THE ORGANIZATION OF THE INDIVIDUAL 13 



erties. This classification is still useful, not so much for its distinction 

 between the features of living as opposed to nonliving matter as for the 

 emphasis it places upon the adaptive end results of the innumerable and 

 still incompletely analyzed chemical and physical processes and relation- 

 ships found in protoplasm. 



Vital properties of protoplasm. Vital properties owe their name to 

 the once widely held view that they were controlled by some peculiar 

 life principle inexplicable by ordinary chemical and physical laws. They 

 may still be termed vital in that they are only exhibited by living or- 

 ganisms; our inability to explain them is probably attributable only to 

 their complexity. "Vital" properties are the end results of chemical and 

 physical relationships too complex and involved to be completely analyzed 

 in the present state of scientific knowledge. 



1. Metabolism. All protoplasm, unless temporarily dormant, is con- 

 stantly capturing energy and utilizing it for activity or growth. The 

 storage of energy and its subsequent use may add an intermediate step 

 to the process. This ability to capture energy and particularly the ability 

 to change energy (food) into more protoplasm (growth) are capacities 

 peculiar to protoplasm. The term metabolism comprises all the chemical 

 processes occurring in living things and includes a great variety of 

 chemical reactions. It is convenient to divide metabolism into two phases 

 — anabolism, which includes the .synthetic processes by which simple 

 substances are changed into the complex materials of protoplasm (as- 

 similation), and catabolism, which includes the breaking-down processes 

 by which complex substances are split into simpler ones, liberating energy 

 and yielding wastes (disintegration). Anabolism results in storage of 

 energy and in growth; catabolism furnishes energy for heat, motion, and 

 chemical syntheses. 



2. Reproduction. All units of protoplasm have the power to divide 

 and produce new units of the same kind. This is in part dependent upon 

 growth, but it also involves the maintenance of a constant type and size 

 of organization in spite of growth. 



3. Irritability or Reactiveness. This is the property of being able to 

 respond to stimuli. A large variety of stimuli, both external and internal, 

 may produce adaptive responses on the part of protoplasm. Protoplasm 

 may react to the stimuli of light, temperature change, contact, gravity, 

 electric current, change in degree of acidity or alkalinity, and various 

 chemical substances. 



Nonvital properties of protoplasm. Many of the characteristic 

 phenomena shown by protoplasm were found to occur also in nonliving 

 substances and systems. Here they are more susceptible to experiment 

 and measurement, and they were soon recognized as expressions of 

 physical and chemical properties and relationships. Diffusion, osmosis, 



