88 THE PHILOSOPHY OF BIOLOGY 



We are thus led to the consideration of the second 

 principle of energetics and its limitations, but before 

 entering upon this discussion we must consider the 

 nature of the activities of the organism. 



By the term ' metabolism ' we understand the 

 totality of the physico-chemical changes which occur 

 in the living substance of the organism. In physio- 

 logical writings we usually find that two categories 

 of metabolic changes are described : (i) anabolic 

 processes, in the course of which simple chemical 

 compounds possessing relatively little energy are built 

 up into much more complex substances, containing 

 a relatively large quantity of available energy, and 

 therefore capable of doing work. The transformations 

 constituting an anabolic change must be accompanied 

 by corresponding compensatory energy-transform- 

 ations, to account for the energy which becomes 

 potential in the substances formed. The formation 

 of starch from carbon dioxide and water, by the green 

 plant, is such an anabolic change, and the compensatory 

 energy-transformation is the absorption of radiation 

 from the ether by the cells of the plant. A further 

 anabolic change in the plant organism is the formation 

 of amido-substances from the ammonia or nitrate 

 absorbed from the soil, and from the soluble carbo- 

 hydrates formed from the starch manufactured in the 

 green cells. 



The typical activities of the chlorophyll-containing 

 organism are of this nature ; they are anabolic. The 

 organism may be a green land-plant ; a marine green, 

 red, or brown alga ; a yellow-green diatom, a yellow, 

 green, red, or brown peridinian or other holophytic 

 protozoan ; an ascidian, mollusc, echinoderm, polyzoan, 

 worm, or coral containing " symbiotic algae ' (that 

 is the chlorophyll - containing cells of some plant 



