It is easy to see, in a general way, what the raw materials are which the plant works up, for the plant get 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.

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.

The fowl in the farmyard 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.

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.