a form, rather than an individual, that we see. Its
permanence altogether depends on the permanence of the external
conditions. If they change, it also changes, and a new form is the
result.

[Sidenote: Characteristics of animal life.] An animal is therefore a
form through which material substance is visibly passing and suffering
transmutation into new products. In that act of transmutation force is
disengaged. That which we call its life is the display of the manner in
which the force thus disengaged is expended.

[Sidenote: Matter and force.] A scientific examination of animal life
must include two primary facts. It must consider whence and in what
manner the stream of material substance has been derived, in what manner
and whither it passes away. And, since force can not be created from
nothing, and is in its very nature indestructible, it must determine
from what source that which is displayed by animals has been obtained,
in what manner it is employed, and what disposal is made of it
eventually.

[Sidenote: Force is derived from the sun.] The force thus expended is
originally derived from the sun. Plants are the intermedium for its
conveyance. The inorganic material of a saline nature entering into
their constitution is obtained from the soil in which they grow, as is
also, for the most part, the water they require; but their organic
substance is derived from the surrounding atmosphere, and hence it is
strictly true that they are condensations from the air.

[Sidenote: Mode in which plants obtain material substance.] These
statements may be sufficiently illustrated, and the relation between
plants and animals shown, by tracing the course of any one of the
ingredients entering into the vegetable composition, and derived, as has
been said, from the air. For this purpose, if we select their chief
solid element, carbon, the remarks applicable to the course it follows
will hold good for other accompanying elements. It is scarcely necessary
to embarrass the brief exposition of vegetable life now to be given by
any historical details, since these will come with more propriety
subsequently. It is sufficient to mention that the chemical explanations
of vegetable physiology rest essentially on the discovery of oxygen gas
by Priestley, of the constitution of carbonic acid by Lavoisier, and of
water by Cavendish and Watt.

[Sidenote: Action of a plant on the air.] While the sun is shining, the
green parts of plants, espe