256 DOCTRINE OF EVOLUTION 



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 estab- 

 lished 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 con- 

 stituent cells, but no one of the varied specialized ele- 

 ments 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 or- 

 ganisms 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 con- 

 stituted aggregate of cells, differing as a whole from 



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