radiation pressure of light, as well as of temperature equalization
and heat concentration, the evolution of the world can continue in an
eternal cycle, in which there is neither beginning nor end, and in
which life may exist and continue forever and undiminished.
VIII
THE SPREADING OF LIFE THROUGH THE UNIVERSE
We have just recognized the probability of the assumption that solar
systems have been evolved from nebulæ, and that nebulæ are produced
by the collision of suns. We likewise consider it probable that there
circulate about the newly formed suns smaller celestial bodies which
cool more rapidly than the central sun. When these satellites have
provided themselves with a solid crust, which will partly be covered
by water, they may, under favorable conditions, harbor organic life,
as the earth and probably also Venus and Mars do. The satellites would
thereby gain a greater interest for us than if we had to imagine them
as consisting entirely of lifeless matter.
The question naturally arises whether we may believe that life can
really originate on a celestial body as soon as circumstances are
favorable for its evolution and propagation. This question will occupy
us in this last chapter.
Men have been pondering over these problems since the remotest ages.
All living beings, past ages recognized, must have been generated and
they had to die after a certain shorter or longer life. Somewhat later,
and yet still in a very early epoch, experience must have taught men
that organisms of one kind can only generate other organisms of the
same kind; that the species are invariable, as we now express it. The
idea was that all species originally came from the hands of the Creator
endowed with their present qualities. This view may still be said to
represent the general or “orthodox” doctrine.
This view has also been called the Linnæan thesis, because Linné, in
the fifth edition of his _Genera Plantarum_, adheres to it strictly:
“Species tot sunt, quot diversas formas ab initio produxit Infinitum
Ens, quae deinde formae secundum generationis inditas leges produxere
plures, at sibi semper similes, ut species nunc nobis non sint plures
quam fuerunt ab initio.” Which we may render: “There are as many
different kind of species as the Infinite Being has created different
forms in the beginning. These forms have later engendered other beings
according to the laws of inheritance, always resembling them, so that
we have at the present time not any more species than there were from
the beginning.” Time was ripe, however, even then for a less rigid
conception of nature, more in accordance with our present views. The
first foundations of the theory of evolution in the biological sciences
were laid by Lamarck (in 1794), Treviranus (in 1809), Goethe and Oken
(in 1820). But a reaction set in. Cuvier and his authority forced
public opinion back to the ancient stand-point. In his view the now
extinct species of past geological epochs had been destroyed by natural
revolutions, and new species had again been generated by a new act of
the Creator.
Within the last few decades, however, the general belief has rapidly
been revolutionized, and the theory of evolution, especially since the
immortal Charles Darwin came forth with his epoch-making researches,
now meets with universal acceptance.
According to this theory the species adapt themselves in the course of
time to their surroundings, and the changes may become so great that a
new species may be considered to have originated from an old species.
The researches of De Vries have, within quite recent times, further
accentuated this view, so that we now concede cases to be extant where
new species spring forth from old ones under our very eyes. This thesis
has become known as the theory of mutation.
At the present time we accordingly imagine that living organisms,
such as we see around us, have all descended from older organisms,
rather unlike them, of which we still find traces and remnants in the
geological strata which have been deposited during past ages. From this
stand-point all living organisms might possibly have originated from
one single, most simple organism. How that was generated still remains
to be explained.
The common view, to which the ancients inclined, is that the lower
organisms need not necessarily have originated from seeds. It was
noticed that some low-type organisms, larvæ, etc., took rise in putrid
meat; Vergil describes this in his _Georgicas_. It was not until the
seventeenth century that this belief was disproved by many experiments,
among others by those of Swammerdam and Leuwenhoek. The thesis of the
so-called “Generatio spontanea” once more blossomed into new life upon
the discovery of the so-called infusoria, the small animal organisms
which seem to arise spontaneously in infusions and concoctions.
Spallanzani, however, demonstrated in 1777 that when the infusions, and
the vessel containing them, as well as the air above them, were heated
to a sufficiently high temperature to kill all the germs present, the
infusions would remain sterile, and no living organisms could develop
in them. To this fact we owe our ordinary methods of making preserves.
It is true that objections were raised against this demonstration.
The air, it was objected, is so changed by heating that subsequent
development of minute organisms is rendered impossible. But this last
objection was refuted by the chemists Chevreul and Pasteur, as well
as by the physicist Tyndall in the sixties and seventies of the past
century. These scientists demonstrated that no organisms are produced
in air which is freed from the smallest germs by some other means than
heating—_i.e._, by filtration through cotton-wool. The researches of
Pasteur, in particular, and the methods of sterilization which are
based upon them and which are applied every day in bacteriological
laboratories, have more and more forced the conviction upon us that a
germ is indispensable for the origination of life.
