these substances is indicated in a very clear and convincing manner,
when we take into consideration the very small relative amount of
the carbon which these animals consume in the nitrogenised
constituents of their food, which bears no proportion whatever to
the oxygen absorbed through the skin and lungs.
A horse, for example, can be kept in perfectly good condition, if he
obtain as food 15 lbs. of hay and 4 1/2 lbs. of oats daily. If we
now calculate the whole amount of nitrogen in these matters, as
ascertained by analysis (1 1/2 per cent. in the hay, 2.2 per cent.
in the oats), in the form of blood, that is, as fibrine and albumen,
with the due proportion of water in blood (80 per cent.), the horse
receives daily no more than 4 1/2 oz. of nitrogen, corresponding to
about 8 lbs. of blood. But along with this nitrogen, that is,
combined with it in the form of fibrine or albumen, the animal
receives only about 14 1/2 oz. of carbon.
Without going further into the calculation, it will readily be
admitted, that the volume of air inspired and expired by a horse,
the quantity of oxygen consumed, and, as a necessary consequence,
the amount of carbonic acid given out by the animal, are much
greater than in the respiratory process in man. But an adult man
consumes daily abut 14 oz. of carbon, and the determination of
Boussingault, according to which a horse expires 79 oz. daily,
cannot be very far from the truth.
In the nitrogenised constituents of his food, therefore, the horse
receives rather less than the fifth part of the carbon which his
organism requires for the support of the respiratory process; and we
see that the wisdom of the Creator has added to his food the
four-fifths which are wanting, in various forms, as starch, sugar,
&c. with which the animal must be supplied, or his organism will be
destroyed by the action of the oxygen.
It is obvious, that in the system of the graminivora, whose food
contains so small a portion, relatively, of the constituents of the
blood, the process of metamorphosis in existing tissues, and
consequently their restoration or reproduction, must go on far less
rapidly than in the carnivora. Were this not the case, a vegetation
a thousand times more luxuriant than the actual one would not
suffice for their nourishment. Sugar, gum, and starch, would no
longer be necessary to support life in these animals, because, in
that case, the products of the waste, or metamorphosis of the
organised tissues, would contain enough carbon to support the
respiratory process.
LETTER X
My dear Sir,
Let me now apply the principles announced in the preceding letters
to the circumstances of our own species. Man, when confined to
animal food, requires for his support and nourishment extensive
sources of food, even more widely extended than the lion and tiger,
because, when he has the opportunity, he kills without eating.
A nation of hunters, on a limited space, is utterly incapable of
increasing its numbers beyond a certain point, which is soon
attained. The carbon necessary for respiration must be obtained from
the animals, of which only a limited number can live on the space
supposed. These animals collect from plants the constituents of
their organs and of their blood, and yield them, in turn, to the
savages who live by the chase alone. They, again, receive this food
unaccompanied by those compounds, destitute of nitrogen, which,
during the life of the animals, served to support the respiratory
process. In such men, confined to an animal diet, it is the carbon
of the flesh and of the blood which must take the place of starch
and sugar.
But 15 lbs. of flesh contain no more carbon than 4 lbs. of starch,
and while the savage with one animal and an equal weight of starch
should maintain life and health for a certain number of days, he
would be compelled, if confined to flesh alone, in order to procure
the carbon necessary for respiration, during the same time, to
consume five such animals.
It is easy to see, from these considerations, how close the
connection is between agriculture and the multiplication of the
human species. The cultivation of our crops has ultimately no other
object than the production of a maximum of those substances which
are adapted for assimilation and respiration, in the smallest
possible space. Grain and other nutritious vegetables yield us, not
only in starch, sugar, and gum, the carbon which protects our organs
from the action of oxygen, and produces in the organism the heat
which is essential to life, but also in the form of vegetable
fibrine, albumen, and caseine, our blood, from which the other parts
of our body are developed.
Man, when confined to animal food, respires, like the carnivora, at
the expense of the matters produced by the metamorphosis of
organised tissues; and, just as the lion, tiger, hyaena, in the
cages of a menagerie, are compelled to accelerate the waste of the
organised tissues by incessant motion, in order to furnish the
matter necessary for respiration, so, the savage, for the very same
object, is forced to make the most laborious exertions, and go
through a vast amount of muscular exercise. He is compelled to
consume force merely in order to supply matter for respiration.
Cultivation is the economy of force. Science teaches us the simplest
means of obtaining the greatest effect with the smallest expenditure
of power, and with given means to produce a maximum of force. The
unprofitable exertion of power, the waste of force in agriculture,
in other branches of industry, in science, or in social economy, is
characteristic of the savage state, or of the want of knowledge.
In accordance with what I have already stated, you will perceive
that the substances of which the food of man is composed may be
divided into two classes; into nitrogenised and non-nitrogenised.
