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s Place 


Russel Wallace 


Bertram & Jt 



A Study of the Results of Scientific Research 
in Relation to the Unity or Plurality 

of Worlds 



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 






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 


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. 


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 

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 


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 


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 


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 

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 ! 




II. MODERN IDEAS, . ... 7 













USEFUL TO Us ? . . . 282 





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 ? 






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 

The relation of the stars to the sun and moon in 
their respective motions was one of the earliest pro- 



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 


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 


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 


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. 


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. 



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. 


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 


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 


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- 


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. 


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 


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 


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 


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 


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 

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 


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 



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 


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 


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 

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 


has reached as high a development as on our 

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 

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 


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 


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 



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 


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 

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 


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- 


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 


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. 


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 


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 


to be refracted more and others less than the normal 

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 


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 


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 

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 

These groups were established by Father Secchi, 
the Roman astronomer, in 1867, and have been 
adopted with some modifications by Vogel of the 



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 


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. 


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 


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 

That such a change of colour would occur was 


pointed out by Professor Doppler of Prague in 1842, 


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 


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. 



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. 


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 

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 


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- 


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. 



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. 


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- 


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 


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 


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. 



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 


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. 


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 


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 



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 

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 


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 


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. 


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 


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 


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 

* 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 


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 


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 


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 


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 


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 


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. 


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 


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 


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 


i. and ix.) to the Galaxy itself (region v.). If the 
latter were a simple ring of stars surrounding a 


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. 


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. 













J 54 








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 


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. 


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 


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 




is not great, and may probably be due to the clearer 
atmosphere of the Cape of Good Hope as compared 
with that of England. 



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 


is more nearly approached. This fact must, there 
fore, be accepted as indisputable. 


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 


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 


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 


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 


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 

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 


impossible that they may be not very dissimilar in 

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. 




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 


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 


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 


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- 


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 


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. 


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 


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) 


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 



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. 


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 


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. 


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 


starry heavens and the mysterious galaxy, is con 

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 


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 


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- 


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 


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 


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 


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- 


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, 


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. 


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 


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 


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. 



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 


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 


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 


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. 

Having thus obtained an inferior limit for the 


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 


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 



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. 



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 



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 


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 


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 


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- 


eludes, from the whole mass of the evidence, that 
the proofs of a physical connection become over 

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 


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 


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. 


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 


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 

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 


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. 


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. 


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 

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 


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 


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 


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 


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 


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. 


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 


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- 


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, 



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 

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 


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 


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. 


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 


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. 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- 


graphed, but present indications seem to show that a 
considerable proportion of them will exhibit spiral 

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 


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 

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 


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 


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. 


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, 


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 


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 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 


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 


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 

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 


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. 


The clusters of stars, which are tolerably abun 
dant in the heavens and offer so many strange and 


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 


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, 


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 


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 


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 


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 


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, 



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 


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 ; 


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. 


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 


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. 



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 



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. 


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. 



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 


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, 


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 


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 


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 

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 

Both Mr. E. W. Maunder of the Greenwich 
Observatory and Professor W. W. Turner of Oxford 


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, 


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- 


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. 



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 

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 



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 

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 


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 


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. 


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. 


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 


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 


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. 




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 


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 


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- 


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 

(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. 



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 



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. 


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 


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. 


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 


were connected together across the vast interven 
ing abyss, and in all probability not far from the 
central plane of that enclosed space. 


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 


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 


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 



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- 

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 


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 


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 


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. 


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. 


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. 


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 

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 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- 


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 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 


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. 


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 


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 


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. 


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 


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 


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 

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, 


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 


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 



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 


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 

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 


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 


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 


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. 



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. 


One of the most important and far-reaching of the 
discoveries due to the spectroscope is that of the 



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 


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 

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- 


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, 


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 


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. 


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 


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 


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. 



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 



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 


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 

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 



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 


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 


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 


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 


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. 


marvellous results than all the human mechanism 
ever invented. 

But besides absorbing carbonic acid from the at- 


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- 


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 


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. 


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. 


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 


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. 


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 


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. 




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 

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. 


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 



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 


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 

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 


animal which passes its whole life where the tem 
perature never rises above the freezing point. 


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. 




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 


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 


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 


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. 


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 


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 


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. 


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 


purpose, we may conclude that the proportions in 
which they exist in our atmosphere cannot be very 


widely departed from wherever organic forms are 


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 


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 


freezing point of water. It is doubtful whether any 
hieh forms of animal life could have arisen under 


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. 



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 



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 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 


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 


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 


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. 


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 


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 


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. 


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 



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 

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 


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. 


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 


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, 


Area. -28 of area 

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 


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- 


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 ; 


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, 


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. 


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 


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 

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 


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 


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 


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- 


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 


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 


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. 



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 

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- 


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 

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 

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 


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. 



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. 




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 



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) 


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 


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 


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 

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. 



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, 


uniting together, form streams and rivers, which 
return to the seas and oceans whence they were 
originally derived. 


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 


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 


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 

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 


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 

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 


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 


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 


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 

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- 


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 

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 


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. 


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 



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 


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 


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 


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, 


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 


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 ! 




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. 


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 



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 


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. 


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 


greater than at present, in which case the water 
would have sufficed to cover the whole surface 
several miles deep. 


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. 



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 


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 

Venus appears to have a dense atmosphere, and 


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 


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. 


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. 



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 


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. 


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. 


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 



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 



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. 


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. 


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 


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 


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 

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 


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 


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 


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 


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- 


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 

1 M. Camilla Flammarion, in Knowledge, June 1903. 




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. 



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- 


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 


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 


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. 


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 


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. 


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 


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. 


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. 



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 

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- 


sidered a guarantee of the accuracy of the observa 
tions. The following are the chief results obtained :- 




Source of Light. 



IV A A. 



Candle at 10 feet distance, 


Betelgeuse (0-9 mag.), 




Aldebaran (1*1 mag.), 

5 2i 



Procyon (0-5 mag.), 


O 26l 


Alpha Cygni (1-3 mag.), . 




Polaris (2*1 mag.), . 



O Ooy 

i volt. 



Arcturus (0*3 mag.), 



O Oig 

Vega (o i mag.), 




Candle at 10 feet, . 


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 : 






Candle power at 10 ft. 


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 


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 

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 


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 

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 


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. 



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 



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 


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 


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 


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 


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 


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 


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 


...... 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- 


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 


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 

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 


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 


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 


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. 


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 



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. 


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, 


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 ; 


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 


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 


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 


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 


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- 


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 


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 


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 


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 


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. 


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 


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 


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 


going a little further, and believing that the universe 
was actually brought into existence for this very 

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 


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 



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 


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 ! 


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. 


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, 

Carbonic acid gas essential for life, 196. 

Central position of sun, importance of, 


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 



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, 

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, 

Germinal vesicle, M Kendrick on, 


Gill, Sir D., on systematic star-motions, 

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, 


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, 


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, 

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. 



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, 

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, 
Newton, Sir Isaac, on sun s habitability, 


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. 



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, 

Ranyard, on star-discs, 98 ; on infinite 

universe, 137 ; on mass of Orion 

nebula, 173. 
Religious bearing of my conclusions, 


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, 

Stoney, Dr., on atmospheres and gravity, 

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. 


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 

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THE WONDERFUL CENTURY (New and Illustrated Edition).