And yet eminent scientists take up the pen again and again in order to
demonstrate the possibility of the “Generatio spontanea.” In this they
do not rely upon the safe methods of natural science, but they proceed
on philosophical lines of argument. Life, they suggest, must once have
had a beginning, and we are hence forced to believe that spontaneous
generation, even if not realizable under actual conditions, must have
once occurred. Considerable interest was excited when the great English
physiologist Huxley believed he had discovered in the mud brought
up from the very bottom of the sea an albuminoid substance which he
called “Bathybius Haeckelii,” in honor of the zealous German Darwinist
Haeckel. In this bathybius (deep-sea organism) one fancied for a time
that the primordial ooze, which had originated from inorganic matter
and from which all organisms might have been evolved, and of which Oken
had been dreaming, had been discovered. But the more exact researches
of the chemist Buchanan demonstrated that the albuminoid substance in
this primordial ooze consisted of flocks of gypsum precipitated by
alcohol.
People then had recourse to the most fantastic speculations. Life, it
was argued, might possibly have had its origin in the incandescent mass
of the interior of the earth. At high temperatures organic compounds
of cyanogen and its derivatives might be formed which would be the
carriers of life (Pflüger). There is, however, little need of our
entering into any of these speculations until they have been provided
with an experimental basis.
Almost every year the statement is repeated in biological literature
that we have at last succeeded in producing life from dead matter.
Among the most recent assertions of this kind, the discovery claimed
by Butler-Burke has provoked much comment. He asserted that he had
succeeded, with the aid of the marvellous substance radium, in
instilling life into lifeless matter—namely, a solution of gelatine.
Criticism has, however, relegated this statement, like all similar
ones, to the realm of fairy tales.
We fully share the opinion which the great natural philosopher
Lord Kelvin has expressed in the following words: “A very ancient
speculation, still clung to by many naturalists (so much so that I have
a choice of modern terms to quote in expressing it), supposes that,
under meteorological conditions very different from the present, dead
matter may have run together or crystallized or fermented into ‘germs
of life,’ or ‘organic cells,’ or ‘protoplasm.’ But science brings a
vast mass of inductive evidence against this hypothesis of spontaneous
generation. Dead matter cannot become living without coming under the
influence of matter previously alive. This seems to me as sure a
teaching of science as the law of gravitation.”
Although the latter verdict may be a little dogmatic, it yet
demonstrates how strongly many scientists feel the necessity of
finding another way of solving the problem. The so-called theory of
panspermia really shows a way. According to this theory life-giving
seeds are drifting about in space. They encounter the planets, and fill
their surfaces with life as soon as the necessary conditions for the
existence of organic beings are established.
This view was probably foreshadowed long ago. Definite suggestions in
this direction we find in the writings of the Frenchman Sales-Guyon de
Montlivault (1821), who assumed that seeds from the moon had awakened
the first life on the surface of the earth. The German physician H. E.
Richter attempted to supplement the doctrine of Darwin by combining the
conception of panspermia with it. Flammarion’s book on the plurality
of inhabited worlds suggested to Richter the idea that seeds had come
from some other inhabited world to our earth. He emphasizes the fact
that carbon has been found in meteorites which move in orbits similar
to those of the comets which wander about in space; and in this carbon
he sees the rests of organic life. There is no proof at all for this
latter opinion. The carbon found in meteorites has never exhibited any
trace of organic structure, and we may well imagine the carbon—_e.g._,
that which appears to occur in the sun—to be of inorganic origin.
Still more fantastic is his idea that organisms floating high in our
atmosphere are caught by the attraction of meteorites flying past
our planet, and are in this way carried out into universal space and
deposited upon other celestial bodies. As the surface of meteorites
becomes incandescent in their flight through the atmosphere, any germs
which they might possibly have caught would be destroyed; and if, in
spite of that, a meteorite should become the conveyor of live germs,
those germs would be burned in the atmosphere of the planet on which
they descended.
In one point, however, we must agree with Richter. There is logic
in his statement that “The infinite space is filled with, or (more
correctly) contains, growing, mature, and dying celestial bodies. By
mature worlds we understand those which are capable of sustaining
organic life. We regard the existence of organic life in the universe
as eternal. Life has always been there; it has always propagated itself
in the shape of living organisms, from cells and from individuals
composed of cells.” Man used to speculate on the origin of matter, but
gave that up when experience taught him that matter is indestructible
and can only be transformed. For similar reasons we never inquire into
the origin of the energy of motion. And we may become accustomed to the
idea that life is eternal, and hence that it is useless to inquire into
its origin.
