the minute seed of one of the great tropical fig-trees, and another
seed of a strawberry, or of garden cress. Both will be about the same
size and shape, and the most acute microscopist would not find any
difference in the internal structure that could intelligibly account
for the different results when these little grains of protoplasm are
exposed to identical conditions. For, even if planted near each other,
and exposed to the same amount of heat and moisture, to the very same
atmosphere, and the same kind of water, as well as identically the
same soil, yet invariably the one will grow into a large tree, the
other into a small herb, and in the course of time, still with no
change whatever of the physical conditions to which both are exposed,
each will produce its peculiar foliage, and flowers, and fruit, very
different in all their characters from those of the other. Were this
result not so common as to seem to us “natural,” we should call it
a miracle; and it is really and essentially as inexplicable as many
things which are termed miracles only because they are unfamiliar and
inexplicable.
Now, this wonderful substance, the physical base of all life--and as
it is the only base that exists, or has ever existed, on the earth, we
may fairly assume that no other is possible--can only maintain itself
and perform its functions under certain very definite conditions, which
conditions are now maintained on our earth’s surface, and must have
been maintained throughout the long geological periods during which
life has been slowly developing. What these conditions are we will now
proceed to show.
[Sidenote: The First Essential for Life]
The first essential for organic life is a certain very limited range of
temperature. We are so accustomed to consider the change of temperature
from winter to summer, from day to night, and that which occurs when we
pass from the tropics to the Polar regions as being very great, that we
do not realise what a small proportion such changes bear to the whole
range of temperature that exists in the known universe. The absolute
zero of temperature is calculated to be minus 461° F., while the heat
of the sun has been determined to be over 10,000° F., and many of the
stars are known to be much hotter than the sun. The actual range of
temperature is therefore enormous; but any development of organic life
is possible only within the very narrow limits of the freezing and
boiling points of water, since within those temperatures only is the
existence of liquid water possible. But a much less range than this
is really required, because albumen, one of the commonest forms of
protoplasm, is coagulated or solidified at a temperature of about 160°
F. Now, if, as is generally believed, the earth has been once a liquid
or even a gaseous mass and has since cooled to its present temperature
on the surface, and the sun is undergoing a similar process of cooling,
we are able to understand that the very limited range of temperature
within which life development is possible implies an equally limited
period of time as compared with that occupied by the whole process of
solar and planetary development.
[Sidenote: We Live by the Heat of the Sun]
It must be understood, however, that the present temperature of the
earth’s surface is due entirely to sun-heat, and that if that were
withdrawn or greatly diminished the whole surface of the globe would
be permanently far below the freezing point and all the oceans be
frozen for a considerable depth; so that all organic life would become
extinct. Under such conditions no renewed development of life would be
possible; and it is therefore quite certain that the sun has actually
maintained the uniform moderate temperature required, and must continue
to maintain it for whatever future period man is destined to continue
his existence upon the earth.
But it is not only a certain amount of heat that is required, but also
a sufficient quantity of light; and this implies a further restriction
of conditions, because light is due to vibrations of a limited range of
wave-length, and without these particular rays plants cannot take the
carbon from the carbonic acid in the atmosphere, and by its means build
up the wonderful series of carbon compounds, including protoplasm,
which are essential for the life of animals. What is commonly termed
dark heat, therefore, would not be sufficient for the development
of any but the lowest forms of life, even though it produced the
necessary temperature during a sufficient period of time.
All organisms, from the lowest to the highest, whether plant or animal,
consist very largely of water, and its constant presence either in the
liquid or gaseous form is essential for organic life. On our earth
oceans and seas occupy the greater part of the surface, while their
average depth is so great that the quantity of water is sufficient to
cover the whole of the globe free from inequalities two miles deep.
It is this enormous amount of water that supplies the air with ample
moisture, such as renders the life of the tropics so luxuriant. Yet
even now the inequality of water-supply is such that large areas in all
parts of the earth are what we term deserts, only supporting a very few
forms of life that have become specially adapted to them, and certainly
unfitted for the continuous development of life from lower to higher
forms.
[Sidenote: Water and the Atmosphere]
Water is also of immense importance as an equaliser of temperature, the
currents of the ocean conveying the warmth of the tropics to ameliorate
the severity of temperate and Polar regions, while the amount of
water-vapour in the atmosphere acts as a retainer of heat during the
night, without which it is probable that the surface of the earth would
freeze every night even in the tropics. When we consider that water
consists of two gases--oxygen and hydrogen--in definite proportions,
and that without their presence in these proportions and in the
necessary quantity the development of organic life would have been
impossible, we find that we have here a remarkable and very complex set
of conditions which must be fulfilled in any planet to enable it to
develop life.
