ey have ejected their
missiles; like soldiers participating in a forlorn hope, they sacrifice
their lives in one supreme effort of service to the cell-community of
which they are members.

These and similar facts prove conclusively that _Hydra_ is a true
community even in the human sense, and that the laws of biological
association are established at a point far below the level of the insects.
The individuality of the unit is still maintained, and each cell must
guard its own interests to a certain degree, but the original independence
of the unit has become so altered by differentiation and division of labor
that a close interdependent relation has come about. The complete
individual is now the _whole_ aggregate; it is the entire _Hydra_ itself
which must obey the primary commands of nature to live efficiently and to
perpetuate its kind. True it is that the life of the higher individual is
the sum total of the activities performed by its constituent cells, but no
one of the varied specialized elements is biologically perfect by itself
or equivalent to the whole. And, as we have seen, the welfare of the
complete animal takes precedence over that of any one of its parts, just
as the existence of a nation may be preserved only by the death of
soldiers warring for its honor and life.

If, now, we should pass on to the more complex organisms like worms and
insects and vertebrates, and should disregard the communal relations of
some of these animals, each individual proves to be like _Hydra_ as
regards the principles underlying its make-up and workings. A single bee,
like a man, is a definitely constituted aggregate of cells, differing as a
whole from _Hydra_ only in the _degree of differentiation_ exhibited by
its constituent elements. Instead of a loose network of nerve-cells there
is the far more complex nervous system whose evolution has been outlined
in the sixth chapter. The blood-vascular and respiratory and excretory
systems have become well organized, in response, so to speak, to the
demands on the part of the nervous and alimentary organs that they may be
relieved of the tasks of circulation and respiration and the discharge of
ash-wastes. Therefore the cells which make up an insect and a man are more
diverse, they have more varied interrelationships, and they are far more
interdependent then in the case of the components of _Hydra_. Yet all the
many-celled organisms that we are so accustomed to regard as individua