the success of American astronomers. Sixty years ago it could not be
said that there was a well-known observatory on the American continent.
The cultivation of astronomy was confined to a professor here and
there, who seldom had anything better than a little telescope with
which he showed the heavenly bodies to his students. But during the
past thirty years all this has been changed. The total quantity of
published research is still less among us than on the continent of
Europe, but the number of men who have reached the highest success
among us may be judged by one fact. The Royal Astronomical Society of
England awards an annual medal to the English or foreign astronomer
deemed most worthy of it. The number of these medals awarded to
Americans within twenty-five years is about equal to the number awarded
to the astronomers of all other nations foreign to the English. That
this preponderance is not growing less is shown by the award of medals
to Americans in three consecutive years--1904, 1905, and 1906. The
recipients were Hale, Boss, and Campbell. Of the fifty foreign
associates chosen by this society for their eminence in astronomical
research, no less than eighteen--more than one-third--are Americans.
VII
LIFE IN THE UNIVERSE
So far as we can judge from what we see on our globe, the production of
life is one of the greatest and most incessant purposes of nature. Life
is absent only in regions of perpetual frost, where it never has an
opportunity to begin; in places where the temperature is near the
boiling-point, which is found to be destructive to it; and beneath the
earth's surface, where none of the changes essential to it can come
about. Within the limits imposed by these prohibitory conditions--that
is to say, within the range of temperature at which water retains its
liquid state, and in regions where the sun's rays can penetrate and
where wind can blow and water exist in a liquid form--life is the
universal rule. How prodigal nature seems to be in its production is
too trite a fact to be dwelt upon. We have all read of the millions of
germs which are destroyed for every one that comes to maturity. Even
the higher forms of life are found almost everywhere. Only small
islands have ever been discovered which were uninhabited, and animals
of a higher grade are as widely diffused as man.
If it would be going too far to claim that all conditions may have
forms of life appropriate to them, it would be going as much too far in
the other direction to claim that life can exist only with the precise
surroundings which nurture it on this planet. It is very remarkable in
this connection that while in one direction we see life coming to an
end, in the other direction we see it flourishing more and more up to
the limit. These two directions are those of heat and cold. We cannot
suppose that life would develop in any important degree in a region of
perpetual frost, such as the polar regions of our globe. But we do not
find any end to it as the climate becomes warmer. On the contrary,
every one knows that the tropics are the most fertile regions of the
globe in its production. The luxuriance of the vegetation and the
number of the animals continually increase the more tropical the
climate becomes. Where the limit may be set no one can say. But it
would doubtless be far above the present temperature of the equatorial
regions.
It has often been said that this does not apply to the human race, that
men lack vigor in the tropics. But human vigor depends on so many
conditions, hereditary and otherwise, that we cannot regard the
inferior development of humanity in the tropics as due solely to
temperature. Physically considered, no men attain a better development
than many tribes who inhabit the warmer regions of the globe. The
inferiority of the inhabitants of these regions in intellectual power
is more likely the result of race heredity than of temperature.
We all know that this earth on which we dwell is only one of countless
millions of globes scattered through the wilds of infinite space. So
far as we know, most of these globes are wholly unlike the earth, being
at a temperature so high that, like our sun, they shine by their own
light. In such worlds we may regard it as quite certain that no
organized life could exist. But evidence is continually increasing that
dark and opaque worlds like ours exist and revolve around their suns,
as the earth on which we dwell revolves around its central luminary.
Although the number of such globes yet discovered is not great, the
circumstances under which they are found lead us to believe that the
actual number may be as great as that of the visible stars which stud
the sky. If so, the probabilities are that millions of them are
essentially similar to our own globe. Have we any reason to believe
that life exists on these other worlds?
The reader will not expect me to answer this question positively. It
must be admitted that, scientifically, we have no light upon the
question, and therefore no positive grounds for reaching a conclusion.
We can only reason by analogy and by what we know of the origin and
conditions of life around us, and assume that the same agencies which
are at play here would be found at play under similar conditions in
other parts of the universe.
If we ask what the opinion of men has been, we know historically that
our race has, in all periods of its history, peopled other regions with
beings even higher in the scale of development than we are ourselves.
