THE HIGHER ORGANISMS 149 



not only advantageous but indispensable. The more 

 highly specialized a cell group becomes, the more isolated 

 it becomes, and the more imperative becomes the neces- 

 sity for intercommunication, control, and coordination. 



In loosely organized cellular combinations, such as 

 Epistylus and Microgromia, little advantage is to be 

 gained for any cell from impulses derived from other 

 cells, though the general irritability, contractility, and 

 conductivity of the protoplasm may enable impulses to 

 pass from cell to cell. 



When one cell of a simple colony is disturbed a defen- 

 sive reaction is sometimes manifested by its fellows, and 

 in the case of Carchesium may result in escape from danger 

 through the contraction of the stalk, which draws away 

 all the members of the colony. But in such cases, as well 

 as in the case of the sponges and the hydras, there is 

 usually little danger to cells other than those immediately 

 menaced. 



If a portion of a sponge or a tentacle of a hydra be bitten 

 off or torn away, the whole organism is scarcely affected 

 and the damage is soon repaired by regeneration. 



Among plants the specialization of structure rarely 

 reaches a point at which injury to a part seriously affects 

 the whole, which fact, added to the immobility of plants 

 in general, has determined that their entire evolution has 

 taken place without the development of any recognizable 

 regulating or communicating system making its appear- 

 ance even in the highest forms. 



But in animals that do move about in search of the 

 particular food upon which they live, and finding it, must 

 apprehend it, comminute it, ingest it, digest it, circulate 

 it, and assimilate it, directing, regulating, communicating, 

 and coordinating mechanisms become essential, and are 

 developed in the form of the nervous system, through which 

 impulses received from without enable the animal to adapt 

 itself to the conditions of its environment fly from danger, 

 pursue its food, etc. and impulses received from within 

 enable it to adapt itself to internal conditions so that its 