The former are capable of conversion into blood; the latter are
incapable of this transformation.
Out of those substances which are adapted to the formation of blood,
are formed all the organised tissues. The other class of substances,
in the normal state of health, serve to support the process of
respiration. The former may be called the plastic elements of
nutrition; the latter, elements of respiration.
Among the former we reckon--
Vegetable fibrine.
Vegetable albumen.
Vegetable caseine.
Animal flesh.
Animal blood.
Among the elements of respiration in our food, are--
Fat. Pectine.
Starch. Bassorine.
Gum. Wine.
Cane sugar. Beer.
Grape sugar. Spirits.
Sugar of milk.
The most recent and exact researches have established as a universal
fact, to which nothing yet known is opposed, that the nitrogenised
constituents of vegetable food have a composition identical with
that of the constituents of the blood.
No nitrogenised compound, the composition of which differs from that
of fibrine, albumen, and caseine, is capable of supporting the vital
process in animals.
The animal organism unquestionably possesses the power of forming,
from the constituents of its blood, the substance of its membranes
and cellular tissue, of the nerves and brain, and of the organic
part of cartilages and bones. But the blood must be supplied to it
perfect in everything but its form--that is, in its chemical
composition. If this be not done, a period is rapidly put to the
formation of blood, and consequently to life.
This consideration enables us easily to explain how it happens that
the tissues yielding gelatine or chondrine, as, for example, the
gelatine of skin or of bones, are not adapted for the support of the
vital process; for their composition is different from that of
fibrine or albumen. It is obvious that this means nothing more than
that those parts of the animal organism which form the blood do not
possess the power of effecting a transformation in the arrangement
of the elements of gelatine, or of those tissues which contain it.
The gelatinous tissues, the gelatine of the bones, the membranes,
the cells and the skin suffer, in the animal body, under the
influence of oxygen and moisture, a progressive alteration; a part
of these tissues is separated, and must be restored from the blood;
but this alteration and restoration are obviously confined within
very narrow limits.
While, in the body of a starving or sick individual, the fat
disappears and the muscular tissue takes once more the form of
blood, we find that the tendons and membranes retain their natural
condition, and the limbs of the dead body their connections, which
depend on the gelatinous tissues.
On the other hand, we see that the gelatine of bones devoured by a
dog entirely disappears, while only the bone earth is found in his
excrements. The same is true of man, when fed on food rich in
gelatine, as, for example, strong soup. The gelatine is not to be
found either in the urine or in the faeces, and consequently must
have undergone a change, and must have served some purpose in the
animal economy. It is clear that the gelatine must be expelled from
the body in a form different from that in which it was introduced as
food.
When we consider the transformation of the albumen of the blood into
a part of an organ composed of fibrine, the identity in composition
of the two substances renders the change easily conceivable. Indeed
we find the change of a dissolved substance into an insoluble organ
of vitality, chemically speaking, natural and easily explained, on
account of this very identity of composition. Hence the opinion is
not unworthy of a closer investigation, that gelatine, when taken in
the dissolved state, is again converted, in the body, into cellular
tissue, membrane and cartilage; that it may serve for the
reproduction of such parts of these tissues as have been wasted, and
for their growth.
And when the powers of nutrition in the whole body are affected by a
change of the health, then, even should the power of forming blood
remain the same, the organic force by which the constituents of the
blood are transformed into cellular tissue and membranes must
necessarily be enfeebled by sickness. In the sick man, the intensity
of the vital force, its power to produce metamorphoses, must be
diminished as well in the stomach as in all other parts of the body.
In this condition, the uniform experience of practical physicians
shows that gelatinous matters in a dissolved state exercise a most
decided influence on the state of the health. Given in a form
adapted for assimilation, they serve to husband the vital force,
just as may be done, in the case of the stomach, by due preparation
of the food in general.
Brittleness in the bones of graminivorous animals is clearly owing
to a weakness in those parts of the organism whose function it is to
convert the constituents of the blood into cellular tissue and
membrane; and if we can trust to the reports of physicians who have
resided in the East, the Turkish women, in their diet of rice, and
in the frequent use of enemata of strong soup, have united the
conditions necessary for the formation both of cellular tissue and
of fat.
LETTER XI
My dear Sir,
In the immense, yet limited expanse of the ocean, the animal and
vegetable kingdoms are mutually dependent upon, and successive to
each other. The animals obtain their constituent elements from the
plants, and restore them to the water in their original form, when
they again serve as nourishment to a new generation of plants.
The oxygen which marine animals withdraw in their respiration from
the air, dissolved in sea water, is returned to the water by the
vital processes of sea plants; that air is richer in oxygen than
atmospheric air, containing 32 to 33 per cent. Oxygen, also,
combines with the products of the putrefaction of dead animal
bodies, changes their carbon into carbonic acid, their hydrogen into
water, and their nitrogen assumes again the form of ammonia.
