GENERAL CHARACTERS OF LIVING ORGANISMS 29 



relatively little utilizable and much non-utilizable 

 material. In such a case self -maintenance demands, in 

 addition to the ability to move from place to place, a 

 selective power of reaction by which food materials 

 may be picked out and incorporated. Accordingly the 

 responsiveness to external changes (irritability, motor 

 activity) reaches its highest development in this group 

 of organisms. The development of locomotor powers 

 is especially characteristic of animals; related to this 

 is their great variety of reactions and instincts. From 

 such general considerations we may see in a general way 

 how the distinguishing or prevailing characters of each 

 group have arisen in evolution in correspondence with 

 the differences in their methods of nutrition. 



In all organisms this selection of assimilable material 

 from the environment and its transformation into living 

 protoplasm proceed automatically and are regulated in 

 correspondence with the physiological requirements, as 

 these vary with the changes of activity and of external 

 conditions. Both the automaticity and the regulated 

 character of these activities are well illustrated by the 

 changes in the reaction of animals to food materials 

 during periods of ''hunger." Consumption of the 

 energy-yielding reserves within the living protoplasm 

 leads to an increased reactivity of the whole organism 

 to these substances. Through this means the mainte- 

 nance of the metabolic equilibrium is assured under the 

 usual conditions. Regulation of this kind is shown to 

 a greater or less degree by all organisms, and constitutes 

 a fundamental condition of self-preservation; typically 

 if the organism is deprived of any substance or condition 

 necessary for maintenance, its reactivity and behavior 