The gods and demons of an earlier age all wielded powers greater than
those granted to man--powers which they could use to determine human
destiny. But, up to the time that Copernicus showed that the planets
were other worlds, the location of these imaginary beings was rather
indefinite. It was therefore quite natural that when the moon and
planets were found to be dark globes of a size comparable with that of
the earth itself, they were made the habitations of beings like unto
ourselves.
The trend of modern discovery has been against carrying this view to
its extreme, as will be presently shown. Before considering the
difficulties in the way of accepting it to the widest extent, let us
enter upon some preliminary considerations as to the origin and
prevalence of life, so far as we have any sound basis to go upon.
A generation ago the origin of life upon our planet was one of the
great mysteries of science. All the facts brought out by investigation
into the past history of our earth seemed to show, with hardly the
possibility of a doubt, that there was a time when it was a fiery mass,
no more capable of serving as the abode of a living being than the
interior of a Bessemer steel furnace. There must therefore have been,
within a certain period, a beginning of life upon its surface. But, so
far as investigation had gone--indeed, so far as it has gone to the
present time--no life has been found to originate of itself. The living
germ seems to be necessary to the beginning of any living form. Whence,
then, came the first germ? Many of our readers may remember a
suggestion by Sir William Thomson, now Lord Kelvin, made twenty or
thirty years ago, that life may have been brought to our planet by the
falling of a meteor from space. This does not, however, solve the
difficulty--indeed, it would only make it greater. It still leaves open
the question how life began on the meteor; and granting this, why it
was not destroyed by the heat generated as the meteor passed through
the air. The popular view that life began through a special act of
creative power seemed to be almost forced upon man by the failure of
science to discover any other beginning for it. It cannot be said that
even to-day anything definite has been actually discovered to refute
this view. All we can say about it is that it does not run in with the
general views of modern science as to the beginning of things, and that
those who refuse to accept it must hold that, under certain conditions
which prevail, life begins by a very gradual process, similar to that
by which forms suggesting growth seem to originate even under
conditions so unfavorable as those existing in a bottle of acid.
But it is not at all necessary for our purpose to decide this question.
If life existed through a creative act, it is absurd to suppose that
that act was confined to one of the countless millions of worlds
scattered through space. If it began at a certain stage of evolution by
a natural process, the question will arise, what conditions are
favorable to the commencement of this process? Here we are quite
justified in reasoning from what, granting this process, has taken
place upon our globe during its past history. One of the most
elementary principles accepted by the human mind is that like causes
produce like effects. The special conditions under which we find life
to develop around us may be comprehensively summed up as the existence
of water in the liquid form, and the presence of nitrogen, free perhaps
in the first place, but accompanied by substances with which it may
form combinations. Oxygen, hydrogen, and nitrogen are, then, the
fundamental requirements. The addition of calcium or other forms of
matter necessary to the existence of a solid world goes without saying.
The question now is whether these necessary conditions exist in other
parts of the universe.
The spectroscope shows that, so far as the chemical elements go, other
worlds are composed of the same elements as ours. Hydrogen especially
exists everywhere, and we have reason to believe that the same is true
of oxygen and nitrogen. Calcium, the base of lime, is almost universal.
So far as chemical elements go, we may therefore take it for granted
that the conditions under which life begins are very widely diffused in
the universe. It is, therefore, contrary to all the analogies of nature
to suppose that life began only on a single world.
It is a scientific inference, based on facts so numerous as not to
admit of serious question, that during the history of our globe there
has been a continually improving development of life. As ages upon ages
pass, new forms are generated, higher in the scale than those which
preceded them, until at length reason appears and asserts its sway. In
a recent well-known work Alfred Russel Wallace has argued that this
development of life required the presence of such a rare combination of
conditions that there is no reason to suppose that it prevailed
anywhere except on our earth. It is quite impossible in the present
discussion to follow his reasoning in detail; but it seems to me
altogether inconclusive. Not only does life, but intelligence, flourish
on this globe under a great variety of conditions as regards
temperature and surroundings, and no sound reason can be shown why
under certain conditions, which are frequent in the universe,
intelligent beings should not acquire the highest development.
