204 DOCTRINE OF EVOLUTION 



such as the Httle protozoon Amoeba. We have become 

 famihar with this organism as one that carries on all of 

 the vital functions within the limits of a single struc- 

 tural unit ; it is a mass of protoplasm enclosing a nu- 

 cleus, and as a biological individual it must perform all 

 of the eight tasks that are essential for life. It does 

 not possess a digestive tract, but it does digest ; it does 

 not have breathing organs, but it does respire ; and it 

 is particularly noteworthy that it must coordinate the 

 different activities of its parts, and maintain definite 

 relations with the environment, even though its coor- 

 dination and sensation are not accomplished by any 

 special parts that would deserve the name of elementary 

 nervous organs. Its many activities are simple re- 

 sponses to stimuli that reach it from without, and its 

 reactions to such stimuli are called reflex processes. 

 Should the light become too strong, it will slowly crawl 

 to a shady place ; should the water in which it lives 

 become warmer, it responds by displaying greater 

 activity. It exhibits, in a word, the property of irri- 

 tability, — that is, simply the power of receiving and 

 reacting to stimuli ; and being only a single cell this 

 property is held in common by all of its parts. 



We come next to a simple many-celled animal like 

 the polyp Hydra, or a jellyfish. In such an animal 

 the body is composed of numerous cells which are not 

 all alike either in their make-up or in their functions. 

 Some of them are concerned primarily with digestion, 

 others with protection, while still others are exempt 

 from these tasks and as sense-cells they devote all their 

 energies to the reception of stimuli from without, or, 

 beneath the outer sheet of cells of the two-layered body, 



