CHAPTER V 



THE INDIVIDUAL AND THE SPECIES 



What is an individual organism ? A Protozoan, 

 such as an Amcsba or a Paramcecium, is a single cell : 

 it is an aggregate of physical and chemical parts, 

 nucleus, cytoplasm, etc., and no one of these parts 

 can be removed if the organism is to continue to live. 

 The cell can be mutilated to some extent, but, in 

 general, its life depends on the integrity of its essential 

 structures, and it cannot be divided without ceasing 

 to be what it was. It contains the minimum number 

 of parts which are necessary for continued organic 

 existence. 



Such an organism as a Hydra consists of an aggre- 

 gate of cells which are not all of the same kind. The 

 outer layer, or ectoderm, is sensory and protective, 

 and contains organs of aggression ; while the inner 

 layer consists of cells which subserve the functions of 

 digestion and assimilation. All these parts are, in 

 general, necessary for the life of the Hydra. They can 

 be mutilated ; the animal can be cut into two parts, 

 and each of these parts may regenerate, by growth, 

 the part that was removed. Yet the existence of 

 ectoderm and endoderm, in a certain minimum of 

 mass, is necessary for this regeneration. The higher 

 animal, or Metazoon, is therefore an aggregate of 

 cells, each of which is equivalent to the individual 

 Protozoon ; but these cells are not all alike — that is, 



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