the vibration of an atom in a
ray of the sun, and our planet is but an island floating
in the celestial archipelago, to which no thought will
ever place any bounds. *
In place of such wild and whirling words, I have
endeavoured to state the sober conclusions of the
best workers and thinkers as to the nature and origin
of the world in which we live, and of the universe
which on all sides surrounds us. I leave it to my
readers to decide which is the more trustworthy
guide.
1 M. Camilla Flammarion, in Knowledge, June 1903.
CHAPTER XV
THE STARS HAVE THEY PLANETARY SYSTEMS?
ARE THEY BENEFICIAL TO US?
MOST of the writers on the Plurality of Worlds,
from Fontenelle to Proctor, taking into consideration
the enormous number of the stars and their apparent
uselessness to our world, have assumed that many
of them must have systems of planets circling round
them, and that some of these planets, at all events,
must possess inhabitants, some, perhaps, lower, but
others no doubt higher than ourselves. One of our
well-known modern astronomers, writing only ten
years ago, adopts the same view. He says : The
suns which we call stars were clearly not created
for our benefit. They are of very little practical
use to the earth s inhabitants. They give us very
little light ; an additional small satellite one con
siderably smaller than the moon would have been
much more useful in this respect than the millions
of stars revealed by the telescope. They must there
fore have been formed for some other purpose. . . .
We may therefore conclude, with a high degree of
probability, that the stars at least those with spectra
of the solar type form centres of planetary systems
somewhat similar to our own. 1 The author then
1 The Worlds of Space , by J. E. Gore, chapter iii.
282
xv.] THE STARS IN RELATION TO LIFE 283
discusses the conditions necessary for life analogous
to that of our earth, as regards temperature, rotation,
mass, atmosphere, water, etc., and he is the only
writer I have met with who has considered these
conditions ; but he touches on them very briefly,
and he arrives at the conclusion that, in the case
of the stars of solar type, it is probable that one
planet, situated at a proper distance, would be fitted
to support life. He estimates roughly that there are
about ten million stars of this type, that is, closely
resembling our sun, and that if only one in ten of
these has a planet at the proper distance and properly
constituted in other respects, there will be one million
worlds fitted for the support of animal life. He
therefore concludes that there are probably many stars
having life-bearing planets revolving round them.
There are, however, many considerations not taken
account of by this writer which tend to reduce very
considerably the above estimate. It is now known
that immense numbers of the stars of smaller magni
tudes are nearer to us than are the majority of the
stars of the first and second magnitudes, so that it
is probable that these, as well as a considerable pro
portion of the very faint telescopic stars, are really
of small dimensions. We have evidence that many
of the brightest stars are much larger than our
sun, but there are probably ten times as many that
are much smaller. We have seen that the whole of
the past light and heat-giving duration of our sun
has, according to the best authorities, been only just
sufficient for the development of life upon the earth.
But the duration of a sun s heat-giving power will
depend mainly upon its mass, together with its con-
284 MAN S PLACE IN THE UNIVERSE [CHAP.
stituent elements. Suns which are much smaller
than ours are, therefore, from that cause alone,
unsuited to give adequate light and heat for a
sufficient time, and with sufficient uniformity, for
life-development on planets, even if they possess
any at the right distance, and with the extensive
series of nicely adjusted conditions which I have
shown to be necessary.
Again, we must, probably, rule out as unfitted for
life-development the whole region of the Milky Way,
on account of the excessive forces there in action, as
shown by the immense size of many of the stars,
their enormous heat-giving power, the crowding of
stars and nebulous matter, the great number of star-
clusters, and, especially, because it is the region of
new stars/ which imply collisions of masses of
matter sufficiently large to become visible from the
immense distance we are from them, but yet exces
sively small as compared with suns the duration of
whose light is to be measured by millions of years.
Hence the Milky Way is the theatre of extreme
activity and motion ; it is comparatively crowded
with matter undergoing continual change, and is
therefore not sufficiently stable for long periods to
be at all likely to possess habitable worlds.
We must, therefore, limit our possible planetary
systems suitable for life-development, to stars situated
inside the circle of the Milky Way and far removed
from it- -that is, to those composing the solar cluster.
These have been variously estimated to consist of
a few hundred or many thousand stars- -at all
events to a very small number as compared with
the hundreds of millions in the whole stellar
xv.] THE STARS IN RELATION TO LIFE 285
universe. But even here we find that only a por
tion are probably suitable. Professor Newcomb
arrives at the conclusion as have some other astro
nomers- -that the stars in general have a much
smaller mass in proportion to the light they give
than our sun has ; and, after an elaborate discus
sion, he finally concludes that the brighter stars
are, on the average, much less dense than our sun.
In all probability, therefore, they cannot give light
and heat for so long a period, and as this period
in the case of our sun has only been just sufficient,
the number of suns of the solar type and of a sufficient
mass may be very limited. Yet further, even among
stars having a similar physical constitution to our
sun, and of an equal or greater mass, only a portion
of their period of luminosity would be suitable for
the support of planetary life. While they are in
process of formation by accretions of solid or gaseous
masses, they would be subject to such fluctuations of
temperature, and to such catastrophic outbursts when
any larger mass than usual was drawn towards them,
that the whole of this period perhaps by far the
longest portion of their existence must be left out
of the account of planet-producing suns. Yet all
these are to us stars of various degrees of brilliancy.
It is almost certain that it is only when the growth
of a sun is nearly completed, and its heat has attained
a maximum, that the epoch of life-development is
likely to begin upon any planets it may possess at
the most suitable distance, and upon which all the
requisite conditions should be present.
It may be said that there are great numbers of
stars beyond our solar cluster and yet within the
286 MAN S PLACE IN THE UNIVERSE [CHAP.
circle of the Milky Way, as well as others towards
the poles of the Milky Way, which I have not here
referred to. But of these regions very little is known,
because it is impossible to tell whether stars in these
directions are situated in the outer portion of the
solar cluster, or in the regions beyond it. Some
astronomers appear to think that these regions may
be nearly empty of stars, and I have endeavoured
to represent what seems to be the general view on
this very difficult subject in the two diagrams of the
stellar universe at pp. 300, 301. The regions beyond
our cluster and above or below the plane of the Milky
Way are those where the small irresolvable nebulae
abound, and these may indicate that sun-formation
is not yet active in those regions. The two charts
of Nebulae and Clusters at the end of the volume
illustrate, and perhaps tend to support this view.
DOUBLE AND MULTIPLE STAR SYSTEMS
We have already seen, in our sixth chapter, how
rapid and extraordinary has been the discovery of
what are termed spectroscopic binaries pairs of stars
so close together as to appear like a single star in
the most powerful telescopes. The systematic search
for such stars has only been carried on for a few
years, yet so many have been already found, and
their numbers are increasing so rapidly, as to quite
startle astronomers. One of the chief workers in this
field, Professor Campbell of the Lick Observatory,
has stated his opinion that, as accuracy of measure
ment increases, these discoveries will go on till-
4 the star that is not a spectroscopic binary will prove
xv.] THE STARS IN RELATION TO LIFE 287
to be the rare exception, and other astronomers of
eminence have expressed similar views. But these
close revolving star-systems are generally admitted
to be out of the category of life-producing suns.
The tidal disturbances mutually produced must be
enormous, and this must be inimical to the develop
ment of planets, unless they were very close to each
sun, and thus in the most unfavourable position for life.
We thus see that the result of the most recent
researches among the stars is entirely opposed to
the old idea that the countless myriads of stars all
had planets circulating round them, and that the
ultimate purpose of their existence was, that they
should be supporters of life, as our sun is the sup
porter of life upon the earth. So far is this from
being the case, that vast numbers of stars have to
be put aside as wholly unfitted for such a purpose ;
and when by successive eliminations of this nature
we have reduced the numbers which may possibly
be available to a few millions, or even to a few
thousands, there comes the last startling discovery,
that the entire host of stars is found to contain
binary systems in such rapidly increasing numbers,
as to lead some of the very first astronomers of the
day to the conclusion that single stars may some
day be found to be the rare exception! But this
tremendous generalisation would, at one stroke, sweep
away a large proportion of the stars which other suc
cessive disqualifications had spared, and thus leave
our sun, which is certainly single, and perhaps two
or three companion orbs, alone among the starry
host as possible supporters of life on some one of
the planets which circulate around them.
288 MAN S PLACE IN THE UNIVERSE [CHAP.
But we do not really know that any such suns
exist. If they exist we do not know that they
possess planets. If any do possess planets these
may not be at the proper distance, or be of the
proper mass, to render life possible. If these
primary conditions should be fulfilled, and if there
should possibly be not only one or two, but a dozen
or more that so far fulfil the first few conditions
which are essential, what probability is there that all
the other conditions, all the other nice adaptations,
all the delicate balance of opposing forces that we
have found to prevail upon the earth, and whose
combination here is due to exceptional conditions
which exist in the case of no other known planet
should all be again combined in some of the possible
planets of these possibly existing suns ?
