below, and in the center the bony supports of the whole limb. Like the
organs, these tissues are differentiated, structurally and functionally,
and they also are so placed and related as to exhibit the kind of
mechanical adjustment which we call adaptation. The tissues, then, in
their relations to the organs are like the organs in their relations to
the whole creature, i.e. adapted to specific situations where they may
most satisfactorily perform their tasks.

Finally, in the last analysis, all organisms and organs and tissues can be
resolved into elements which are called _cells_. They are not little
hollow cases, it is true, although for historical reasons we employ a word
that implies such a condition. They are unitary masses of living matter
with a peculiar central body or nucleus, and every tissue of every living
thing is composed of them.

The cells of bone differ from those of cartilage mainly in the different
consistency of the substances secreted by the cells to lie between them;
skin cells are soft-walled masses lying close together; even blood is a
tissue, although it is fluid and its cells are the corpuscles which float
freely in a liquid serum. Thus an organism proves to be a complex
mechanism composed of cells as structural units, just as a building is
ultimately a collection of bricks and girders and bolts, related to one
another in definite ways.

Our analysis reveals the living creature in an entirely new light, not
only as a machinelike structure whose parts are marvelously formed and
coordinated in material respects, but also as one whose activities or
workings are ultimately cellular in origin. Structure and function are
inseparable, and if an animal or a plant is an aggregate of cells, then
its whole varied life must be the sum total of the lives of its