Now let us look at the subject from the view of the mathematical theory
of probabilities. A fundamental tenet of this theory is that no matter
how improbable a result may be on a single trial, supposing it at all
possible, it is sure to occur after a sufficient number of trials--and
over and over again if the trials are repeated often enough. For
example, if a million grains of corn, of which a single one was red,
were all placed in a pile, and a blindfolded person were required to
grope in the pile, select a grain, and then put it back again, the
chances would be a million to one against his drawing out the red
grain. If drawing it meant he should die, a sensible person would give
himself no concern at having to draw the grain. The probability of his
death would not be so great as the actual probability that he will
really die within the next twenty-four hours. And yet if the whole
human race were required to run this chance, it is certain that about
fifteen hundred, or one out of a million, of the whole human family
would draw the red grain and meet his death.
Now apply this principle to the universe. Let us suppose, to fix the
ideas, that there are a hundred million worlds, but that the chances
are one thousand to one against any one of these taken at random being
fitted for the highest development of life or for the evolution of
reason. The chances would still be that one hundred thousand of them
would be inhabited by rational beings whom we call human. But where are
we to look for these worlds? This no man can tell. We only infer from
the statistics of the stars--and this inference is fairly well
grounded--that the number of worlds which, so far as we know, may be
inhabited, are to be counted by thousands, and perhaps by millions.
In a number of bodies so vast we should expect every variety of
conditions as regards temperature and surroundings. If we suppose that
the special conditions which prevail on our planet are necessary to the
highest forms of life, we still have reason to believe that these same
conditions prevail on thousands of other worlds. The fact that we might
find the conditions in millions of other worlds unfavorable to life
would not disprove the existence of the latter on countless worlds
differently situated.
Coming down now from the general question to the specific one, we all
know that the only worlds the conditions of which can be made the
subject of observation are the planets which revolve around the sun,
and their satellites. The question whether these bodies are inhabited
is one which, of course, completely transcends not only our powers of
observation at present, but every appliance of research that we can
conceive of men devising. If Mars is inhabited, and if the people of
that planet have equal powers with ourselves, the problem of merely
producing an illumination which could be seen in our most powerful
telescope would be beyond all the ordinary efforts of an entire nation.
An unbroken square mile of flame would be invisible in our telescopes,
but a hundred square miles might be seen. We cannot, therefore, expect
to see any signs of the works of inhabitants even on Mars. All that we
can do is to ascertain with greater or less probability whether the
conditions necessary to life exist on the other planets of the system.
The moon being much the nearest to us of all the heavenly bodies, we
can pronounce more definitely in its case than in any other. We know
that neither air nor water exists on the moon in quantities sufficient
to be perceived by the most delicate tests at our command. It is
certain that the moon's atmosphere, if any exists, is less than the
thousandth part of the density of that around us. The vacuum is greater
than any ordinary air-pump is capable of producing. We can hardly
suppose that so small a quantity of air could be of any benefit
whatever in sustaining life; an animal that could get along on so
little could get along on none at all.
But the proof of the absence of life is yet stronger when we consider
the results of actual telescopic observation. An object such as an
ordinary city block could be detected on the moon. If anything like
vegetation were present on its surface, we should see the changes which
it would undergo in the course of a month, during one portion of which
it would be exposed to the rays of the unclouded sun, and during
another to the intense cold of space. If men built cities, or even
separate buildings the size of the larger ones on our earth, we might
see some signs of them.
In recent times we not only observe the moon with the telescope, but
get still more definite information by photography. The whole visible
surface has been repeatedly photographed under the best conditions. But
no change has been established beyond question, nor does the photograph
show the slightest difference of structure or shade which could be
attributed to cities or other works of man. To all appearances the
whole surface of our satellite is as completely devoid of life as the
lava newly thrown from Vesuvius. We next pass to the planets. Mercury,
the nearest to the sun, is in a position very unfavorable for
observation from the earth, because when nearest to us it is between us
and the sun, so that its dark hemisphere is presented to us. Nothing
satisfactory has yet been made out as to its condition. We cannot say
with certainty whether it has an atmosphere or not. What seems very
probable is that the temperature on its surface is higher than any of
our earthly animals could sustain. But this proves nothing.
We know that Venus has an atmosphere. This was very conclusively shown
during the transits of Venus in 1874 and 1882. But this atmosphere is
so filled with clouds or vapor that it does not seem likely that we
ever get a view of the solid body of the planet through it. Some
observers have thought they could see spots on Venus day after day,
while others have disputed this view. On the whole, if intelligent
inhabitants live there, it is not likely that they ever see sun or
stars. Instead of the sun they see only an effulgence in the vapory sky
which disappears and reappears at regular intervals.