I submit that the probability is now all the other
way. So long as we could assume that all the stars
might be, in all essentials, like our sun, it seemed
almost ludicrous to suppose that our sun alone should
be in a position to support life. But when we find
that enormous classes like the gaseous stars of small
density, the solar stars while increasing in size and
temperature, the stars which are much smaller than
our sun, the nebulous stars, probably all the stars of
the Milky Way, and lastly that enormous class of
spectroscopic doubles veritable Aaron s rods which
threaten to swallow up all the rest that all these
are for various reasons unlikely to have attendant
planets adapted to develop life, then the probabili
ties seem to be enormously against there being any
considerable number of suns possessing attendant
habitable earths. Just as the habitability of all the
xv.] THE STARS IN RELATION TO LIFE 289
planets and larger satellites, once assumed as so
extremely probable as to amount almost to a
certainty, is now generally given up, so that in
speculating on life in stellar systems Mr. Gore
assumes that only one planet to each sun can be
habitable ; in like manner it may, and I believe will,
turn out, that of all the myriad stars, the more we
learn about them, the smaller and smaller will become
the scanty residue which, with any probability, we
can suppose to illuminate and vivify habitable earths.
And when with this scanty probability we combine
the still scantier probability that any such planet
will possess simultaneously, and for a sufficiently
long period, all the highly complex and delicately
balanced conditions known to be essential for a full
life-development, the conception that on this earth
alone has such development been completed will not
seem so wildly improbable a conjecture as it has
hitherto been held to be.
ARE THE STARS BENEFICIAL TO Us?
When I suggested in my first publication on this
subject that some emanations from the stars might
be beneficial or injurious, and that a central position
might be essential in order to render these emana
tions equable, one of my astronomical critics laughed
the idea to scorn, and declared that we might
wander into outer space without losing anything
more serious than we lose when the night is cloudy
and we cannot see the stars. How my critic knows
1 The Fortnightly Review, April 1903, p. 60.
T
290 MAN S PLACE IN THE UNIVERSE [CHAP.
that this is so he does not tell us. He states it
positively, with no qualification, as if it were an
established fact. It may be as well to inquire,
therefore, if there is any evidence bearing upon the
point at issue.
Astronomers are so fully occupied with the vast
number and variety of the phenomena presented by
the stellar universe and the various difficult problems
arising therefrom, that many lesser but still in
teresting inquiries have necessarily received little
attention. Such a minor problem is the determina
tion of how much heat or other active radiation we
receive from the stars ; yet a few observations have
been made with results that are of considerable
interest.
In the years 1900 and 1901 Mr. E. F. Nichols of the
Yerkes Observatory made a series of experiments
with a radiometer of special construction, to deter
mine the heat emitted by certain stars. The result
arrived at was, that Vega gave about 2 oooo^ o^TT
of the heat of a candle at one metre distance, and
Arcturus about 2*2 times as much.
In 1895 and 1896 Mr. G. M. Minchin made a
series of experiments on the Electrical Measurement
of Starlight, by means of a photo-electric cell of
peculiar construction which is sensitive to the whole
of the rays in the spectrum, and also to some of the
ultra-red and ultra-violet rays. Combined with this
was a very delicate electrometer. The telescope
employed to concentrate the light was a reflector of
two feet aperture. Mr. Minchin was assisted in the
experiments by the late Professor G. F. Fitzgerald,
F.R.S., of Trinity College, Dublin, which may be con-
xv.] THE STARS IN RELATION TO LIFE 291
sidered a guarantee of the accuracy of the observa
tions. The following are the chief results obtained :-
Deflection
Light
EM F
Source of Light.
in
Millimetres.
in
Candles.
IV A A.
Volts.
1896
Candle at 10 feet distance,
1870
Betelgeuse (0-9 mag.),
12*80
0-685
0*026
Aldebaran (1*1 mag.),
5 2i
0*279
OI2
Procyon (0-5 mag.),
4-89
O 26l
O OII
Alpha Cygni (1-3 mag.), .
4-90
0*262
0*011
Polaris (2*1 mag.), .
3-10
0*166
O Ooy
i volt.
432-00
1895
Arcturus (0*3 mag.),
8*2
roi
O Oig
Vega (o i mag.),
*s
1*42
0*026
Candle at 10 feet, .
8-1
N.B. The standard candle shone directly on the cell, whereas
the star s light was concentrated by a 2-foot mirror.
The sensitive surface on which the light of the stars
was concentrated was -^ inch in diameter. We must
therefore diminish the amount of candle light in this
table in the proportion of the square of the diameter of
the mirror (in -^j of an inch) to one, equal to ^Jitzny-
If we make the necessary reduction in the case of
Vega, and also equalise the distance at which the
candle was placed, we find the following result :
Observer.
Minchin.
Nichols.
Star.
Vega.
it
Candle power at 10 ft.
i
This enormous difference in the result is no doubt
largely due to the fact that Mr. Nichols s apparatus
measured heat alone, whereas Mr. M inchin s cell
measured almost all the rays. And this is further
292 MAN S PLACE IN THE UNIVERSE [CHAP.
shown by the fact that, whereas Mr. Nichols found
Arcturus a red star, hotter than Vega a white one,
Mr. Minchin, measuring also the light-giving and
some of the chemical rays, found Vega considerably
more energetic than Arcturus. These comparisons
also suggest that other modes of measurement might
give yet higher results, but it will no doubt be urged
that such minute effects must necessarily be quite
inoperative upon the organic world.
There are, however, some considerations which
tend the other way. Mr. Minchin remarks on the
unexpected fact that Betelgeuse produces more than
double the electrical energy of Procyon, a much
brighter star. This indicates that many of the stars
of smaller visual magnitudes may give out a large
amount of energy, and it is this energy, which we
now know can take many strange and varied forms,
that would be likely to influence organic life. And
as to the quantity being too minute to have any
effect, we know that the excessively minute amount of
light from the very smallest telescopic stars produces
such chemical changes on a photographic plate as to
form distinct images, with comparatively small lenses
or reflectors and with an exposure of two or three
hours. And if it were not that the diffused light of
the surrounding sky also acts upon the plate and
blurs the faint images, much smaller stars could be
photographed.
We know that not all the rays, but a portion only,
are capable of producing these effects ; we know also
that there are many kinds of radiation from the stars,
and probably some yet undiscovered comparable with
the X rays and other new forms of radiation. We
xv.] THE STARS IN RELATION TO LIFE 293
must also remember the endless variety and the
extreme instability of the protoplasmic products in
the living organism, many of which are perhaps as
sensitive to special rays as is the photographic plate.
And we are not here limited to action for a few
minutes or a few hours, but throughout the whole
night and day, and continued whenever the sky is
clear for months or years. Thus the cumulative
effect of these very weak radiations may become
important. It is probable that their action would be
most influential on plants, and here we find all the
conditions requisite for its accumulation and utilisa
tion in the large amount of leaf-surface exposed to it.
A large tree must present some hundreds of super
ficial feet of receptive surface, while even shrubs and
herbs often have a leaf-area of greater superficial ex
tent than the object-glasses of our largest telescopes.
Some of the highly complex chemical processes that
go on in plants may be helped by these radiations,
and their action would be increased by the fact that,
coming from every direction over the whole surface
of the heavens, the rays from the stars would be
able to reach and act upon every leaf of the densest
masses of foliage. The large amount of growth that
takes place at night may be in part due to this
agency.
Of course all this is highly speculative ; but I
submit, in view of the fact that the light of the very
faintest stars does produce distinct chemical changes,
that even the very minute heat-effects are measure-
able, as well as the electro-motive forces caused
by them ; and further, that when we consider the
millions, perhaps hundreds of millions of stars, all
294 MAN S PLACE IN THE UNIVERSE [CHAP.XV.
acting simultaneously on any organism which may
be sensitive to them, the supposition that they do
produce some effect, and possibly a very important
effect, is not one to be summarily rejected as alto
gether absurd and not worth inquiring into.
It is not, however, these possible direct actions of
the stars upon living organisms to which I attach
much weight as regards our central position in the
stellar universe. Further consideration of the subject
has convinced me that the fundamental importance
of that position is a physical one, as has already been
suggested by Sir Norman Lockyer and some other
astronomers. Briefly, the central position appears
to be the only one where suns can be sufficiently
stable and long-lived to be capable of maintaining the
long process of life-development in any of the planets
they may possess. This point will be further de
veloped in the next (and concluding) chapter.
CHAPTER XVI
STABILITY OF THE STAR-SYSTEM : IMPORTANCE OF OUR
CENTRAL POSITION : SUMMARY AND CONCLUSION
ONE of the greatest difficulties with regard to the
vast system of stars around us is the question of
its permanence and stability, if not absolutely and
indefinitely, yet for periods sufficiently long to allow
for the many millions of years that have certainly
been required for our terrestrial life-development.
This period, in the case of the earth, as I have
sufficiently shown, has been characterised throughout
by extreme uniformity, while a continuance of that
uniformity for a few millions of years in the future is
almost equally certain.
But our mathematical astronomers can find no
indications of such stability of the stellar universe
as a whole, if subject to the law of gravitation alone.
