458 GENERAL CONSIDERATIONS 



the degree of dependence of one upon the other is not sufficient to counter- 

 balance the essential individuality of each and the two are considered as 

 separate organisms. 



459. Income and Outgo. — An organism is continually carrying on 

 metabolic activity. When this reaches a low level and the animal is 

 incapable of immediate action, it is said to be dormant; and when 

 metabolism ceases entirely, death has occurred. It was pointed out 

 long ago that life was essentially the result of combustion under com- 

 plicated circumstances, and Mach has referred suggestively to a living 

 organism as that which is able to "keep itself going, produce its own 

 combustion temperature, bring neighboring bodies up to that tempera- 

 ture and thereby drag them into the process, assimilate and grow, expand 

 and propagate itself." 



Metabolism involves the addition to the organism of matter and 

 energy and also the giving off of both. The income and outgo of the 

 organism may be expressed as follows: 



Income. 



1. Material income. 



a. Food. 



b. Oxygen. 



2. Energy income. 



a. Chemical energy, contained in the food. 



b. Light energy, received naturally from the sun. 



c. Heat energy, received from the sun and the earth. 



Outgo. 



1. Material outgo. 



a. Solid wastes, egested. 



b. Liquid wastes, eUminated. 



c. Gaseous wastes, expired. 



2. Energy outgo. 



a. Mechanical energy, evident in muscular activity. 



b. Heat, produced as a result of chemical changes in the body. 



c. A small amount of electrical energy, freed during these same changes. 



d. A small amount of light in organisms which are luminescent. 



If the income exceeds the outgo, then the difference represents growth 

 in substance and an increase of the potential energy contained in the 

 body. If the reverse is true, both the size of the body and the energy 

 contained in it diminish. Theoretically it should be possible, taking 

 into account the condition of the organism at the beginning and end of a 

 given period, to reach a perfect balance between income and outgo for 

 that period. Some of the most recent and most successful experi- 

 ments have approached such a balance within a fraction of one part in a 

 thousand. 



460. Differentiation. — It has been seen that differentiation, accom- 

 panied by division of labor, presents itself in organisms in several ways 