Thus we observe in the ocean a circulation takes place without the
addition or subtraction of any element, unlimited in duration,
although limited in extent, inasmuch as in a confined space the
nourishment of plants exists in a limited quantity.
We well know that marine plants cannot derive a supply of humus for
their nourishment through their roots. Look at the great sea-tang,
the Fucus giganteus: this plant, according to Cook, reaches a height
of 360 feet, and a single specimen, with its immense ramifications,
nourishes thousands of marine animals, yet its root is a small body,
no larger than the fist. What nourishment can this draw from a naked
rock, upon the surface of which there is no perceptible change? It
is quite obvious that these plants require only a hold,--a fastening
to prevent a change of place,--as a counterpoise to their specific
gravity, which is less than that of the medium in which they float.
That medium provides the necessary nourishment, and presents it to
the surface of every part of the plant. Sea-water contains not only
carbonic acid and ammonia, but the alkaline and earthy phosphates
and carbonates required by these plants for their growth, and which
we always find as constant constituents of their ashes.
All experience demonstrates that the conditions of the existence of
marine plants are the same which are essential to terrestrial
plants. But the latter do not live like sea-plants, in a medium
which contains all their elements and surrounds with appropriate
nourishment every part of their organs; on the contrary, they
require two media, of which one, namely the soil, contains those
essential elements which are absent from the medium surrounding
them, i.e. the atmosphere.
Is it possible that we could ever be in doubt respecting the office
which the soil and its component parts subserve in the existence and
growth of vegetables?--that there should have been a time when the
mineral elements of plants were not regarded as absolutely essential
to their vitality? Has not the same circulation been observed on the
surface of the earth which we have just contemplated in the
ocean,--the same incessant change, disturbance and restitution of
equilibrium?
Experience in agriculture shows that the production of vegetables on
a given surface increases with the supply of certain matters,
originally parts of the soil which had been taken up from it by
plants--the excrements of man and animals. These are nothing more
than matters derived from vegetable food, which in the vital
processes of animals, or after their death, assume again the form
under which they originally existed, as parts of the soil. Now, we
know that the atmosphere contains none of these substances, and
therefore can replace none; and we know that their removal from a
soil destroys its fertility, which may be restored and increased by
a new supply.
Is it possible, after so many decisive investigations into the
origin of the elements of animals and vegetables, the use of the
alkalies, of lime and the phosphates, any doubt can exist as to the
principles upon which a rational agriculture depends? Can the art of
agriculture be based upon anything but the restitution of a
disturbed equilibrium? Can it be imagined that any country, however
rich and fertile, with a flourishing commerce, which for centuries
exports its produce in the shape of grain and cattle, will maintain
its fertility, if the same commerce does not restore, in some form
of manure, those elements which have been removed from the soil, and
which cannot be replaced by the atmosphere? Must not the same fate
await every such country which has actually befallen the once
prolific soil of Virginia, now in many parts no longer able to grow
its former staple productions--wheat and tobacco?
In the large towns of England the produce both of English and
foreign agriculture is largely consumed; elements of the soil
indispensable to plants do not return to the fields,--contrivances
resulting from the manners and customs of English people, and
peculiar to them, render it difficult, perhaps impossible, to
collect the enormous quantity of the phosphates which are daily, as
solid and liquid excrements, carried into the rivers. These
phosphates, although present in the soil in the smallest quantity,
are its most important mineral constituents. It was observed that
many English fields exhausted in that manner immediately doubled
their produce, as if by a miracle, when dressed with bone earth
imported from the Continent. But if the export of bones from Germany
is continued to the extent it has hitherto reached, our soil must be
gradually exhausted, and the extent of our loss may be estimated, by
considering that one pound of bones contains as much phosphoric acid
as a hundred-weight of grain.
The imperfect knowledge of Nature and the properties and relations
of matter possessed by the alchemists gave rise, in their time, to
an opinion that metals as well as plants could be produced from a
seed. The regular forms and ramifications seen in crystals, they
imagined to be the leaves and branches of metal plants; and as they
saw the seed of plants grow, producing root, stem and leaves, and
again blossoms, fruit and seeds, apparently without receiving any
supply of appropriate material, they deemed it worthy of zealous
inquiry to discover the seed of gold, and the earth necessary for
its development. If the metal seeds were once obtained, might they
not entertain hopes of their growth?
Such ideas could only be entertained when nothing was known of the
atmosphere, and its participation with the earth, in administering
to the vital processes of plants and animals. Modern chemistry
indeed produces the elements of water, and, combining them, forms
water anew; but it does not create those elements--it derives them
from water; the new-formed artificial water has been water before.