In reply to some questions on this point, my friend
Professor George Darwin writes as follows:- - A
symmetrical annular system of bodies might revolve
in a circle with or without a central body. Such a
system would be unstable. If the bodies are of
unequal masses and not symmetrically disposed, the
break - up of the system would probably be more
rapid than in the ideal case of symmetry.
This would imply that the great annular system of
295
296 MAN S PLACE IN THE UNIVERSE [CHAP.
the Milky Way is unstable. But if so, its existence
at all is a greater mystery than ever. Although in
detail its structure is very irregular, as a whole it is
wonderfully symmetrical ; and it seems quite impos
sible that its generally circular ring- like form can be
the result of the chance aggregation of matter from
any pre-existing different form. Star-clusters are
equally unstable, or, rather, nothing is known or can
be predicated about their stability or instability,
according to Professors Newcomb and Darwin.
Mr. E. T. Whittaker (Secretary to the Royal
Astronomical Society), to whom Professor G. Darwin
sent my questions, writes : I doubt whether the
principal phenomena of the stellar universe are
consequences of the law of gravitation at all. I have
been working myself at spiral nebulae, and have got
a first approximation to an explanation- -but it is
electro-dynamical and not gravitational. In fact, it
may be questioned whether, for bodies of such
tremendous extent as the Milky Way or nebulas,
the effect which we call gravitation is given by
Newton s law ; just as the ordinary formulae of
electrostatic attraction break down when we consider
charges moving with very great velocities.
Accepting these statements and opinions of two
mathematicians who have given special attention to
similar problems, we need not limit ourselves to the
laws of gravitation as having determined the present
form of the stellar universe ; and this is the more
important because we may thus escape from a
conclusion which many astronomers seem to think
inevitable, viz. that the observed proper motions of
the stars cannot be explained by the gravitative
xvi.] STABILITY OF THE STAR-SYSTEM 297
forces of the system itself. In chapter VIII. of this
work I have quoted Professor Newcomb s calculation
as to the effect of gravitation in a universe of 100
million stars, each five times the mass of our
sun, and spread over a sphere which it would take
light 30,000 years to cross ; then, a body falling
from its outer limits to the centre could at the utmost
acquire a velocity of twenty-five miles a second ;
and therefore, any body in any part of such a
universe having a greater velocity would pass away
into infinite space. Now, as several stars have,
it is believed, much more than this velocity, it
follows not only that they will inevitably escape
from our universe, but that they do not belong to it,
as their great velocity must have been acquired
elsewhere. This seems to have been the idea of
the astronomer who stated that, even at the very
moderate speed of our sun, we should in five million
years be deep in the actual stream of the Milky
Way. To this I have already sufficiently replied ;
but I now wish to bring before my readers an
excellent illustration of the importance of the late
Professor Huxley s remark, that the results you got
out of the mathematical mill depend entirely on
what you put into it.
In the Philosophical Magazine (January 1902) is
a remarkable article by Lord Kelvin, in which he
discusses the very same problem as that which
Professor Newcomb had discussed at a much earlier
date, but, starting from different assumptions,
equally based on ascertained facts and probabilities
deduced from them, brings out a very different
result.
298 MAN S PLACE IN THE UNIVERSE [CHAP.
Lord Kelvin postulates a sphere of such a radius
that a star at its confines would have a parallax of
one-thousandth part of a second (o"-ooi), equivalent
103215 light-years. Uniformly distributed through
this sphere there is matter equal in mass to 1000
million suns like ours. If this matter becomes
subject to gravitation, it all begins to move at first
with almost infinite slowness, especially near its
centre ; but nevertheless, in twenty-five million years
many of these suns would have acquired velocities of
from twelve to twenty miles a second, while some
would have less and some probably more than
seventy miles a second. Now such velocities as
these agree generally with the measured velocities
of the stars, hence Lord Kelvin thinks there may be
as much matter as 1000 million suns within the above-
named distance. He then states that if we suppose
there to be 10,000 million suns within the same
sphere, velocities would be produced very much
greater than the known star-velocities ; hence it is
probable that there is very much less matter than
10,000 million times the sun s mass. He also states
that if the matter were not uniformly distributed
within the sphere, then, whatever was the irregu
larity, the acquired motions would be greater ; again
indicating that the 1000 million suns would be ample
to produce the observed effects of stellar motion.
He then calculates the average distance apart of each
of the 1000 million stars, which he finds to be about
300 millions of millions of miles. Now the nearest
star to our sun is about twenty-six million million of
miles distant, and, as the evidence shows, is situated
in the denser part of the solar cluster. This gives
XVL] STABILITY OF THE STAR-SYSTEM 299
ample allowance for the comparative emptiness of
the space between our cluster and the Milky Way,
as well as of the whole region towards the poles of
the Milky Way (as shown by the diagrams in
chapter IV.), while the comparative density of exten
sive portions of the Galaxy itself may serve to make
up the average.
Now, previous writers have come to a different
conclusion from the same general line of argument,
because they have started with different assumptions.
Professor Newcomb, whose statement made some
years back is usually followed, assumed 100 million
stars each five times as large as our sun, equal to
500 million suns in all, and he distributed them
equally throughout a sphere 30,000 light-years in
diameter. Thus he has half the amount of matter
assumed by Lord Kelvin, but nearly five times the
extent, the result being that gravity could only
produce a maximum speed of twenty-five miles a
second ; whereas on Lord Kelvin s assumption a
maximum speed of seventy miles a second would be
produced, or even more. By this latter calculation
we find no insuperable difficulty in the speed of any
of the stars being beyond the power of gravitation to
produce, because the rates here given are the direct
results of gravitation acting on bodies almost
uniformly distributed through space. Irregular dis
tribution, such as we see everywhere in the universe,
might lead to both greater and less velocities ; and
if we further take account of collisions and near
approaches of large masses resulting in explosive
disruptions, we might have almost any amount of
motion as the result, but as this motion would be
300 MAN S PLACE IN THE UNIVERSE [CHAP,
produced by gravitation within the system, it could
equally well be controlled by gravitation.
In order that my readers may better understand
the calculations of Lord Kelvin, and also the general
DIAGRAM OF STELLAR UNIVERSE (Plan).
1. Central part of Solar Cluster.
2. Sun s Orbit (Black spot).
3. Outer limit of Solar Cluster.
4. Milky Way.
conclusions of astronomers as to the form and dimen
sions of the stellar universe, I have drawn two
diagrams, one showing a plan on the central plane
xvi.] STABILITY OF THE STAR-SYSTEM 301
of the Milky Way, the other a section through its
poles. Both are on the same scale, and they show
the total diameter across the Milky Way as being
3600 light-years, or about half that postulated by
Lord Kelvin for his hypothetical universe. I do this
DIAGRAM OF STELLAR UNIVERSE (Section).
...... Pole of Galaxy. .
^j&?\^^:^ -^* : :^fc *^^i
...,V^v!>|^ ^ : ^>;V \\ -- : :;:- ."- ;!> ::>;:?&:,-.:
-v ^-.?^;^ /. - . ~ JfelmleR aJ>uruldn^ : : : \- -
Section through Poles of Milky Way.
because the dimensions given by him are those which
are sufficient to lead to motions near the centre such as
the stars now possess in a minimum period of twenty-
five million years after the initial arrangement he
supposes, at which later epoch which we are now
supposed to have reached, the whole system would
of course be greatly reduced in extent by aggrega-
302 MAN S PLACE IN THE UNIVERSE [CHAP.
tions towards and near the centre. These dimen
sions also seem to accord sufficiently with the actual
distances of stars as yet measured. The smallest
parallax which has been determined with any certainty,
according to Professor Newcomb s list, is that of
Gamma Cassiopeise, which is one-hundredth of a
second (o"-oi), while Lord Kelvin gives none smaller
than o"-O2, and these will all be included within the
solar cluster as I have shown it.
It must be clearly understood that these two
illustrations are merely diagrams to show the main
features of the stellar universe according to the best
information available, with the proportionate dimen
sions of these features, so far as the facts of the
distribution of the stars and the views of those
astronomers who have paid most attention to the
subject can be harmonised. Of course it is not sug
gested that the whole arrangement is so regular as
here shown, but an attempt has been made by means
of the dotted shading to represent the comparative
densities of the different portions of space around us,
and a few remarks on this point may be needed.
The solar cluster is shown very dense at the
central portion, occupying one-tenth of its diameter,
and it is near the outside of this dense centre that
our sun is supposed to be situated. Beyond this
there seems to be almost a vacuity, beyond which
again is the outer portion of the cluster consisting of
comparatively thinly scattered stars, thus forming a
kind of ring-cluster, resembling in shape the beauti
ful ring-nebula in Lyra, as has been suggested by
several astronomers. There is some direct evidence
for this ring-form. Professor Newcomb in his recent
xvi.] STABILITY OF THE STAR-SYSTEM 303
book on The Stars gives a list of all stars of which the
parallax is fairly well known. These are sixty-nine
in number ; and on arranging them in the order of the
amount of their parallax, I find that no less than thirty-
five of them have parallaxes between o"- 1 and o"-4 of a
second, thus showing that they constitute part of the
dense central mass ; while three others, from o"-4 to
o"-75, indicate those which are our closest companions
at the present time, but still at an enormous distance.
