her seven in that the performance of its task is of far less
importance to the individual than it is to the race as a whole. It is the
reproductive system, with a function that must be always biologically
supreme. We can very readily see why this must be so; it is because nature
has no place for a species which permits the performance of any individual
function to gain ascendency over the necessary task of perpetuating the
kind. Nature does not tolerate race suicide.

All organisms must perform these eight functions in one way or another.
The bacterium, the simplest animal, the lowest plant, the higher plants
and animals,--all of these have a biological problem to solve which
comprises eight terms or parts, no more and no less. This is surely an
astonishing agreement when we consider the varied forms of living
creatures. And perhaps when we see that this is true we may understand why
adaptation is a characteristic of all organisms, for they all have similar
biological problems to solve, and their lives must necessarily be adjusted
in somewhat similar ways to their surroundings.

Carrying the analysis of organic structure one step further, it is found
that the various organisms are themselves complex, being composed of
_tissues_. A frog's leg as an organ of locomotion is composed of the
protecting skin on the outside, the muscles, blood vessels, and nerves
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