directly as such. The animal cell can absorb its carbohydrate and
proteid food only in the form of carbohydrate and proteid; it is
dependent, in fact, on the pre-existence of these organic substances,
themselves the products of living matter, and in this respect the
animal is essentially a parasite on existing animal and plant life.
The plant, on the other hand, if it be a green plant, containing
chlorophyll, is capable, in the presence of light, of building up both
carbohydrate material and proteid material from inorganic salts; if
it be a fungus, devoid of chlorophyll, whilst it is dependent on
pre-existing carbohydrate material and is capable of absorbing,
like an animal, proteid material as such, it is able to build up its
proteid food from material chemically simpler than proteid. On these
basal differences are founded most of the characters which make the
higher forms of animal and plant life so different. The animal body,
if it be composed of many cells, follows a different architectural
plan; the compact nature of its food, and the yielding nature of its
cell-walls, result in a form of structure consisting essentially of
tubular or spherical masses of cells arranged concentrically round the
food-cavity. The relatively rigid nature of the plant cell-wall, and
the attenuated inorganic food-supply of plants, make possible and
necessary a form of growth in which the greatest surface is exposed to
the exterior, and thus the plant body is composed of flattened laminae
and elongated branching growths. The distinctions between animals and
plants are in fact obviously secondary and adaptive, and point clearly
towards the conception of a common origin for the two forms of life, a
conception which is made still more probable by the existence of many
low forms in which the primary differences between animals and plants
fade out.

An animal may be defined as a living organism, the protoplasm of which
does not secrete a cellulose cell-wall, and which requires for its
existence proteid material obtained from the living or dead bodies of
existing plants or animals. The common use of the word animal as
the equivalent of mammal, as opposed to bird or reptile or fish, is
erroneous.

The classification of the animal kingdom is dealt with in the article
ZOOLOGY.

(P.C.M.)

ANIMAL HEAT. Under this heading is discussed the physiology of the
temperature of the animal body.

The higher animals have within their bodies certai