Those which have parallaxes of less than the tenth
and down to one-hundredth of a second are only
thirty-one in all ; but as they are spread over a
sphere ten times the diameter, and therefore a thou
sand times the cubic content of the sphere containing
those above one-tenth of a second, they ought to be
immensely more numerous even if very much more
thinly scattered. The interesting point, however,
is, that till we get down to a parallax of o"-o6, there
are only three stars as yet measured, whereas those
between o"-O2 and o"-o6, an equal range of parallax,
are twenty-six in number, and as these are scat
tered in all directions they indicate an almost
vacant space followed by a moderately dense outer
ring.
In the enormous space between our cluster and
the Milky Way, and also above and below its plane
to the poles of the Galaxy, stars appear to be very
thinly scattered, perhaps more densely in the plane
of the Milky Way than above and below it where the
irresolvable nebulae are so numerous ; and there may
not improbably be an almost vacant space beyond
our cluster for a considerable distance, as has been
supposed, but this cannot be known till some means
304 MAN S PLACE IN THE UNIVERSE [CHAP.
are discovered of measuring parallaxes of from one-
hundredth to one five-hundredth of a second.
These diagrams also serve to indicate another
point of considerable importance to the view here
advocated. By placing the solar system towards
the outer margin of the dense central portion of
the solar cluster (which may very possibly include a
large proportion of dark stars and thus be much
more dense towards the centre than it appears to
us), it may very well be supposed to revolve, with
the other stars composing it, around the centre of
gravity of the cluster, as the force of gravity towards
that centre might be perhaps twenty or a hundred
times greater than towards the very much less dense
and more remote outer portions of the cluster. The
sun, as indicated on the diagrams, is about thirty
light-years from that centre, corresponding to a
parallax of a little more than one-tenth of a second,
and an actual distance of 190 millions of millions of
miles, equal to about 70,000 times the distance of the
sun from Neptune. Yet we see that this position
is so little removed from the exact centre of the whole
stellar universe, that if any beneficial influences are
due to that central position in regard to the Galaxy,
it will receive them perhaps to as full an extent as if
situated at the actual centre. But if it is situated as
here shown, there is no further difficulty as to its
proper motion carrying it from one side to the other
of the Milky Way in less time than has been required
for the development of life upon the earth. And if
the solar cluster is really sub-globular, and suffici
ently condensed to serve as a centre of gravity
for the whole of the stars of the cluster to revolve
XVL] IMPORTANCE OF CENTRAL POSITION 305
around, all the component stars which are not situ
ated in the plane of its equator (and that of the Milky
Way) must revolve obliquely at various angles up to
an angle of 90. These numerous diverging motions,
together with the motions of the nearer stars outside
o
the cluster, some of which may revolve round other
centres of gravity made up largely of dark bodies,
would perhaps sufficiently account for the apparent
random motions of so many of the stars.
UNIFORM HEAT-SUPPLY DUE TO CENTRAL POSITION
We now come to a point of the greatest interest as
regards the problem we are investigating. We have
seen how great is the difference in the estimates of
geologists and those of physicists as to the time
that has elapsed during the whole development of
life. But the position we have now found for the
sun, in the outer portion of the central star-cluster,
may afford a clue to this problem. What we require
is, some mode of keeping up the sun s heat during
the enormous geological periods in which we have
evidence of a wonderful uniformity in the earth s
temperature, and therefore in the sun s heat-emission.
The great central ring-cluster with its condensed
central mass, which presumably has been forming for
a much longer period than our sun has been giving
heat to the earth, must during all this time have
been exerting a powerful attraction on the diffused
matter in the spaces around it, now apparently almost
void as compared with what they may have been.
Some scanty remnants of that matter we see in the
numerous meteoric swarms which have been drawn
into our system. A position towards the outside of
u
306 MAN S PLACE IN THE UNIVERSE [CHAP.
this central aggregation of suns would evidently be
very favourable for the growth by accretion of any
considerable mass. The enormous distance apart of
the outer components (the outer ring) of the cluster
would allow a large amount of the inflowing meteoritic
matter to escape them, and the larger suns situated
near the surface of the inner dense cluster would
draw to themselves the greater part of this matter. 1
The various planets of our system were no doubt
built up from a portion of the matter that flowed in
near the plane of the ecliptic, but much of that which
came from all other directions would be drawn to
wards the sun itself or to its neighbouring suns.
Some of this would fall directly into it ; other masses
coming from different directions and colliding with
each other would have their motion checked, and
thus again fall into the sun ; and so long as the
matter falling in were not in too large masses, the
slow additions to the sun s bulk and increase of its
heat would be sufficiently gradual to be in no way
prejudicial to a planet at the earth s distance.
The main point I wish to suggest here is, that by
far the greater portion of the matter of the whole
stellar universe has, either through gravitation or in
combination with electrical forces, as suggested by
1 Since writing this chapter I have seen a paper by Luigi d Auria
dealing mathematically with Stellar Motion, etc., 3 and am pleased to
see that, from quite different considerations, he has found it necessary
to place the solar system at a distance from the centre not very much
more remote than the position I have given it. He says : We have
good reasons to suppose that the solar system is rather near the centre
of the Milky Way, and as this centre would, according to our hypo
thesis, coincide with the centre of the Universe, the distance of 159
light years assumed is not too great, nor can it be very much smaller.
Journal of the Franklin Institute, March 1903.
xvi.] IMPORTANCE OF CENTRAL POSITION 307
Mr. Whittaker, become drawn together into the vast
ring-formed system of the Milky Way, which is, pre
sumably, slowly revolving, and has thus been checked
in its original inflow towards the centre of mass of
the stellar universe. It has also probably drawn
towards itself the adjacent portions of the scattered
material in the spaces around it in all directions.
Had the vast mass of matter postulated by Lord
Kelvin acquired no motion of revolution, but have
fallen continuously towards the centre of mass, the
motions developed when the more distant bodies
approached that centre would have been extremely
rapid ; while, as they must have fallen in from every
direction, they would have become more and more
densely aggregated, and collisions of the most
catastrophic nature would frequently have occurred,
and this would have rendered the central portion of
the universe the least stable and the least fitted to
develop life.
But, under the conditions that actually prevail, the
very reverse is the case. The quantity of matter
remaining between our cluster and the Milky Way
being comparatively small, the aggregation into suns
has gone on more regularly and more slowly. The
motions acquired by our sun and its neighbours have
been rendered moderate by two causes: (i) their
nearness to the centre of the very slowly aggregat
ing cluster where the motion due to gravitation is
least in amount ; and (2) the slight differential attrac
tion away from the centre by the Milky Way on the
side nearest to us. Again, this protective action of
the Milky Way has been repeated, on a smaller scale,
by the formation of the outer ring of the solar cluster,
308 MAN S PLACE IN THE UNIVERSE [CHAP.
which has thus preserved the inner central cluster
itself from a too abundant direct inflow of large
masses of matter.
But although the matter composing the outer portion
of the original universe has been to a large extent
aggregated into the vast system of the Milky Way,
it seems probable, perhaps even certain, that some
portion would escape its attractive forces and would
pass through its numerous open spaces indicated
by the dark rifts, channels, and patches, as already
described and thus flow on unchecked towards the
centre of mass of the whole system. The quantity
of matter thus reaching the central cluster from the
enormously remote spaces beyond the Milky Way
might be very small in comparison with what was
retained to build up that wonderful star-system ; but
it might yet be so large in total amount as to play an
important part in the formation of the central group
of suns. It would probably flow inwards almost con
tinuously, and when it ultimately reached the solar
cluster, it would have attained a very high velocity.
If, therefore, it were widely diffused, and consisted of
masses of small or moderate size as compared with
planets or stars, it would furnish the energy requisite
for bringing these slowly aggregating stars to the
required intensity of heat for forming luminous suns.
Here, then, I think, we have found an adequate
explanation of the very long-continued light- and heat-
emitting capacity of our sun, and probably of many
others in about the same position in the solar cluster.
These would at first gradually aggregate into con
siderable masses from the slowly moving diffused
matter of the central portions of the original universe ;
XVL] IMPORTANCE OF CENTRAL POSITION 309
but at a later period they would be reinforced by a
constant and steady inrush of matter from its very
outer regions, and therefore possessing such high
velocities as to materially aid in producing and main
taining the requisite temperature of a sun such as
ours, during the long periods demanded for continuous
life-development. The enormous extension and mass
of the original universe of diffused matter (as postu
lated by Lord Kelvin) is thus seen to be of the
greatest importance as regards this ultimate product
of evolution, because, without it, the comparatively
slow-moving and cool central regions might not have
been able to produce and maintain the requisite
energy in the form of heat ; while the aggregation
of by far the larger portion of its matter in the great
revolving ring of the galaxy was equally important,
in order to prevent the too great and too rapid inflow
of matter to these favoured regions.