When we come to Mars, we have more definite knowledge, and there seems
to be greater possibilities for life there than in the case of any
other planet besides the earth. The main reason for denying that life
such as ours could exist there is that the atmosphere of Mars is so
rare that, in the light of the most recent researches, we cannot be
fully assured that it exists at all. The very careful comparisons of
the spectra of Mars and of the moon made by Campbell at the Lick
Observatory failed to show the slightest difference in the two. If Mars
had an atmosphere as dense as ours, the result could be seen in the
darkening of the lines of the spectrum produced by the double passage
of the light through it. There were no lines in the spectrum of Mars
that were not seen with equal distinctness in that of the moon. But
this does not prove the entire absence of an atmosphere. It only shows
a limit to its density. It may be one-fifth or one-fourth the density
of that on the earth, but probably no more.
That there must be something in the nature of vapor at least seems to
be shown by the formation and disappearance of the white polar caps of
this planet. Every reader of astronomy at the present time knows that,
during the Martian winter, white caps form around the pole of the
planet which is turned away from the sun, and grow larger and larger
until the sun begins to shine upon them, when they gradually grow
smaller, and perhaps nearly disappear. It seems, therefore, fairly well
proved that, under the influence of cold, some white substance forms
around the polar regions of Mars which evaporates under the influence
of the sun's rays. It has been supposed that this substance is snow,
produced in the same way that snow is produced on the earth, by the
evaporation of water.
But there are difficulties in the way of this explanation. The sun
sends less than half as much heat to Mars as to the earth, and it does
not seem likely that the polar regions can ever receive enough of heat
to melt any considerable quantity of snow. Nor does it seem likely that
any clouds from which snow could fall ever obscure the surface of Mars.
But a very slight change in the explanation will make it tenable. Quite
possibly the white deposits may be due to something like hoar-frost
condensed from slightly moist air, without the actual production of
snow. This would produce the effect that we see. Even this explanation
implies that Mars has air and water, rare though the former may be. It
is quite possible that air as thin as that of Mars would sustain life
in some form. Life not totally unlike that on the earth may therefore
exist upon this planet for anything that we know to the contrary. More
than this we cannot say.
In the case of the outer planets the answer to our question must be in
the negative. It now seems likely that Jupiter is a body very much like
our sun, only that the dark portion is too cool to emit much, if any,
light. It is doubtful whether Jupiter has anything in the nature of a
solid surface. Its interior is in all likelihood a mass of molten
matter far above a red heat, which is surrounded by a comparatively
cool, yet, to our measure, extremely hot, vapor. The belt-like clouds
which surround the planet are due to this vapor combined with the rapid
rotation. If there is any solid surface below the atmosphere that we
can see, it is swept by winds such that nothing we have on earth could
withstand them. But, as we have said, the probabilities are very much
against there being anything like such a surface. At some great depth
in the fiery vapor there is a solid nucleus; that is all we can say.
The planet Saturn seems to be very much like that of Jupiter in its
composition. It receives so little heat from the sun that, unless it is
a mass of fiery vapor like Jupiter, the surface must be far below the
freezing-point.
We cannot speak with such certainty of Uranus and Neptune; yet the
probability seems to be that they are in much the same condition as
Saturn. They are known to have very dense atmospheres, which are made
known to us only by their absorbing some of the light of the sun. But
nothing is known of the composition of these atmospheres.
To sum up our argument: the fact that, so far as we have yet been able
to learn, only a very small proportion of the visible worlds scattered
through space are fitted to be the abode of life does not preclude the
probability that among hundreds of millions of such worlds a vast
number are so fitted. Such being the case, all the analogies of nature
lead us to believe that, whatever the process which led to life upon
this earth--whether a special act of creative power or a gradual course
of development--through that same process does life begin in every part
of the universe fitted to sustain it. The course of development
involves a gradual improvement in living forms, which by irregular
steps rise higher and higher in the scale of being. We have every
reason to believe that this is the case wherever life exists. It is,
therefore, perfectly reasonable to suppose that beings, not only
animated, but endowed with reason, inhabit countless worlds in space.
It would, indeed, be very inspiring could we learn by actual
observation what forms of society exist throughout space, and see the
members of such societies enjoying themselves by their warm firesides.
But this, so far as we can now see, is entirely beyond the possible
reach of our race, so long as it is confined to a single world.
VIII
HOW THE PLANETS ARE WEIGHED