But this is not all. The atmosphere is so intimately associated with
water in its life-relations, and is itself so absolutely essential to
the existence from moment to moment of the higher animals, that the two
require to be duly proportioned to each other and to the globe of which
they form a part.
[Sidenote: How Water Protects Earth by Night]
In the first place the atmosphere must be of a sufficient density,
this being needed in order that it may be an adequate storer up of
solar heat, and also in order that it may be able to supply sufficient
oxygen, water-vapour, and carbonic-acid gas for the requirements of
both vegetable and animal life. We have a striking example of the use
of air as a storer-up and distributor of heat and moisture in the
very different character of our south-west and north-east winds. The
effect of the density of the air is equally well shown when we ascend
lofty mountains where we find perpetual snow and ice, due simply to
the fact that the air is not dense enough to retain the heat of the
sun--which is actually greater than at low levels--so that at night
the temperature regularly falls below the freezing point. On the other
hand a very much denser atmosphere would absorb so much water vapour as
probably to shut out the light of the sun, and thus have a prejudicial
effect on vegetable life.
Again, there is good reason to believe that the proportions of the
various gases in the atmosphere are, within certain narrow limits, such
as are most favourable not only for the life that actually exists, but
for any life that could be developed from the elements that constitute
the universe. Oxygen has properties which seem absolutely essential to
organic life; but nitrogen, though only serving to dilute the oxygen
so far as the higher animals are directly concerned, is yet indirectly
essential for them, since it is in vegetables a constituent of that
protoplasm which is the very substance of their bodies.
[Sidenote: Use of Thunderstorms]
[Sidenote: The Wonder of the Atmosphere]
Now, plants obtain their nitrogen mainly from the minute proportion
of ammonia that exists in the atmosphere, and this ammonia is formed
by the union of the nitrogen of the air with the hydrogen of the
water-vapour under the influence of electric discharges--that is,
of thunderstorms. It is evident, then, that the required amount of
this essential compound will depend upon a due adjustment of the
quantities of nitrogen and aqueous vapour always present; while the
electric discharges seem to be due to the friction of various strata
of air with each other and with the earth’s surface, due to the winds
and storms; and winds are due to highly complex causes, involving
the rate of the earth’s rotation, the rise and fall of the tide, the
density of the atmosphere, the quantity of its aqueous vapour, and the
amount of solar heat which it receives. Unless all these very diverse
factors existed in their due proportion, some of the results might be
highly prejudicial if not quite inimical to the development of life.
To these various adaptations of our gaseous envelope we must add one
other. Carbonic acid gas in the atmosphere is absolutely essential
to vegetable life, while it is directly antagonistic to that of the
higher animals. Its quantity must, therefore, be strictly proportionate
to the needs of both; and that beneficial proportion must have been
preserved throughout the whole period of the existence of the higher
air-breathing animals.
These various considerations show us that our atmosphere, consisting
as it does mainly of two common gases mixed together, and therefore
seeming to most people one of the simplest things possible, is really a
wonderfully complex arrangement which is adapted to serve the purposes
of living organisms in a great variety of ways. But this by no means
exhausts the subject of its adaptation to support and develop organic
life, because its very existence on the earth in a suitable quantity
and composed of the essential elements can be shown to depend on other
and deeper relations which will now be pointed out.
The older writers on the subject of the habitability of the planets
took no account whatever of the importance of size, distance from the
sun, period of rotation, and obliquity of the ecliptic as determining
the possibility of organic life, but simply assumed that, because the
earth possessed an abundant life-development, all the other planets
must also possess it. But we know that the above-mentioned factors are
of very high importance, as we will proceed briefly to point out.
[Sidenote: Earth’s Envelope of Gas]
It is now believed that the amount of atmosphere possessed by a
planet is due mainly, perhaps entirely, to the planet’s mass, and its
consequent gravitative power. Spectrum-analysis has shown that vast
masses of gaseous matter exist in the universe, and it is probable
that, in a state of extreme tenuity, these are very widely diffused.
Just as meteoric dust is constantly attracted to the earth, and
periodically in larger quantities, so are gases, and supposing the
aggregations of free gaseous matter to have been distributed with some
approach to uniformity, then, as planets grew in size, they would also
tend to secure a larger amount of the diffused gases, thus forming
deeper atmospheres. The observed facts agree with this view. The
largest planets, Jupiter and Saturn, have such a depth of atmosphere as
permanently to obscure any solid interior they may possess. The only
planet closely approaching the earth in size and density--Venus--has
an atmosphere which appears to be loftier than ours, but it may be
composed of different gases. Mars, which has only one-ninth the mass
of the earth, has a lofty but very tenuous atmosphere, and probably no
water, the Polar snows being due probably to the freezing of some dense
gas. The climate and physical condition of Mars is, however, still a
subject of much controversy, which I hope to discuss in a separate work
dealing with the arguments of Professor Lowell [see page 105]. In that
volume the reader will find, fully set forth my reasons, on scientific
grounds, against the supposed habitability of Mars.