It appears, then, that if we admit as probable some
such process of development as I have here indicated,
we can dimly see the bearing of all the great features
of the stellar universe upon the successful develop
ment of life. These are, its vast dimensions; the form
it has acquired in the mighty ring of the Milky Way;
and our position near to, but not exactly in, its centre.
We know that the star-system has acquired these
forms, presumably from some simple and more diffused
condition. We know that we are situated near the
centre of this vast system. We know that our sun
has emitted light and heat, almost uniformly, for
periods incompatible with rapid aggregation and the
equally rapid cooling which physicists consider inevit
able. I have here suggested a mode of development
3io MAN S PLACE IN THE UNIVERSE [CHAR
which would lead to a very slow but continuous
growth of the more central suns ; to an excessively
long period of nearly stationary heat-giving power ;
and lastly, an equally long period of very gradual
cooling a period the commencement of which our
sun may have just entered upon.
Descending now to terrestrial physics, I have
shown that, owing to the highly complex nature of
the adjustments required to render a world habitable
and to retain its habitability during the aeons of time
requisite for life-development, it is in the highest
degree improbable that the required conditions and
adaptations should have occurred in any other planets
of any other suns, which might occupy an equally
favourable position with our own, and which were of
the requisite size and heat-giving power.
Lastly, I submit that the whole of the evidence
I have here brought together leads to the conclusion
that our earth is almost certainly the only inhabited
planet in our solar system ; and, further, that there is
no inconceivability no improbability even in the
conception that, in order to produce a world that
should be precisely adapted in every detail for the
orderly development of organic life culminating in
man, such a vast and complex universe as that which
we know exists around us, may have been absolutely
required.
SUMMARY OF ARGUMENT
As the last ten chapters of this volume embody a
connected argument leading to the conclusion above
stated, it may be useful to my readers to summarise
rather fully the successive steps of this argument, the
XVL] SUMMARY OF ARGUMENT 311
facts on which it rests, and the various subsidiary
conclusions arrived at.
(1) One of the most important results of modern
astronomy is to have established the unity of the
vast stellar universe which we see around us. This
rests upon a great mass of observations, which de
monstrate the wonderful complexity in detail of the
arrangement and distribution of stars and nebulae,
combined with a no less remarkable general sym
metry, indicating throughout a single inter-dependent
system, not a number of totally distinct systems so
far apart as to have no physical relations with each
other, as was once supposed.
(2) This view is supported by numerous conver
ging lines of evidence, all tending to show that the
stars are not infinite in number, as was once generally
believed, and which view is even now advocated by
some astronomers. The very remarkable calcula
tions of Lord Kelvin, referred to in the early part
of this chapter, give a further support to this view,
since they show that if the stars extended much
beyond those we see or can obtain direct knowledge
of, and with no very great change in their average
distance apart, then the force of gravitation towards
the centre would have produced on the average more
rapid motions than the stars generally possess.
(3) An overwhelming consensus of opinion among
the best astronomers establishes the fact of our nearly
central position in the stellar universe. They all
agree that the Milky Way is nearly circular in form.
They all agree that our sun is situated almost exactly
in its medial plane. They all agree that our sun,
icrh not situated at the exact centre of the
312 MAN S PLACE IN THE UNIVERSE [CHAP.
galactic circle, is yet not very far from it, because
there are no unmistakable signs of our being nearer
to it at any one point and farther away from the
opposite point. Thus the nearly central position
of our sun in the great star-system is almost uni
versally admitted.
On the question of the solar-cluster there is more
difference of opinion ; though here, again, all are
agreed that there is such a cluster. Its size, form,
density, and exact position are somewhat uncertain,
but I have, as far as possible, been guided by the
best available evidence. If we adopt Lord Kelvin s
general idea of the gradual condensation of an
enormous diffused mass of matter towards its common
centre of gravity, that centre would be approximately
the centre of this cluster. Also, as gravitational force
at and near this centre would be comparatively small,
the motions produced there would be slow, and colli
sions, being due only to differential motions, when
they did occur would be very gentle. We might
therefore expect many dark aggregations of matter
here, which may explain why we do not find any
special crowding of visible stars in the direction of
this centre ; while, as no star has a sensible disc, the
dark stars if at great distances would hardly ever
be seen to occult the bright ones. Thus, it seems
to me, the controlling force may be explained which
has retained our sun in approximately the same
orbit around the centre of gravity of this central
cluster during the whole period of its existence as
a sun and our existence as a planet ; and has thus
saved us from the possibility perhaps even the
certainty of disastrous collisions or disruptive ap-
xvi.] SUMMARY OF ARGUMENT 313
preaches to which suns, in or near the Milky Way,
and to a less extent elsewhere, are or have been
exposed. It seems quite probable that in that region
of more rapid and less controlled motions and more
crowded masses of matter, no star can remain in
a nearly stable condition as regards temperature for
sufficiently long periods to allow of a complete system
of life-development on any planet it may possess.
(4) The various proofs are next stated that assure
us of the almost complete uniformity of matter, and
of material physical and chemical laws, throughout
our universe. This I believe no one seriously dis
putes ; and it is a point of the greatest importance
when we come to consider the conditions required
for the development and maintenance of life, since
it assures us that very similar, if not identical, con
ditions must prevail wherever organic life is or can
be developed.
(5) This leads us on to the consideration of the
essential characteristics of the living organism, con
sisting as it does of some of the most abundant
and most widely distributed of these material ele
ments, and being always subject to the general laws
of matter. The best authorities in physiology are
quoted, as to the extreme complexity of the chemical
compounds which constitute the physical basis for
the manifestation of life ; as to their great instability ;
their wonderful mobility combined with permanence
of form and structure ; and the altogether marvellous
powers they possess of bringing about unique chemical
transformations and of building up the most com
plicated structures from simple elements.
I have endeavoured to put the broad phenomena
3H MAN S PLACE IN THE UNIVERSE [CHAP.
of vegetable and animal life in a way that will enable
my readers to form some faint conception of the
intricacy, the delicacy, and the mystery of the myriad
living forms they see everywhere around them. Such
a conception will enable them to realise how supremely
grand is organic life, and to appreciate better, perhaps,
the absolute necessity for the numerous, complex and
delicate adaptations of inorganic nature, without which
it is impossible for life either to exist now, or to have
been developed during the immeasurable past.
(6) The general conditions which are absolutely
essential for life thus manifested on our planet are
then discussed, such as, solar light and heat ; water
universally distributed on the planet s surface and in
the atmosphere ; an atmosphere of sufficient density,
and composed of the several gases from which alone
protoplasm can be formed ; some alternations of light
and darkness, and a few others.
(7) Having treated these conditions broadly, and
explained why they are important and even indis
pensable for life, we next proceed to show how they
are fulfilled upon the earth, and how numerous, how
complex, and often how exact are the adjustments
needed to bring them about, and maintain them
almost unchanged throughout the vast aeons of time
occupied in the development of life. Two chapters
are devoted to this subject ; and it is believed that
they contain facts that will be new to many of my
readers. The combinations of causes which lead to
this result are so varied, and in several cases depen
dent on such exceptional peculiarities of physical
constitution, that it seems in the highest degree im
probable that they can all be found again combined
xvi.] SUMMARY OF ARGUMENT 315
either in the solar system or even in the stellar
universe. It will be well here just to enumerate
these conditions, which are all essential within more
or less narrow limits :
Distance of planet from the sun.
Mass of planet.
Obliquity of its ecliptic.
Amount of water as compared with land.
Surface distribution of land and water.
Permanence of this distribution, dependent pro
bably on the unique origin of our moon.
An atmosphere of sufficient density, and of suitable
component gases.
An adequate amount of dust in the atmosphere.
Atmospheric electricity.
Many of these act and react on each other, and lead
to results of great complexity.
(8) Passing on to other planets of the solar system,
it is shown that none of them combine all the com
plex conditions which are found to work harmoni
ously together on the earth ; while in most cases
there is some one defect which alone removes them
from the category of possible life-producing and life-
supporting planets. Among these are the small size
and mass of Mars, being such that it cannot retain
aqueous vapour ; and the fact that Venus rotates on
its axis in the same time as it takes to revolve round
the sun. Neither of these facts was known when
Proctor wrote upon the question of the habitability of
the planets. All the other planets are now given
up and were given up by Proctor himself as
possible life-bearers in their present stage ; but he
and others have held that, if not suitable now, some
316 MAN S PLACE IN THE UNIVERSE [CHAP.
of them may have been the scene of life-development
in the past, while others will be so in the future.
In order to show the futility of this supposition,
the problem of the duration of the sun as a stable
heat-giver is discussed ; and it is shown that it is
only by reducing the periods claimed by geologists
and biologists for life-development upon the earth, and
by extending the time allowed by physicists to its
utmost limits, that the two claims can be harmonised.
It follows that the whole period of the sun s duration
as a light- and heat-giver has been required for the
development of life upon the earth ; and that it
is only upon planets whose phases of development
synchronise with that of the earth that the evolution
of life is possible. For those whose material evolu
tion has gone on quicker or slower, there has not
been, or will not be, time enough for the development
of life.
