ILLUSTRATED AMONG PROTOZOA. 107 



soft mass and dissolved ; and if they be of a certain 

 chemical composition, operate as nutriment, and lib- 

 erate new forces internally or add to the potential 

 energy of the living mass. The forces of light, heat, 

 electricity, and gravity, all act upon the organism. To 

 all of these forces the living mass reacts in a purely 

 mechanical fashion. The reality of the effects of 

 these forces upon inanimate matter is well known, 

 and living matter, as we have seen, is also affected 

 by them. The effects upon living matter are much 

 more complicated than upon inanimate matter. This 

 is due to the peculiar constitution of living matter, 

 it being a viscous, semi-fluid mass, composed of a 

 physical mixture of water and various chemical com- 

 pounds of varying degrees of density and viscosity. 



Under the continued action of the forces of the 

 environment, the mass of living matter, like all 

 other bodies, tends to attain a state of rest. In 

 the nature of things, however, this equilibrium 

 toward which the organism and its environment are 

 tending cannot be attained, for the reason that the 

 environment does not remain the same, but is con- 

 tinually changing. There is an alternation of day 

 and night, light and darkness. The energy of the 

 sun causes changes of temperature and of density 

 in the medium in which the organism exists, be it 

 either air or water. These changes of temperature 



