30 PROTOPLASMIC ACTION AND NERVOUS ACTION 



are altered in a manner tending to compensate or remove 

 the deficiency. Thus hunger is, physiologically speaking, 

 increased reactivity to food materials; thirst is increased 

 reactivity to water; the respiratory center of vertebrates 

 increases its rhythm as CO2 accumulates in the blood; 

 when the oxygen in the water is decreased, the gill-cilia 

 of the fresh-water clam beat more vigorously.^ These 

 and many other instances illustrate the manner in which 

 a physiological deficiency may itself furnish the means 

 of setting in motion some physiological mechanism which 

 remedies the deficiency.^ The end-effect of all such 

 regulatory responses is to further the persistence of the 

 organism in its environment. As already mentioned, 

 the term adaptive is usually applied to those special 

 peculiarities of structure and activity by which the 

 organism is automatically conserved in spite of environ- 

 mental change; hence, from the present generalized 

 point of view any active adaptation may be regarded 

 as a special kind of regulation. It is evident that all 

 such regulations are based upon a highly developed 

 irritability; this fundamental property of irritability, 

 therefore, controls all of the active relations between 

 organism and environment, including the interchange of 

 material and energy which is the essential feature of such 

 relations. 



SPECIFIC CHARACTERS OF ORGANISMS 



The constructive metabolic processes which build up 

 the living system involve the synthesis of a multiplicity 

 of new chemical compounds from the food materials and 



^ Cf. Babak, Z. allg. Physiol, XV (1913), 184. 



^ In Pfliiger's aphorism, in living organisms "the cause of the need 

 is the cause of the satisfaction of the need." 