(9) The problem of the stars as possibly having
life-supporting planets is next dealt with, and reasons
are given why in only a minute portion of the whole
is this possible. Even in that minute portion, reduced
probably to a few of the component suns of the solar-
cluster, a large proportion seems likely to be ruled out
by being close binary systems, and another large
portion by being in process of aggregation. In those
remaining, whether they may be reckoned by tens or
by hundreds we cannot say, the chances against the
same complex combination of conditions as those
which we find on the earth occurring on any planet
of any other sun are enormously great.
(10) I then refer, briefly, to some recent measure
ments of star-radiation, and suggest that they may
XVL] CONCLUSIONS 317
thus possibly have important effects on the develop
ment of vegetable and animal life ; and, finally, I
discuss the problem of the stability of the stellar
universe and the special advantage we derive from
our central position, suggested by some of the latest
researches of our great mathematician and physicist
Lord Kelvin.
CONCLUSIONS
Having thus brought together the whole of the
available evidence bearing upon the questions treated
in this volume, I claim that certain definite con
clusions have been reached and proved, and that
certain other conclusions have enormous probabilities
in their favour.
The conclusions reached by modern astronomers
are: (i) That the stellar universe forms one con
nected whole ; and, though of enormous extent, is
yet finite, and its extent determinable.
(2) That the solar system is situated in the plane
of the Milky Way, and not far removed from the
centre of that plane. The earth is therefore nearly
in the centre of the stellar universe.
(3) That this universe consists throughout of the
same kinds of matter, and is subjected to the same
physical and chemical laws.
The conclusions which I claim to have shown to
have enormous probabilities in their favour are
(4) That no other planet in the solar system than
our earth is inhabited or habitable.
(5) That the probabilities are almost as great
against any other sun possessing inhabited planets.
(6) That the nearly central position of our sun is
3i8 MAN S PLACE IN THE UNIVERSE [CHAP.
probably a permanent one, and has been specially
favourable, perhaps absolutely essential, to life-de
velopment on the earth.
These latter conclusions depend upon the com
bination of a large number of special conditions, each
of which must be in definite relation to many of the
others, and must all have persisted simultaneously
during enormous periods of time. The weight to be
given to this kind of reasoning depends upon a full
and fair consideration of the whole evidence as I
have endeavoured to present it in the last seven
chapters of this book. To this evidence I appeal.
This completes my work as a connected argument,
founded wholly on the facts and principles accumu
lated by modern science ; and it leads, if my facts are
substantially correct and my reasoning sound, to one
great and definite conclusion- -that man, the culmi
nation of conscious organic life, has been developed
here only in the whole vast material universe we see
around us. I claim that this is the logical outcome
of the evidence, if we consider and weigh this evi
dence without any prepossessions whatever. I main
tain that it is a question as to which we have no
right to form a priori opinions not founded upon
evidence. And evidence opposed to this conclusion,
or even as to its improbability, we have absolutely
none whatever.
But, if we admit the conclusion, nothing that need
alarm either the scientific or the religious mind
necessarily follows, because it can be explained or
accounted for in either of two distinct ways. One
XVL] CONCLUSION 319
considerable body, including probably the majority
of men of science, will admit that the evidence does
apparently lead to this conclusion, but will explain it
as due to a fortunate coincidence. There might have
been a hundred or a thousand life-bearing planets,
had the course of evolution of the universe been a
little different, or there might have been none at all.
They would probably add, that, as life and man have
been produced, that shows that their production was
possible ; and therefore, if not now then at some
other time, if not here then in some other planet of
some other sun, we should be sure to have come into
existence ; or if not precisely the same as we are, then
something a little better or a little worse.
The other body, and probably much the largest,
would be represented by those who, holding that
mind is essentially superior to matter and distinct from
it, cannot believe that life, consciousness, mind, are
products of matter. They hold that the marvellous
complexity of forces which appear to control matter,
if not actually to constitute it, are and must be mind-
products ; and when they see life and mind apparently
rising out of matter and giving to its myriad forms
an added complexity and unfathomable mystery, they
see in this development an additional proof of the
supremacy of mind. Such persons would be inclined
to the belief of the great eighteenth century scholar,
Dr. Bentley, that the soul of one virtuous man is of
greater worth and excellency than the sun and all his
planets and all the stars in the heavens ; and when
they are shown that there are strong reasons for
thinking that man is the unique and supreme product
of this vast universe, they will see no difficulty in
320 MAN S PLACE IN THE UNIVERSE [CHAP.
going a little further, and believing that the universe
was actually brought into existence for this very
purpose.
With infinite space around us and infinite time
before and behind us, there is no incongruity in this
conception. A universe as large as ours for the
purpose of bringing into existence many myriads of
living, intellectual, moral, and spiritual beings, with
unlimited possibilities of life and happiness, is surely
not more out of proportion than is the complex
machinery, the lifelong labour, the ingenuity and
invention which we have bestowed upon the produc
tion of the humble, the trivial, pin. Neither is the
apparent waste of energy so great in such a universe,
comparatively, as the millions of acorns, produced
during its life by an oak, every one of which might
grow to be a tree, but of which only one does
actually, after several hundred years, produce the one
tree which is to replace the parent. And if it is
said that the acorns are food for bird and beast, yet
the spores of ferns and the seeds of orchids are not
so, and countless millions of these go to waste for
every one which reproduces the parent form. And
all through the animal world, especially among the
lower types, the same thing is seen. For the great
majority of these entities we can see no use what
ever, either of the enormous variety of the species,
or the vast hordes of individuals. Of beetles alone
there are at least a hundred thousand distinct species
now living, while in some parts of sub-arctic America
mosquitoes are sometimes so excessively abundant
that they obscure the sun. And when we think of
the myriads that have existed through the vast ages
XVL] CONCLUSION 321
of geological time, the mind reels under the im
mensity of, to us, apparently useless life.
All nature tells us the same strange, mysterious
story, of the exuberance of life, of endless variety,
of unimaginable quantity. All this life upon our
earth has led up to and culminated in that of man.
It has been, I believe, a common and not unpopular
idea that during the whole process of the rise and
growth and extinction of past forms, the earth has
been preparing for the ultimate Man. Much of the
wealth and luxuriance of living things, the infinite
variety of form and structure, the exquisite grace and
beauty in bird and insect, in foliage and flower, may
have been mere by-products of the grand mechanism
we call nature the one and only method of de
veloping humanity.
And is it not in perfect harmony with this grandeur
of design (if it be design), this vastness of scale, this
marvellous process of development through all the
ages, that the material universe needed to produce
this cradle of organic life, and of a being destined to
a higher and a permanent existence, should be on
a corresponding scale of vastness, of complexity, of
beauty ? Even if there were no such evidence as I
have here adduced for the unique position and the
exceptional characteristics which distinguish the
earth, the old idea that all the planets were in
habited, and that all the stars existed for the sake of
other planets, which planets existed to develop life,
would, in the light of our present knowledge, seem
utterly improbable and incredible. It would intro
duce monotony into a universe whose grand character
and teaching is endless diversity. It would imply
x
322 MAN S PLACE IN THE UNIVERSE [CHAP.
that to produce the living soul in the marvellous and
glorious body of man man with his faculties, his
aspirations, his powers for good and evil that this
was an easy matter which could be brought about
anywhere, in any world. It would imply that man is
an animal and nothing more, is of no importance in
the universe, needed no great preparations for his
advent, only, perhaps, a second-rate demon, and a
third or fourth-rate earth. Looking at the long and
slow and complex growth of nature that preceded
his appearance, the immensity of the stellar universe
with its thousand million suns, and the vast aeons of
time during which it has been developing all these
seem only the appropriate and harmonious surround
ings, the necessary supply of material, the sufficiently
spacious workshop for the production of that planet
which was to produce first, the organic world, and
then, Man.
In one of his finest passages our great world-poet
gives us his conception of the grandeur of human
nature What a piece of work is man! How
noble in reason! How infinite in faculty! In form
and moving, how express and admirable! In action
how like an angel! In apprehension how like a
god ! And for the development of such a being
what is a universe such as ours? However vast it
may seem to our faculties, it is as a mere nothing in
the ocean of the infinite. In infinite space there
may be infinite universes, but I hardly think they
would be all universes of matter. That would indeed
be a low conception of infinite power! Here, on
earth, we see millions of distinct species of animals,
millions of different species of plants, and each and
xvi.] CONCLUSION 323
every species consisting often of many millions of
individuals, no two individuals exactly alike ; and
when we turn to the heavens, no two planets, no two
satellites alike ; and outside our system we see the
same law prevailing no two stars, no two clusters,
no two nebulae alike. Why then should there be
other universes of the same matter and subject to the
same laws as is implied by the conception that the
stars are infinite in number, and extend through
o
infinite space?
Of course there may be, and probably are, other
universes, perhaps of other kinds of matter and sub
ject to other laws, perhaps more like our conceptions
of the ether, perhaps wholly non-material, and what
we can only conceive of as spiritual. But, unless
these universes, even though each of them were a
million times vaster than our stellar universe, were
also infinite in number, they could not fill infinite
space, which would extend on all sides beyond them,
so that even a million million such universes would
shrink to imperceptibility when compared with the
vast beyond !