The ideas of Richter were taken up again in a popular lecture delivered
in 1872 by the famous botanist Ferdinand Cohn. The best-known
expression of opinion on the subject, however, is that of Sir William
Thomson (later Lord Kelvin) in his presidential address to the British
Association at Edinburgh in 1871:
“When two great masses come into collision in space, it is certain
that a large part of each is melted; but it seems also quite certain
that in many cases a large quantity of débris must be shot forth in
all directions, much of which may have experienced no greater violence
than individual pieces of rock experience in a landslip or in blasting
by gunpowder. Should the time when this earth comes into collision
with another body, comparable in dimensions to itself, be when it is
still clothed as at present with vegetation, many great and small
fragments carrying seed and living plants and animals would undoubtedly
be scattered through space. Hence, and because we all confidently
believe that there are at present, and have been from time immemorial,
many worlds of life besides our own, we must regard it as probable
in the highest degree that there are countless seed-bearing meteoric
stones moving about through space. If at the present instant no life
existed upon this earth, one such stone falling upon it might, by what
we blindly call _natural_ causes, lead to its becoming covered with
vegetation. I am fully conscious of the many objections which may be
urged against this hypothesis. I will not tax your patience further by
discussing any of them on the present occasion. All I maintain is that
I believe them to be all answerable.”
Unfortunately we cannot share Lord Kelvin’s optimism regarding this
point. It is, in the first instance, questionable whether living beings
would be able to survive the violent impact of the collision of two
worlds. We know, further, that the meteorite in its fall towards the
earth becomes incandescent all over its surface, and any seeds on it
would therefore be deprived of their germinating power. Meteorites,
moreover, show quite a different composition from that of the fragments
from the surface of the earth or a similar planet. Plants develop
almost exclusively in loose soil, and a lump of earth falling through
our atmosphere would, no doubt, be disintegrated into a shower of small
particles by the resistance of the atmosphere. Each of these particles
would by itself flash up like a shooting-star, and could not reach the
earth in any other shape than that of burned dust. Another difficulty
is that such collisions, which, as we presume, are responsible for the
flashing-up of so-called new stars, are rather rare phenomena, so that
little likelihood remains of small seeds being transported to our earth
in this manner.
The question has, however, entered into a far more favorable stage
since the effects of radiation have become understood.
Bodies which, according to the deductions of Schwarzschild, would
undergo the strongest influence of solar radiation must have a diameter
of 0.00016 mm., supposing them to be spherical. The first question is,
therefore: are there any living seeds of such extraordinary minuteness?
The reply of the botanist is that the so-called permanent spores of
many bacteria have a size of 0.0003 or 0.0002 mm., and there are, no
doubt, much smaller germs which our microscopes fail to disclose. Thus,
yellow-fever in man, rabies in dogs, the foot-and-mouth disease in
cattle, and the so-called mosaic disease—common to the tobacco plant
in Netherlandish India, and also observed in other countries—are, no
doubt, parasitical diseases; but the respective parasites have not yet
been discovered, presumably because they are too minute to be visible
under the microscope.[20]
[Footnote 20: Meanwhile a large number of organisms which are
invisible under the ordinary microscope have been rendered
visible by the aid of the ultra-microscope, among others the
presumable microbe of the foot-and-mouth disease.]
It is, therefore, very probable that there are organisms so small
that the radiation pressure of a sun would push them out into space,
where they might give rise to life on planets, provided they met with
favorable conditions for their development.
We will, in the first instance, make a rough calculation of what would
happen if such an organism were detached from the earth and pushed out
into space by the radiation pressure of our sun. The organism would,
first of all, have to cross the orbit of Mars; then the orbits of the
smaller and of the outer planets; and, having passed the last station
of our solar system, the orbit of Neptune, it would drift farther into
infinite space towards other solar systems. It is not so difficult to
estimate the time which the smallest particles would require for this
journey. Let their specific gravity be that of water, which will very
fairly correspond to the facts. The organisms would cross the orbit of
Mars after twenty days, the Jupiter orbit after eighty days, and the
orbit of Neptune after fourteen months. Our nearest solar system, Alpha
Centauri, would be reached in nine thousand years. These calculations
have been made under the supposition that the radiation pressure is
four times as strong as gravitation, which would be nearly correct
according to the figures of Schwarzschild.[21]
[Footnote 21: The radiation pressure has here been assumed to
be somewhat greater than on page 103, because the spores are
here regarded as opaque, while the drops of hydrocarbons have
been regarded as partially translucid to luminous rays.]
