The wheat plant in the field is said to be a =living= thing; the fowl
running about the farm-yard is also said to be a =living= thing. If the
plant is plucked up, and if the fowl is knocked on the head, they soon
die and become =dead= things. Both the fowl and the wheat plant, as we
have seen, are composed of the same elements as those which enter into
the composition of mineral matter, though united into compounds which do
not exist in the mineral world. Why then do we distinguish this matter
when it takes the shape of a wheat plant, or a fowl, as =living matter=?
61. =The Living Plant increases in size, by adding to the Substances
which compose its Body, like Substances; these, however, are not derived
from without, but are manufactured within the Body of the Plant from
simpler Materials.=
In the spring, a wheat-field is covered with small green plants. These
grow taller and taller until they attain many times the size which they
had when they first appeared; and they produce the heads of flowers
which eventually change into ears of corn.
In so far as this is a process of growth, accompanied by the assumption
of a definite form, it might be compared with the growth of a crystal of
salt in brine: but, on closer examination, it turns out to be something
very different. For the crystal of salt grows by taking to itself the
salt contained in the brine, which is added to its exterior; whereas the
plant grows by addition to its interior: and there is not a trace of the
characteristic compounds of the plant’s body, albumin, gluten, starch,
or cellulose, or fat, in the soil, or in the water, or in the air.
Yet the plant creates nothing (§ 50) and, therefore, the matter of the
proteids and amyloids and fats which it contains must be supplied to it,
and simply manufactured, or combined in new fashions, in the body of the
plant.
It is easy to see, in a general way, what the raw materials are which
the plant works up, for the plant gets nothing but the materials
supplied to it by the atmosphere and by the soil. The atmosphere
contains oxygen and nitrogen, a little carbonic acid gas, a minute
quantity of ammoniacal salts, and a variable proportion of water. The
soil contains clay and sand (silica), lime, iron, potash, phosphorus,
sulphur, ammoniacal salts, and other matters which are of no importance.
Thus, between them, the soil and the atmosphere contain all the
elementary bodies which we find in the plant: but the plant has to
separate them and join them together afresh.
Moreover, the new matter, by the addition of which the plant grows, is
not applied to its outer surface, but is manufactured in its interior;
and the new molecules are diffused among the old ones.
62. =The Living Plant, after it has grown up, detaches part of its
Substance, which has the Power of developing into a similar Plant, as a
Seed.=
The grain of wheat is a part of the flower of the wheat plant, which,
when it becomes ripe, is easily separated. It contains a minute and
rudimentary plant; and, when it is sown, this gradually grows, or
becomes =developed= into, the perfect plant, with its stem, roots,
leaves and flowers, which again give rise to similar seeds. No mineral
body runs through a regular series of changes of form and size and then
gives off parts of its substance which take the same course. Mineral
bodies present no such =development= and give off no seeds or =germs=.
They do not reproduce their kind.
63. =The Living Animal increases in Size by adding to the Substances
which compose its Body, like Substances; these, however, are chiefly
derived directly from other Animals or from Plants.=
The fowl in the farm-yard is incessantly pecking about and swallowing
now a grain of corn, and now a fly or a worm. In fact, it is feeding,
and, as every one knows, would soon die if not supplied with food. It is
also a matter of every day knowledge that it would not be of much use to
give a fowl the soil of a cornfield, with plenty of air and water, to
eat.
In this respect, the fowl is like all other animals; it cannot
manufacture the proteid materials of its body, but it has to take them
ready made, or in a condition which requires but very slight
modification, by devouring the bodies either of other animals or of
plants. The animal or vegetable substances devoured are taken into the
animal’s stomach; they are there digested or dissolved; and thus they
are fitted to be distributed to all parts of the fowl’s own body, and
applied to its maintenance and growth.
64. =The Living Animal, after it has grown up, detaches part of its
Substance, which has the Power of growing into a similar Animal, as an
Egg.=
The fowl’s egg is formed in the body of the hen, and is, in fact, part
of her body inclosed in a shell and detached. It contains a minute
rudiment of a fowl; and when it is kept at a proper temperature by the
hen’s sitting upon it, or otherwise, for three weeks, this rudiment
grows or develops, at the expense of the materials contained in the yolk
and the white, into a small bird, the chick, which is then hatched and
grows into a fowl. The animal, therefore, is produced by the development
of a germ in the same way as the plant; and, in this respect, all plants
and all animals agree with one another and differ from all mineral
matter.
65. =Living Bodies differ from Mineral Bodies in their Essential
Composition, in the manner of their Growth, and in the fact that they
are reproduced by Germs.=
Thus there is a very broad distinction between mineral matter and living
matter. The elements of living matter are identical with those of
mineral bodies; and the fundamental laws of matter and motion apply as
much to living matter as to mineral matter; but every living body is, as
it were, a complicated piece of mechanism which “goes,” or lives, only
under certain conditions. The germ contained in the fowl’s egg requires
nothing but a supply of warmth, within certain narrow limits of
temperature, to build the molecules of the egg into the body of the
chick. And the process of development of the egg, like that of the seed,
is neither more nor less mysterious than that, in virtue of which, the
molecules of water, when it is cooled down to the freezing-point, build
themselves up into regular crystals.
The further study of living bodies leads to the province of =Biology=,
of which there are two great divisions—=Botany=, which deals with
plants, and =Zoology=, which treats of animals.
Each of these divisions has its subdivisions—such as =Morphology=, which
treats of the form, structure, and development of living beings, and
=Physiology=, which explains their actions or functions, besides others.