Of infinity in any of its aspects we can really
know nothing, but that it exists and is inconceivable.
It is a thought that oppresses and overwhelms. Yet
many speak of it glibly as if they knew what it con
tains, and even use that assumed knowledge as an
argument against views that are unacceptable to
themselves. To me its existence is absolute but
unthinkable that way madness lies.
O night ! O stars, too rudely jars
The finite with the infinite !
324 MAN S PLACE IN THE UNIVERSE [CHAP.
I will conclude with one of the finest passages
relating to the infinite that I am acquainted with, from
the pen of the late R. A. Proctor :
* Inconceivable, doubtless, are these infinities of
time and space, of matter, of motion, and of life.
Inconceivable that the whole universe can be for all
time the scene of the operation of infinite power,
omnipresent, all-knowing. Utterly incomprehensible
how Infinite Purpose can be associated with endless
material evolution. But it is no new thought, no
modern discovery, that we are thus utterly powerless
to conceive or comprehend the idea of an Infinite
Being, Almighty, All-knowing, Omnipresent, and
Eternal, of whose inscrutable purpose the material
universe is the unexplained manifestation. Science
is in presence of the old, old mystery ; the old, old
questions are asked of her " Canst thou by search
ing find out God ? Canst thou find out the Almighty
unto perfection ? It is as high as heaven ; what
canst thou do ? deeper than hell ; what canst thou
know? And science answers these questions as
they were answered of old " As touching the
Almighty we cannot find Him out."
The following beautiful lines among the latest pro
ducts of Tennyson s genius so completely harmonise
with the subject-matter of the present volume, that
no apology is needed for quoting them here :
xvi.] CONCLUSION 325
(The Question)
Will my tiny spark of being
Wholly vanish in your deeps and heights?
Must my day be dark by reason,
O ye Heavens, of your boundless nights,
Rush of Suns and roll of systems,
And your fiery clash of meteorites ?
(The Answer)
Spirit, nearing yon dark portal
At the limit of thy human state,
Fear not thou the hidden purpose
Of that Power which alone is great,
Nor the myriad world, His shadow,
Nor the silent Opener of the Gate.
INDEX
ADRIANUS TOLLIUS on stone axes, 203.
Air criminally poisoned by us, 260.
Albedo explained, 162.
Algol and its companion, 39 ; change
of colour of, 41.
Allen, Prof. F. J., on living matter,
193 ; on importance of nitrogen,
195 ; on physical conditions essential
for life, 196.
Alpha Centauri, nearest star, 74.
Ammonia, importance of, to life, 195.
Anaximander s cosmic theory, 2.
Angles of a minute and second, So.
Arcturus, rapid motion of, 172.
Argument of book, summary of, 310.
Astronomers, the first, 2.
Astronomy, the new, 24.
Astrophysics, a new science, 32.
Atmosphere, qualities requisite for life,
210; requisite composition of, 212;
aqueous vapour in, 214; and life,
243 ; effects of density of, 245 ; a
complex structure, 259; its vital
importance to us, 260.
BALL, Sir R., on dark stars, 143;
Time and Tide, 233.
Barnham, S. W., on double stars, 123.
Blue of sky due to dust, 251.
Boeddicker s map of Milky Way, 164.
Brewster, Sir D. , against Whewell, 15.
CAMPBELL, Prof., on spectroscopic
binaries, 125 ; on uncertainty of
sun s motions, 179; on number of
binary systems, 286.
Carbon compounds, vast numbers of,
194.
Carbonic acid gas essential for life, 196.
326
Central position of sun, importance of,
^305-
Chaldeans the first astronomers, 2.
Chalmers,, Dr., on plurality of worlds,
Chamberlin, T. C, origin of nebulae,
120; on stellar disruption, 186.
Chromosphere, the sun s, 107.
Clerke, Miss A. M., on limits of star
system, 138; on Milky Way, 158,
160; on solar cluster, 165; on un
certainty of the sun s motion, 177.
Climate, persistence of mild, 222.
Clouds, importance of, to life, 248.
Clusters in relation to Galaxy, 67.
Comte, on impossibility of real know
ledge of the stars, 25.
Conclusions of the book, 317; bearing
of, on science and on religion, 319.
Corona of sun, 108.
Criticisms of article in Fortnightly
Review > 168, 180.
DARWIN, Prof. G., on meteoritic
hypothesis, 133 ; on origin of moon,
233 ; on instability of annular
systems, 295.
Day and night, uses of, 215.
Diagrams of star-distribution, 62, 66.
Diffraction-gratings, 30.
Disruption of stellar bodies, 187.
Doppler principle, the, 37.
Double stars, evolution of, 123 ; not
fitted for life, 286.
Dust, importance of, 249.
Dust-free air, results of, 254.
EARTH, first measured, 5 ; in relation
to life, 218; the only habitable
INDEX
327
planet, 262 ; cannot retain hydrogen,
264 ; supposed extreme conditions
of, 271.
Earth s mass, how related to life, 265.
Ecliptic, obliquity of, in relation to life,
219.
Electricity, effects of atmospheric, 257;
atmospheric, how caused, 258.
Elements, change in spectra of, 129;
in the sun, 184 ; in meteorites, 185 ;
in organic structures, 201.
Empedocles an early astronomer, 3.
Eudoxus on motions of planets, 3.
Evolution of the stars, 128.
Explanations of life-processes, 202.
FACUL^E of sun, 105.
Fisher, Rev. O., on oceanic basins,
234 ; on thin sub-oceanic crust, 237.
Fizeau measures speed of light, 79.
Flammarion, C, on universality of
life, 274, 281.
Fontenelle on plurality of worlds, 9.
GALILEO on star measurement, 74.
Geological climates, 222.
Geologists on duration of sun s heat,
275-
Germinal vesicle, M Kendrick on,
202.
Gill, Sir D., on systematic star-motions,
178.
Globular clusters, stability of, 126;
and variables, 127.
Gore, Mr. J. E., on stars in Galaxy, 60 ;
on mass of binary stars, 97 ; on
remoteness of bright stars, 140 ; on
limits of star system, 145 ; on limited
number of stars, 151 ; on life on
planets of other suns, 282, 289.
Gould on solar cluster, 165.
Gould s map of Milky Way, 164.
Gravitation, motions produced by, on
Lord Kelvin s hypothesis, 298.
HALIBURTON, Professor W. D., on
proteids, 200.
Heat and cold on earth s surface, 207.
Heat-supply, our long-continued, ac
counted for, 305.
Herschel, Sir J., on Milky Way, 50;
on limits of the star-system, 147.
Heliometer, description of, 89.
Huggins, Sir W., on spectra of stars,
32 ; measures radial motion, 37.
Huxley, Prof., on protoplasm, 198 ; on
duration of life, 278.
Hydrogen, why not in atmosphere, 240 ;
escapes from earth, 264.
INFINITY, unknowable, 323 ; Proctor
on, 324.
JUPITER S satellites show speed of
light, 79.
KAPTEYN on solar cluster, 166.
Kelvin, Lord, on the sun s age, 279 ;
on a suggested primitive form of
star-system, 298.
Kirchhoff, discovers spectrum-analysis,
28.
LAWS of matter uniform throughout
universe, 187.
Leaves, importance of, 197.
Lee, Dr., on origin of double stars, 123.
Lewis, on remote bright stars, 141.
Life, unity of organic, 189; definitions
of, 191 ; conditions essential for, 206 ;
water essential for, 210; atmosphere
for, 210; dependent on temperature,
218 ; now improbable in stars, 288 ;
conditions essential for, summarised,
314.
Life-processes, explanations of, 202.
Light, velocity of measured, 79 ; neces
sity of solar, 209 ; from sky due to
dust, 252.
Light-journey explained, 75.
Light-ratio shows stars to be limited,
152.
Living bodies, essential points in, 192.
Lockyer, Sir N., on inorganic evolution,
117; on evolution of stars, 130; on
Milky Way, 159 ; on position of solar
system, 161.
Luigi d Auria on stellar motion, 306.
M KENDRICK, Prof., on germinal
vesicle, 202.
328
MAN S PLACE IN THE UNIVERSE
Magnetism and sun-spots, 106.
Man, Shakespeare on, 322.
Mars, has no water, 266 ; excessive
temperatures on, 267.
Matter of universe uniform, 183.
Maunder on dark stars, 143.
Maxwell Hall, Mr., on star-motions,
178.
Measurement of star-distances, 85 ;
difficulty of, 86.
Mercury not habitable, 266.
Meteorites, elements in, 185 ; not primi
tive bodies, 186.
Meteoritic hypothesis, 113; Proctor
on, 114; explains nebi.lse, 116; Dr.
Roberts on, 119.
Milky Way, the, 48 ; form of, 51, 159 ;
description of, 52 ; telescopic view
of, 57 ; stars in relation to, 59 ; Mr.
Gore on, 60 ; density of stars in,
6 1 ; clusters and nebulae in relation
to, 67 ; probable distance of, 96 ;
forms a great circle, 157, 162; Prof.
