PROTOPLASM 15 



cells. For instance, in an animal, the cells lining certain regions of 

 the alimentary canal are differentiated for the digestion and ab- 

 sorption of food. They are not concerned with nor would they 

 find it possible to function, for example, in hearing. 



5. Irritability and Adaptation 



Inasmuch as metabolism is a fundamental feature of protoplasm, 

 it follows that organisms must have an environment which sup- 

 plies the necessary materials together with favorable conditions 

 for the continuous and successful operation of all these essential 

 life processes. If an environment is too hot or too cold or too dry, 

 metabolism may be hindered or entirely stopped. If water, oxy- 

 gen, proteins, and other necessary materials are not in adequate 

 supply, death results. In this connection another unique feature of 

 protoplasm should be emphasized, namely, irritability, by which 

 is meant that living matter responds to its environment and is 

 able thereby to determine whether or not the situation is advan- 

 tageous or detrimental. In the higher animals, a differentiated 

 nervous system is developed which is equipped to receive the 

 external stimuli, translate them, and then to incite the proper 

 coordinated response, or adaptation, on the part of the organism. 

 Thus a continuous and exquisite adjustment is maintained between 

 the organism and its environment. Furthermore, protoplasm 

 possesses a certain amount of plasticity with regard to the sur- 

 rounding conditions which we may call adaptation. That is to 

 say, protoplasm can adjust or adapt its metabolic processes to 

 various environmental factors, such as temperature, so long as 

 certain limits are not passed. Thus, for example, in a Frog, the 

 life processes exhibit their highest rate of activity in the spring 

 and summer when the temperature conditions are favorable. 

 During the winter season when the environmental conditions are 

 unfavorable the life processes are reduced to a minimum — the 

 animal adapts itself to the situation and hibernates. 



6. Movement 



In the first place it should be noted that the power of movement, 

 which is commonly associated with living organisms, is not peculiar 

 to protoplasm. It is apparently inherent in all types of matter, 

 and if we were able to examine the ultimate structure of a piece of 

 metal or any other inanimate or animate material, a field of con- 



