THE WHEEL OF LIFE
I heard them in their sadness say
The earth rebukes the thought of God;
We are but embers wrapped in clay,
A little nobler than the sod.
But I have touched the lips of clay;
Mother, thy rudest sod to me
Is thrilled with fire of hidden day,
And haunted by all mystery.—A.E.
It has long been known that no definite line of demarcation can be
drawn between the animal and the vegetable worlds. There are lowly
organisms which cannot be decisively referred either to the one or
to the other. It has been more recently shown that the apparently
more strongly marked line between the living and the non-living also
grows wavering and indistinct in places. Metals are known to respond
to stimuli and to show ‘fatigue’ in a manner commonly attributed only
to the nervous system of animals,[16] while some of the phenomena of
crystallization strongly resemble those of vitality.[17] Le Dantec has
uttered the latest word of physics on this subject, where he insists on
the “absence of all essential difference and all absolute discontinuity
between living and non-living matter.”[18] Indeed, one may say of
nature-study in general, that if, as Plato said, the beginning of
knowledge is in definitions and classification, the end of it lies in
getting rid of them. There is probably no such thing as a universally
applicable definition of any group of natural phenomena. _There is
certainly no condition of matter of which we are entitled to say that
Life is impossible without it._ Still, natural groups have well-marked
central features, even if at their margins they melt into something
else. Now the things which in the ordinary sense of the word we call
Living are marked by these characters: Their chemical constituents
are always compounds of carbon. These compounds are what is called
‘unstable’—they ‘consume’ or disintegrate by combining with oxygen in
air or water. In this process organisms obtain the energy necessary
for assimilation and growth. The above characters (carbon-compounds,
chemical instability, and faculty of assimilation) apply to plants and
animals alike. But we find, in general, that plants are able, from
inorganic mineral constituents such as carbonic acid, water, nitrates,
sulphates, etc.,[19] to build up the organic compounds like proteid,
albumin, the carbohydrates, alcohol, fat; while animals use for their
nourishment not the inorganic substances but only organic compounds
already formed by plants or by other animals. A well-developed
vegetable world must therefore, it would seem, have preceded the
appearance of animal life on the globe.[20] It was long believed that
these organic compounds of carbon could only be formed by the vital
action of living vegetation. One of the epochs in the history of modern
chemistry has been the demonstration (first by Wöhler, in 1828,) that
many of them can be produced in the laboratory from inorganic chemical
constituents. But this is only effected by circuitous and difficult
ways, and—a circumstance often overlooked—it only resembles what is
accomplished in nature if we include under nature the directive agency
represented by the chemist himself, as well as the materials with which
he deals.
The characteristic colour of living vegetation is green. This is also
the rarest of colours among the higher animals.[21] It is due in
vegetables to the presence in their cells of grains of the substance
known as chlorophyll, which very few animals possess or have need
of. It is developed normally under the action of sunlight, and plays
a most important part in the economy of the plant. The usual method
by which any organism obtains the energy necessary for its vital
functions is through the oxidization, _i.e._ the slow burning, of its
substance, by combination with the oxygen of the air. The process is
to all intents the same as the more rapid oxidization, under great
heat, of coal in a steam-engine. If a plant can obtain sugar, which
oxidizes easily in contact with atmospheric oxygen, it has thus a fund
of energy to draw on for all the processes of its life. Now sugar is
composed of carbon and water. Carbon exists in the air, in combination
with oxygen, in the form known as carbonic acid. Chlorophyll, in some
way as yet unexplained, enables plants, when acted upon by light, to
take in carbonic acid from the air and to disintegrate it into its
constituents, carbon and oxygen. The oxygen disappears again in the
air, and the released carbon combines with water in the plant to form
sugar,[22] thus giving the plant its needed store of potential energy.
All it does with this energy is to live, grow, and reproduce its kind;
till at length a time comes when the assimilative energy weakens
relatively to the forces of decay, and the plant dies; it is again
resolved into the chemical constituents from which it was built up;
but not without having passed on the flame of life to burn afresh in
its descendants.
Plants which have no chlorophyll, like certain bacteria and moulds, and
which, therefore, cannot decompose the carbonic acid in the air for
their nourishment, offer an interesting example of the manner in which
Nature contrives to get her way, if not by the normal instruments, then
by the utilization of others. They acquire their first store of energy
sometimes like animals, from other organic compounds, or they take
carbon from acetates and tartrates. The nitro-bacteria appear to depend
on ammonia derived from decaying animal matter, and the moulds draw
their energy from sugar, which (as in our jams, etc.) they find already
formed.