[Sidenote: The Earth Selects and Uses Gas]
But, besides attracting cosmic masses of gaseous matter to form its
atmosphere, there is another equally important function of the mass of
a planet--its selective power on the kind of gases it can permanently
retain in a free state. The molecules of gases are in a condition of
rapid motion in all directions, which explains the elastic force they
exhibit. The speed of this motion has been determined for all the chief
gases, and also the gravitative force necessary to prevent them from
continually escaping into space from the upper limit of the atmosphere.
Thus the moon, which has a mass only one-eightieth that of the earth,
can retain no free gas whatever on its surface. Mars can retain only
the very heavy gases, but neither hydrogen nor water-vapour. The earth,
however, has force enough to retain all the gases except hydrogen,
which is just beyond its limit; and this may explain why it is that
there is no free hydrogen in the atmosphere, although this gas is
continually produced in small quantities by submarine volcanoes, is
emitted sometimes from fissures in volcanic regions, and is a product
of decaying vegetation. Once united with oxygen to form water, it
becomes amenable to gravity in the form of invisible aqueous vapour,
and is thenceforth a permanent possession for us in its most valuable
form.
[Illustration: EARLY ICE AGE, WHEN MAMMOTHS ROAMED THE EARTH AND MAN
WAS ARISING]
The very accurate adjustments that render our earth suitable for
the production and long-continued development of organic life,
culminating in man, may be well shown by another consideration. If our
earth had been 9,600 miles instead of 8,000 miles in diameter--a very
small increase in view of the immense range of planetary magnitudes
from Mercury to Jupiter--with a slight proportionate increase in
density, due to its greater force of gravitative compression, its
mass would have been about double what it is now. This would probably
have led to its having attracted and retained double the amount of
gases, in which case the water produced would have been double what
it is--perhaps even more, because hydrogen gas would not then escape
into space as it does now. But the surface of the globe would have
been only one-half greater than at present; so that, unless the ocean
cavities were twice as deep as they actually are, the whole surface of
the earth--except, perhaps, a few tops of submarine volcanoes--would
have been covered several miles deep in water, and all terrestrial life
would have been impossible.
[Sidenote: The Deep Atmosphere of Venus]
From the various considerations here set forth it appears clear to me
that no other planet of the solar system makes any approach to the
conditions essential for the development of a rich and varied organic
life such as adorns our earth. One only--Venus--has a sufficient bulk
and density to give it the needful atmosphere; but as it receives
about twice as much solar heat as does the earth, it is probable that
its very deep atmosphere may be mainly due to the fact that a large
proportion of its water is held in a state of vapour, its seas and
oceans being proportionately reduced in extent. Judging from what
happens on the earth, this would probably lead to an excessive area
of deserts, and thus be inimical to life. But this planet appears to
possess one feature which renders it fundamentally unsuitable for
organic life.
[Sidenote: Why there is no Life on Venus]
Several modern observers have found that the older astronomers were
all in error in giving Venus a rotation-period almost exactly the
same as ours, an error due to the indefinite and variable markings
of its surface. They have now deduced a period about equal to that
of its revolution round the sun--a rate which has been confirmed by
spectrum-analysis, and further confirmed by the fact that this planet
has no measurable polar compression. As during transits of Venus over
the sun’s disc the conditions for the accurate measurement of the
compression, if any exist, are the best possible, and as none has been
found, this alone affords a demonstration that the rate of rotation
must be very slow, because the laws of motion _necessitate_ a definite
amount of equatorial protuberance corresponding to that rate. Half the
surface has, therefore, perpetual day and the other half perpetual
night, leading to violent contrasts of heat and cold for the two
hemispheres with, in all probability, correspondingly violent winds,
rains, and electrical disturbances--conditions so entirely opposed
to the uniformity of temperatures and stability of meteorological
phenomena during long geological epochs which are essential for the
full development of organic life, that such development is perhaps less
probable on this planet than on any other.
I think I have now shown not only that no other planet in the solar
system makes any approach to the possession of the varied and complex
adaptations which are essential for a full development of organic life,
but also that on the Earth itself the conditions are so numerous and so
nicely balanced that very moderate deviations in excess or defect of
what actually exists in the case of any one of them--and of others not
referred to here--might have rendered it equally unsuitable, so that
either no organic life at all, or only a very low type of life, could
have been developed or supported.
