they afford; each kind needs the other, and each performs some distinctive task for the other as well as for itself.  But the parallel thus drawn need not stop here.  In the case of the outer layer, the cells are mostly flat covering elements that are the first to be torn off and injured when the animal is attacked.  Scattered about among them are sense-cells standing like sentinels with delicate upright processes which receive stimuli from without the sense-cells transmit impulses to the network of nerve-cells below, which is a counterpart of the signal corps of an army, keeping all parts of the whole organization in communication with one another.  Most wonderful of all are the stinging-cells of the outer layer; these produce a flask-shaped, poisoned bomb which is discharged by the convulsive contraction of the cell itself so as to stun and injure the enemy or prey.  The bomb-throwing cells die immediately after they 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