Systems of support, comprising bones or shells, occur in many animals
where the other organs are soft or weak. Perhaps the most interesting of
the individual systems of relation is the nervous system. The strands of
its nerve fibers and its groups of cells keep the various organs of the
body properly coördinated, whereas in the second place, through the
sensitive structures at the surface of the body, they receive the
impressions from the outside world and so enable the organism to relate
itself properly to its environment. The last organic system differs from
the other 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