Many of our farmers are like the alchemists of old,--they are
searching for the miraculous seed,--the means, which, without any
further supply of nourishment to a soil scarcely rich enough to be
sprinkled with indigenous plants, shall produce crops of grain a
hundred-fold.
The experience of centuries, nay, of thousands of years, is
insufficient to guard men against these fallacies; our only security
from these and similar absurdities must be derived from a correct
knowledge of scientific principles.
In the first period of natural philosophy, organic life was supposed
to be derived from water only; afterwards, it was admitted that
certain elements derived from the air must be superadded to the
water; but we now know that other elements must be supplied by the
earth, if plants are to thrive and multiply.
The amount of materials contained in the atmosphere, suited to the
nourishment of plants, is limited; but it must be abundantly
sufficient to cover the whole surface of the earth with a rich
vegetation. Under the tropics, and in those parts of our globe where
the most genial conditions of fertility exist,--a suitable soil, a
moist atmosphere, and a high temperature,--vegetation is scarcely
limited by space; and, where the soil is wanting, it is gradually
supplied by the decaying leaves, bark and branches of plants. It is
obvious there is no deficiency of atmospheric nourishment for plants
in those regions, nor are these wanting in our own cultivated
fields: all that plants require for their development is conveyed to
them by the incessant motions of the atmosphere. The air between the
tropics contains no more than that of the arctic zones; and yet how
different is the amount of produce of an equal surface of land in
the two situations!
This is easily explicable. All the plants of tropical climates, the
oil and wax palms, the sugar cane, &c., contain only a small
quantity of the elements of the blood necessary to the nutrition of
animals, as compared with our cultivated plants. The tubers of the
potato in Chili, its native country, where the plant resembles a
shrub, if collected from an acre of land, would scarcely suffice to
maintain an Irish family for a single day (Darwin). The result of
cultivation in those plants which serve as food, is to produce in
them those constituents of the blood. In the absence of the elements
essential to these in the soil, starch, sugar and woody fibre, are
perhaps formed; but no vegetable fibrine, albumen, or caseine. If we
intend to produce on a given surface of soil more of these latter
matters than the plants can obtain from the atmosphere or receive
from the soil of the same surface in its uncultivated and normal
state, we must create an artificial atmosphere, and add the needed
elements to the soil.
The nourishment which must be supplied in a given time to different
plants, in order to admit a free and unimpeded growth, is very
unequal.
On pure sand, on calcareous soil, on naked rocks, only a few genera
of plants prosper, and these are, for the most part, perennial
plants. They require, for their slow growth, only such minute
quantities of mineral substances as the soil can furnish, which may
be totally barren for other species. Annual, and especially summer
plants, grow and attain their perfection in a comparatively short
time; they therefore do not prosper on a soil which is poor in those
mineral substances necessary to their development. To attain a
maximum in height in the short period of their existence, the
nourishment contained in the atmosphere is not sufficient. If the
end of cultivation is to be obtained, we must create in the soil an
artificial atmosphere of carbonic acid and ammonia; and this surplus
of nourishment, which the leaves cannot appropriate from the air,
must be taken up by the corresponding organs, i.e. the roots, from
the soil. But the ammonia, together with the carbonic acid, are
alone insufficient to become part of a plant destined to the
nourishment of animals. In the absence of the alkalies, the
phosphates and other earthy salts, no vegetable fibrine, no
vegetable caseine, can be formed. The phosphoric acid of the
phosphate of lime, indispensable to the cerealia and other
vegetables in the formation of their seeds, is separated as an
excrement, in great quantities, by the rind and barks of ligneous
plants.
How different are the evergreen plants, the cacti, the mosses, the
ferns, and the pines, from our annual grasses, the cerealia and
leguminous vegetables! The former, at every time of the day during
winter and summer, obtain carbon through their leaves by absorbing
carbonic acid which is not furnished by the barren soil on which
they grow; water is also absorbed and retained by their coriaceous
or fleshy leaves with great force. They lose very little by
evaporation, compared with other plants. On the other hand, how very
small is the quantity of mineral substances which they withdraw from
the soil during their almost constant growth in one year, in
comparison with the quantity which one crop of wheat of an equal
weight receives in three months!
It is by means of moisture that plants receive the necessary
alkalies and salts from the soil. In dry summers a phenomenon is
observed, which, when the importance of mineral elements to the life
of a plant was unknown, could not be explained. The leaves of plants
first developed and perfected, and therefore nearer the surface of
the soil, shrivel up and become yellow, lose their vitality, and
fall off while the plant is in an active state of growth, without
any visible cause. This phenomenon is not seen in moist years, nor
in evergreen plants, and but rarely in plants which have long and
deep roots, nor is it seen in perennials in autumn and winter.