[Sidenote: There is Purpose in our World]
If, then, the more superficial indications of design in the relations
of animals to their environment, and of man to the universe, have been
shown by modern science to have required no _special_ interference of
a higher power to bring them about, but that they have been due to
natural laws acting in accordance with and in subordination to the
deeper laws and forces that determine the very constitution of matter
and the unknown power and principle we term “life,”--yet, on the other
hand, we find that a more careful study of the outer universe, or
cosmos, reveals a new set of adaptations not less wonderful or more
easily explicable by chance coincidence than those presented by the
organic world.
Even the very brief sketch of the subject here given suggests the
idea of _purpose_ in a world so precisely and uniquely adapted to
develop organic life, and to support that life during the countless
ages required for the completed evolution of man. But that suggestion
becomes a logical induction when the whole of the available evidence
is set forth, as I have attempted to set it forth in my work on “Man’s
Place in the Universe.” I have there shown not only that the cumulative
evidence for the earth being the only supporter of a fully-developed
organic life within the solar system is irresistible, but that there
is some direct, and much more indirect, evidence that this uniqueness
extends to the whole stellar universe; and it is certain that no
particle of _direct_ evidence for the existence of organic life
elsewhere has been, or is likely to be, adduced.
I have also shown (in an appendix to the second edition of my book)
that the purely biological argument for the uniqueness of the
development of man--as the culminating point of one line of descent
throughout the diverging ramifications of the animal kingdom--is
overwhelmingly strong; hence the logical conclusion from the whole
of the evidence is that man is the one supreme product of the whole
material universe.
My object in the present essay has been limited to showing that,
besides and beyond the special adaptations of the various kinds of
animals and plants to their special environments, there exist in the
earth as a planet, in its various physical and cosmical relations, a
whole series of adaptations of a very remarkable character which, so
far as we can judge, are essential to its function as a life-producing
world. The study of these adaptations, therefore, may be considered
to be appropriate here, as constituting a preliminary chapter in the
natural history of the Earth and of Mankind.
ALFRED RUSSEL WALLACE
[Illustration: IN THE DAYS OF THE SEA MONSTERS
Reproduced from a plate in Hawkins’ “Book of the Great Sea
Dragons.”
]
THE BEGINNING OF LIFE ON THE EARTH
BY DR. C. W. SALEEBY
[Sidenote: The Earth Without Life]
For some decades past we have been faced with a critical difficulty
at the most critical and important point in the history of the earth.
In the first place, it has been definitely established that in the
earlier period of its history there was no life whatever--as the word
is usually understood--upon the earth, as is abundantly shown elsewhere
in this work. None of the conditions that make life possible, as we
know it, were satisfied. As a recent French writer has said, life is
an aquatic phenomenon, absolutely incapable of existence except in the
presence of liquid water; and there was an age of vast duration in the
history of the earth when all its water must have been in the gaseous
state. Other reasons of equal cogency may be at present ignored. The
broad fact is that, however widely students of this matter may differ
on other points, there is absolute agreement upon the cardinal and
initial fact that whereas there is life upon the earth now, there was a
time when there was none.
[Sidenote: A Gap in the Philosophy of Evolution]
Now, in the ever memorable year 1859, Charles Darwin published a
volume, the main thesis of which is now universally accepted, wherein
the following is the last sentence: “There is grandeur in this view of
life, with its several powers, having been originally breathed by the
Creator into a few forms or into one; and that, whilst this planet has
gone cycling on according to the fixed law of gravity, from so simple
a beginning endless forms most beautiful and most wonderful have been,
and are being evolved.” “The Origin of Species” may be said, in a
word, to establish the doctrine of the evolution of living organisms
upon the earth “by laws acting around us”--to use Darwin’s own phrase.
But Darwin’s work begins with and assumes the existence of life as an
established planetary fact. There obviously remains a tremendous gap in
the evolutionary philosophy as it stands in our statement of it thus
far; and the first fact which we have to note is that the existence
and recognition of this supposed gap, so far from being a matter of
common recognition from the earliest times, so far from being an
observation made by the critics of the doctrine of evolution, is, on
the contrary, a special doctrine peculiar to scientific study and of
quite recent origin, being indeed established--as was supposed--within
the memory of many now living.
If we turn to the first chapter of Genesis, we shall see no suggestion
or recognition of the supposed difficulty involved in the beginning of
life upon the earth. In this immortal piece of ancient poetry it is
stated that after the creation of the heaven and the earth, which were
at first “without form and void,” God said, “Let the earth bring forth
grass ... and it was so”; and later God said, “Let the waters bring
forth abundantly the moving creature that hath life ... let the earth
bring forth the living creature after his kind.” Here we have suggested
to us the natural origin of living creatures in earth and sea under the
will and direction of the Creator as conceived by the poet.
[Sidenote: First Ideas on the Origin of Life]
[Sidenote: The Coming of Darwin]