THE PHYSICAL CONDITIONS ESSENTIAL FOR ORGANIC LIFE
The physical conditions on the surface of our earth which appear to be
necessary for the development and maintenance of living organisms may be
dealt with under the following headings:--
1. Regularity of heat-supply, resulting in a limited range of temperature.
2. A sufficient amount of solar light and heat.
3. Water in great abundance, and universally distributed.
4. An atmosphere of sufficient density, and consisting of the gases which
are essential for vegetable and animal life. These are Oxygen,
Carbonic-acid gas, Aqueous vapour, Nitrogen, and Ammonia. These must all be
present in suitable proportions.
5. Alternations of day and night.
SMALL RANGE OF TEMPERATURE REQUIRED FOR GROWTH AND DEVELOPMENT
Vital phenomena for the most part occur between the temperatures of
freezing water and 104° Fahr., and this is supposed to be due mainly to the
properties of nitrogen and its compounds, which 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 continued 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 160° Fahr.
The extreme importance to all the higher organisms 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 condition 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 internal temperature is quite
intelligible when we consider 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 recombinations, should be
effected with the greatest regularity. And though a few of the higher
animals, including man, are so perfectly organised that they can adapt or
protect themselves so as to be able to live under very extreme conditions
as regards temperature, yet this is not the case with the great majority,
or with the lower types, as evidenced by the almost complete absence of
reptiles from the arctic regions.
It must also be remembered that extreme cold and extreme heat are nowhere
perpetual. There is always some diversity of seasons, and there is no land
animal which passes its whole life where the temperature never rises above
the freezing point.
THE NECESSITY OF SOLAR LIGHT
Whether the higher animals and man could have been developed upon the earth
without solar light, even if all the other essential conditions were
present, 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 power of making protoplasm 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.
WATER A FIRST ESSENTIAL OF ORGANIC LIFE
It is hardly necessary to point out the absolute necessity of water, since
it actually constitutes a very large proportion of the material of every
living organism, and about three-fourths of our own bodies. Water,
therefore, must be present everywhere, in one form or another, on any globe
where life is possible. Neither animal nor plant can exist without it. It
must also be present in such quantity and so distributed 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 geological epochs during which
life has been developing. We shall see later on how very special are the
conditions that have secured this continuous distribution of water on our
earth, and we shall also learn that this large amount of water, its wide
distribution, and its arrangement with regard to the land-surface, is an
essential factor in producing that limited range of temperature which, as
we have seen, is a primary condition for the development and maintenance of
life.
THE ATMOSPHERE MUST BE OF SUFFICIENT DENSITY AND COMPOSED OF SUITABLE GASES
The atmosphere of any planet on which life can be developed must have
several qualities which are unconnected with each other, and the
coincidence of which may be a rare phenomenon in the universe. The first of
these is a sufficient density, which is 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 temperature, a rather dense
atmosphere is a first necessity, in co-operation with the large quantity
and wide distribution 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 considerably 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 permanently 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 temperature alone. And as there would certainly
be an added difficulty in the needful supply of oxygen to animals and
carbonic acid to plants, it seems highly probable that a reduction of
density of even one-fourth might be sufficient to render a large portion of
the globe a snow and ice-clad waste, and the remainder liable to such
extremes of climate that only low forms of life could have arisen and been
permanently maintained.
THE GASES OF THE ATMOSPHERE
Coming now to consider the constituent gases of the atmosphere, there is
reason to believe that they form a mixture as nicely balanced in regard to
animal and vegetable life as are the density and the temperature. At a
first view of the subject we might conclude that oxygen is the one great
essential for animal life, and that all else is of little importance. 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 impurities 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.
AQUEOUS VAPOUR IN THE ATMOSPHERE
This water-gas, although it occurs in the atmosphere in largely varying
quantities, is yet, in two distinct ways, essential to organic life. It
prevents the too rapid loss of moisture from the leaves of plants when
exposed to the sun, and it is also absorbed by the upper surface of the
leaf and by the young shoots, which thus obtain both water and minute
quantities of ammonia when the supply by the roots is insufficient. But it
is of even more vital importance in supplying the hydrogen which, when
united with the nitrogen of the atmosphere by electrical discharges,
produces the ammonia, which is the main source of all the proteids of the
plant, which proteids are the very foundation of animal life.
From this brief statement of the purposes served by the various gases
forming our atmosphere, we see that they are to some extent antagonistic,
and that any considerable increase of one or the other would lead to
results that might be injurious either directly or in their ultimate
results. And as the elements which constitute the bulk of all living matter
possess properties which render them alone suitable for the purpose, we may
conclude that the proportions in which they exist in our atmosphere cannot
be very widely departed from wherever organic forms are developed.
THE ALTERNATION OF DAY AND NIGHT
Although it is difficult to decide positively whether alternations of light
and darkness at short intervals are absolutely essential for the
development of the various higher forms of life, or whether a world in
which light was constant might do as well, yet on the whole it seems
probable that day and night are really important factors. All nature is
full of rhythmic movements of infinitely varied kinds, degrees, and
durations. All the motions and functions 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 harmonious 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 100 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 high 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.