These time intervals required for the organisms to reach the different
planets of our solar system are not too long for the germs in question
to preserve their germinating power. The estimate is more unfavorable
in the case of their transference from one planetary system to another,
which will require thousands of years. But we shall see further on that
the very low temperature of those parts of space (about -220° C.) would
suspend the extinction of the germinating power, as it arrests all
chemical reactions.
As regards the period during which the germinating power can be
preserved at ordinary temperature, we have been told that the so-called
“mummy wheat” which had been found in ancient Egyptian tombs was still
capable of germination. Critics, however, have established that the
respective statements of the Arabs concerning the sources of that wheat
are very doubtful. The French scientist Baudoin asserts that bacteria
capable of germination were found in a Roman tomb which had certainly
remained untouched for eighteen hundred years; but this statement is to
be received with caution. It is certain, however, that both seeds of
some higher plants and spores of certain bacteria—_e.g._, anthrax—do
maintain their germinating power for several years (say, twenty), and
thus for periods which are much longer than those we have estimated as
necessary for their transference to our planet.
On the road from the earth the germs would for about a month be exposed
to the powerful light of the sun, and it has been demonstrated that
the most highly refrangible rays of the sun can kill bacteria and
their spores in relatively short periods. As a rule, however, these
experiments have been conducted in such a manner that the spores could
germinate on the moist surface on which they were deposited (for
instance, in Marshall Ward’s experiments). That, however, does not at
all conform to the conditions prevailing in planetary space. For Roux
has shown that anthrax spores, which are readily killed by light when
the air has access, remain alive when the air is excluded. Some spores
do not suffer from insulation at all. That applies, for instance,
according to Duclaux, to _Thyrothrix scaber_, which occurs in milk and
which may live for a full month under the intense light of the sun. All
the botanists that I have been able to consult are of the opinion that
we can by no means assert with certainty that spores would be killed
by the light rays in wandering through infinite space.
It may further be argued that the spores, in their journey through
universal space, would be exposed during most of that period to an
extreme cold which possibly they might not be able to endure. When the
spores have passed the orbit of Neptune, their temperature will have
sunk to -220°, and farther out it will sink still lower. In recent years
experiments have been made in the Jenner Institute, in London, with
spores of bacteria which were kept for twenty hours at a temperature
of -252° in liquid hydrogen. Their germinating power was not destroyed
thereby.
Professor Macfadyen has, indeed, gone still further. He has
demonstrated that micro-organisms may be kept in liquid air (at -200°)
for six months without being deprived of their germinating power.
According to what I was told on the occasion of my last visit to
London, further experiments, continued for still longer periods, have
only confirmed this observation.
There is nothing improbable in the idea that the germinating power
should be preserved at lower temperatures for longer periods than at
our ordinary temperatures. The loss of germinating power is no doubt
due to some chemical process, and all chemical processes proceed at
slower rates at lower temperatures than they do at higher. The vital
functions are intensified in the ratio of 1: 2.5 when the temperature
is raised by 10° C. (18° F.). By the time that the spores reached the
orbit of Neptune and their temperature had been lowered to -220°, their
vital energy would, according to this ratio, react with one thousand
millions less intensity than at 10°. The germinating power of the
spores would hence, at -220°, during the period of three million years,
not be diminished to any greater degree than during one day at 10°.
It is, therefore, not at all unreasonable to assert that the intense
cold of space will act like a most effective preservative upon the
seeds, and that they will in consequence be able to endure much longer
journeys than we could assume if we judged from their behavior at
ordinary temperatures.
It is similar with the drying effect which may be so injurious to plant
life. In interplanetary space, which is devoid of atmosphere, absolute
dryness prevails. An investigation by B. Schröber demonstrates that the
green alga _Pleurococcus vulgaris_, which is so common on the trunks
of trees, can be kept in absolute dryness (over concentrated sulphuric
acid in a desiccator) for twenty weeks without being killed. Seeds and
spores may last still longer in a dry atmosphere.
Now, the tension of water vapor decreases in nearly the same ratio as
the speed of the reaction with lower temperatures. The evaporation of
water—_i.e._, the drying effect—may hence, at a temperature of -220°,
not proceed further in three million years than it will in one day at
10°. We have thus several plausible reasons for concluding that spores
which oppose an effective resistance to drying may well be carried from
one planet to another and from one planetary system to another without
sacrificing their vital energy.
The destructive effect of light is, according to the experiments of
Roux, no doubt due to the fact that the rays of light call forth
an oxidation by the intermediation of the surrounding air. This
possibility is excluded in interplanetary space. Moreover, the
radiation of the sun is nine hundred times fainter in the orbit
of Neptune than in the orbit of the earth, and half-way to the
nearest fixed star, Alpha Centauri, twenty million times feebler.
Light, therefore, will not do much harm to the spores during their
transference.