ing at great length on the
question of relative superiority or inferiority. It may be
unhesitatingly asserted that all animals live, move, and have their
being, in every essential respect, in the same way. Whether one
considers those creatures of microscopic size living in stagnant
ponds, or man himself, it is found that certain qualities characterize
them all. That minute mass of jelly-like substance known as
protoplasm, constituting the one-celled animal amoeba, may be
described as _ingestive_, _digestive_, _secretory_, _excretory_,
_assimilative_, _respiratory_, _irritable_, _contractile_, and
_reproductive_: that is to say, the amoeba must take in food;
must digest it, or change its form; must produce some fluid within
itself which acts on food; must cast out from itself what is no longer
of any use; must convert the digested material into its own
substance--perhaps the most wonderful property of living things; must
take up into its own substance oxygen, and expel carbonic acid gas
(carbon dioxide); and possess the power to respond to a stimulus, or
cause of change, the property of changing form, and, finally, the
ability to bring into being others like itself.

[Illustration: FIG. 1. Muscle-fibres from the heart, much magnified,
showing cross-stripings, nuclei, or the darkly stained central bodies
very important to the life of the cell, also the divisions and points
of union. (Schaefer's _Histology_.)]

[Illustration: FIG. 2. Appearance of a small portion of muscle under a
moderate magnification. Between the muscle-cells proper a form of
binding tissue may be seen.]

[Illustration: FIG. 3. Muscle-cells isolated from the muscular coats
of the intestine. Similar cells are found in some part of most of the
internal organs, including the bronchial tubes. These cells are less
ready in responding to a stimulus, contract more slowly, and tend to
remain longer contracted when they pass into this condition than
striped muscle cells. (Schaefer.)]

Before justifying these statements in detail it will be desirable to
say something of the anatomy or structure of a mammal, and we may
select man himself, though it is to be remembered that one might apply
exactly the same treatment to a dog, pig, mouse, or any other member
of this group of animals. The amoeba and creatures like it live
immersed in water; man, at the bottom of an ocean of air. Both move in
their own medium, the amoeba creeping with extreme slowness, man
movin