Newcomb on, 158; probably no life
in, 284 ; diagrams of, 300 ; revolu
tion of, important to us, 307.
Million, how to appreciate a, 82.
Minchin, G. M., on radiation from
stars, 290.
Monck, Mr. W. H. S., on non-infinity
of stars, 144; on uncertainty of sun s
motion, 177.
Moon, why no atmosphere, 263.
Moon s supposed origin, 233.
Motion, in line of sight, 35.
Motions, imperceptible, 39.
NEBULAE, with gaseous spectra, 43 ; in
relation to Galaxy, 66 ; distribution
of, 69 ; many forms of, 7 > gaseous,
71 ; meteoritic theory of, 116 ; plane
tary and annular, 175; Dr. Roberts
on spiral, 117, 174; Chamberlin on
origin of, 1 20.
Nebular hypothesis, 98, ill ; objection
to, 112.
Newcomb, Prof. S., on star distribu
tion, 6 1 ; on parallax of stars, 94 ;
on stability of star clusters, 126 ; on
scarcity of single stars, 128 ; on limits
of star system, 138 ; on Milky Way,
158, 160; on solar cluster, 167; on
star velocities, 171 ; on average small
mass of stars, 285 ; on star-motions,
297.
Newton, Sir Isaac, on sun s habitability,
9-
Nichols, E. F., on heat of stars, 290.
Nitrogen, its importance to life, 195.
Non-habitability of great planets, 272.
OCEAN and land, diagram of, 228.
basins, permanence of, 229.
symmetry of, 238.
depths, how produced, 232.
Oceans, effect of, on temperature, 239 ;
curious relations of, 264.
Organic products, diversity of, 195.
PHOTOGRAPHIC astronomy, 43 ; mea
sures of star-distances, 89.
Photosphere, the, 105.
Physicists on sun s duration, 278.
Pickering s measurements of Algol, 40.
Planets, supposed habitability of, 266,
269 ; the great, uninhabitable, 272 ;
internal heat of great, 273 ; a last
argument for habitability of, 274 ;
have probably no life, 315.
Planets motions first explained, 3 ;
mass and atmosphere, 262.
Pleiades, number of stars in, 67 ; a
drifting cluster, 177.
Plurality of worlds, early writers on, 9 ;
Proctor on, 18.
Posidonius measures the earth, 5.
Pritchard s photographic measures of
star-distance, 89.
Proctor, R. A., on other worlds, 18 ;
on form of Galaxy, 51 ; on Herschel s
views, IOI ; on stellar universe, 103 ;
on meteoritic theory, 114; on in
finities, 136; on star-drift, 176;
on life under varied conditions, 271 ?
on infinity, 324.
Proctor s Old and New Astronomy , 46 ;
chart of stars, 60.
Prominences of sun, 107.
Proteids, formation of, 199 J
Haliburton on, 200.
INDEX
329
Protoplasm, complexity of, 194; a
mechanism, 198 ; sensibility of, to
heat, 208.
Ptolemaic system of the heavens, 4.
RADIAL motion, 35.
Radiation from stars, 290.
Rain in the Carboniferous age, 225 ;
dependent on dust, 249.
Ramsay, Prof. , on geological climates,
278.
Ranyard, on star-discs, 98 ; on infinite
universe, 137 ; on mass of Orion
nebula, 173.
Religious bearing of my conclusions,
319.
Reproduction, marvel of, 201.
Reversing layer of sun, 107.
Roberts, A. W., on birth of double
stars, 123.
Dr. I., on limits of star-system,
148; on spiral nebulae, 117; on
meteoritic theory, 119; photographs
of nebulae, 45, 174.
Roche limit explained, 120, 187.
SANDERSON, Prof. Burdon, on living
matter, 192.
Scientific and agnostic opinion on my
conclusions, 318.
Secchi s classification of stars, 33.
Single stars perhaps rare, 128.
Solar apex, position of, 176.
Solar cluster, the, 165 ; diagram show
ing, 300 ; evidence for, 302 ; im
portance to us, 306-7, 312.
Solar system, position of, 304.
Sorby on constitution of meteorites, 1 86.
Spectra, varieties of, 34 ; of elements,
changes in, 129.
Spectroscopic binaries, abundance of,
125 ; great numbers of, 286.
Spectrum analysis, discovery of, 26.
Spencer, H., on status of nebulaa, 102.
Spiral nebulae, origin of, 120.
Stars, proved to be suns, 32 ; invisible,
39 ; classification of, 33 ; spectro-
scopic double, 42 ; distribution of
the, 47 ; number of visible, 48 ; de
scription of Milky Way, 52 ; in relation
to Milky Way, 59; distances of,
74 ; measurement of distance of, 85 ;
mass of binary, 97 ; evolution of
double, 122 ; spectroscopic double,
123; clusters of, 125; evolution of
the, 128; classification of, 130; the
hottest, 131 ; when cooling give more
heat, 132; cycle of evolution and
decay, 133 ; supposed infinite number
of > 135 J not infinite, 138 ; law of
diminishing numbers of, 149 ; sys
tematic motions of, 178; in relation
to life, 282, 287 ; possible use of their
emanations, 289.
Star-clusters and variables, 127.
Star-density, diagram of, 66.
Star-drift, Proctor on, 176.
Starlight, electrical measure of, 290;
possible uses of, 292.
Star-motions, Prof. Newcomb on, 297.
Star-system, limited, 145 ; stability of,
295 ; supposed primitive form of, 297.
Stellar motion, Luigi d Auria on, 306.
universe, shape of, 49 ; unity of,
loo ; evolution of, 103 ; diagrams of,
300.
Stoney, Dr., on atmospheres and gravity,
263.
Sun, a typical star, 104 ; brightness of,
104 ; heat of, 104 ; surface of, 105 ;
surroundings of, 106-110 ; corona of,
108 ; colour of, 1 1 1 ; elements in, 184.
Sun s distance, measure of, 76.
heat, supposed limits of, 275.
life, all required to develop earth-
life, 280.
motion through space, 91, 169.
uncertain, 177.
Sun-spots, nature of, 105.
Symmetry of oceans, cause of, 238.
TEMPERATURE, essential for life, 206 ;
equalised by water, 239 ; as regards
life on planets, 267.
Tennyson on man and the universe, 325.
UNIFORMITY of matter, 183.
Unity of stellar universe, 100.
Universe of stars, how its form has
affected our sun and earth, 308.
330 MAN S PLACE IN THE UNIVERSE
Universe not disproportionate if man is
its sole product, 320.
VENUS, radial motions of, 38 ; diagram
of transit of, 77 ; life barely possible
on, 266 ; adverse climatic conditions
of, 268.
WATER, an essential for life, 210; its
amount and distribution, 227 ; an
equaliser of temperature, 239.
Wave-lengths, how measured, 31.
Whewell, on plurality of worlds, 8, 15 ;
on man as the highest product of the
universe, 14.
Whittaker, Mr. E. T., on gravitative
and electro-dynamical forces, 296.
Winds, importance of, to life, 246.
ZODIACAL light, 109.
Printed by T. and A. CONSTABLE, Printers to His Majesty,
at the Edinburgh University Press
* * *
* .
f
. V, .
**.* % /- s
. . .
* ^
: : - *
- B^
V *
*.>**.
* *
A
1* %
,. .
,r- -JM*.- *
<
*
*
y.- />x y ^"
from Dr. L y |ronl tae 1
SIDNEY; By SJDNEV
.-.. -.
, * . .
THE NEBUL/E AND
Drawn npn an equal sorfi
The Milky vi
NEBUL/E _
RESOLVABLE
CLUSTERS
NEBUb_..
ERS OF THE NORTHERN HEAVENS.
action from Dr. DRKYEB S Cut.Iof u. of 1888.
a Dr. BOEDDICXER S Drawing.
)NEY WATERS.
THE NEBUL/E AND Ct
Drvrn apon an equal *nrfao
The Milky Wy f
By
. .
.. *-
V" ... ..-
;;<: . ?-. -,^ <% .
THE SOUTHERN HEAVENS.
on Iron, ,. DHKYER 8 Catalotfo. of J 88 8.
VKANOMKTRIA AHGENTIiVfl.
Y
WORKS BY THE SAME AUTHOR.
TRAVELS ON THE AMAZON AND RIO-NEGRO.
PALM-TREES OF THE AMAZON, AND THEIR USES (put of print).
THE MALAY ARCHIPELAGO.
NATURAL SELECTION AND TROPICAL NATURE.
THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 2 vols.
ISLAND LIFE, OR INSULAR FAUNAS AND FLORAS.
DARWINISM.
AUSTRALASIA. 2 vols.
BAD TIMES CAUSES OF DEPRESSION OF TRADE (out of print).
LAND NATIONALISATION, ITS NECESSITY AND ITS AIMS.
VACCINATION A DELUSION.
MIRACLES AND MODERN SPIRITUALISM.
THE WONDERFUL CENTURY (New and Illustrated Edition).
THE WONDERFUL CENTURY READER.
STUDIES SCIENTIFIC AND SOCIAL.