There are other plants, such as the fly-eating _Drosera_, which feed
upon organic substances with the aid of digestive juices, exactly as
animals do; and there are animals, such as _Hydra_ and others, of very
primitive form,[23] which produce chlorophyll and are thereby enabled,
like plants, to feed upon carbonic acid. The distinction therefore
which has been drawn between the two kingdoms as regards their modes of
nourishment must, like other definitions of natural groups, be taken
to apply to central and typical forms and not to constitute a distinct
boundary line. Allowing for these exceptional cases, we may say
broadly that the wheel of life makes its full circle in passing from
inorganic matter through plants to animals and thence back to gases and
minerals again. The process of taking in fresh matter, transforming it
chemically into living tissue, and thus repairing the waste occasioned
by the decomposition of the carbon-compounds of that tissue, is
technically known as Metabolism. This is the typical and characteristic
function of organic life.
Now this function of living matter, or Protoplasm, depends upon two
elements: first, its Substance; secondly, its Structure. As regards
the former, we are in this serious difficulty, that living matter can
never be chemically investigated by any means at present known, for
it dies immediately in presence of any of the reagents which are used
to ascertain its chemical composition. It is known that there are no
_elementary_ substances in living matter which are not also found
in the world of inorganic matter, but it is also known that their
synthetic combination in living is different from that which obtains in
dead tissue,[24] and it is precisely through this factor—that of the
grouping or synthesis of elements—that the most remarkable forms of
energy are developed.
The secret of life, therefore, cannot be stated in terms of chemistry,
because we cannot surprise the secret of its chemical synthesis. Even
if we could do this we should still be unable to say why certain
syntheses should appear in living matter and resolve themselves into
others at death.
We find, however, in the investigation of organic tissue (plant or
animal) by such means as are available, that one substance is common
to all the organic and is never found (as such) in the inorganic
world. This is called Proteid. It is composed of five elements—Carbon,
Hydrogen, Sulphur, Nitrogen, and Oxygen, which are combined in
proportions not at present ascertained. Subject to the limitations just
set forth we may say that proteid is the essential stuff of organic
tissue. The two other usual (though not, like proteid, universal)
constituents of this tissue—the Carbohydrates (sugar, starch, etc.) and
the Fats—are, it is believed, formed partly from the products of the
metabolism of proteid.
When we come to deal with the essential Structure of life we are
in much the same difficulty as that in which we found ourselves in
investigating its chemical Substance. We can observe living cells under
the microscope, but the most powerful microscope has never reached
the limits beyond which we can say that there is no structure. There
is another limitation too. The microscope has revealed the fact that
all living tissue is made up of cells, but the internal structure of
the cell, beyond the fact that it is composed of a fluid substance
within which a darker coloured nucleus is usually embedded, could not
be ascertained until the recent device of staining the object with
aniline dyes had been thought of. Different substances in the cell are
found to take these dyes differently, and thus a world of structure
of the most singular kind has been revealed in what formerly seemed a
simple, semi-transparent fluid. Some parts of this structure hover, as
it were, upon the very edge of perceptibility, the most suitable dyes
for bringing them under observation not having been as yet discovered.
There may be others which no dye can reveal, but which are yet active
and necessary parts of the organism. Moreover, here too the cell is
killed by the means taken to observe it, and the processes in which its
structure is engaged can only as a rule be deduced from the observation
of a great number of cells in which their internal movements are
arrested at different stages of completion.
It has been practically demonstrated that all organic life must
be at least duplex if not multiplex in its constituent elements.
In its simplest known form it consists of Protoplasm and Nucleus.
We know that the carrying-on of all vital functions depends on
peculiar relations existing between these two elements, but what
these relations exactly are is still quite obscure. Both protoplasm
and nucleus are compounds of proteid with other chemical substances
not yet fully determined. Protoplasm is a fluid, and has been shown
by the epoch-making observations of Bütschli[25] to have a structure
resembling that of an exceedingly minute foam. The nucleus usually
exists in the form of a single definite body, but it may be scattered
through the protoplasm of the organism in little granules. In the
lowliest of organisms, the _Amœbæ_, we have simply a speck of
protoplasm containing a nucleus, but with no surrounding wall of the
harder substance which protoplasm builds up for itself in the cells
belonging to higher forms of life. Such amœboid forms are the white
corpuscles in the human blood, whose slow changes of form we can
observe under the microscope, and which play so important a part in our
economy by feeding on the noxious bacteria which produce the various
forms of blood-poisoning and zymotic disease.
A more detailed account of the functions and structure of the cell must
be reserved for the next chapter. In considering these and all other
phenomena of vitality let me again recall the warning expressed in the
taunt of Mephistopheles to the young student: the lines are as true
to-day as they were when Goethe wrote them over a hundred years ago:—
“If some living thing you would learn about,
You begin by driving its Spirit out;
There lie the parts of it, one by one,
But the binding Spirit, alas, is gone!”