28 COMPARATIVE PHYSIOLOGY. 



Since all the vital activities of cells can only be manifested when 

 supplied with food, it follows that living organisms convert po- 

 tential or possible energy into kinetic or actual energy. When 

 lifeless, immobile matter is taken in as food and, as a result, is 

 converted by a process of assimilation into the protoplasm of the 

 cell using it, we have an example of potential being converted 

 into actual energy, for one of the properties of all protoplasm is 

 its contractility. Assimilation implies, of course, the absorp- 

 tion of what is to be used, vrith rejection of waste matters. 



The movements of protoplasm of whatever kind, when due 

 to a stimulus, are said to indicate irritability ; while, if inde- 

 pendent of any external source of excitation, they are denomi- 

 nated automatic. 



Among agents that modify the action of all kinds of proto- 

 plasm are heat, moisture, electricity, light, and others in great 

 variety, both chemical and mechanical. It can not be too well 

 remembered that living things are what they are, neither by 

 vLttue of their own organization alone nor through the action 

 of their environment alone (else would they be in no sense dif- 

 ferent from inanimate things), but because of the relation of 

 the organization to the surroundings. 



Protoplasm, then, is contractile, irritable, automatic, absorp- 

 tive, secretory (and excretory), metabolic, and reproductive. 



But when it is affirmed that these are the fundamental prop- 

 erties of all protoplasm, the idea is not to be conveyed that cells 

 exhibiting these properties are identical biologically. No two 

 masses of protoplasm can be quite alike, else would there be no 

 distinction in physiological aemeanor — no individuality. Every 

 cell, could we but behold its inner molecular mechanism, differs 

 from its neighbor. When this difference reaches a certain de- 

 gree in one direction, we have a manifest differentiation leading 

 to physiological division of labor, which may now with advan- 

 tage be treated in the following section. 



THE ANIMAL BODY. 



An animal, as we have learned, may be made up of a single 

 cell in which each part performs much the same work ; or, if 

 there be differences in function, they are ill-deflned as compared 

 with those of higher animals. The condition of things in such 

 an animal as Amoeba may be compared to a civilized commu- 

 nity in a very crude social condition. When each individual 



