148 Life as a Physical Phenomenon. 



There are even naturalists of profound learning and re- 

 search, who declare that certain beings belong to both of 

 these two classes, and are at one period of their existence 

 plants and£t another animals. This is mentioned not with 

 a view of endorsing the theory, but to illustrate the ex- 

 treme difficulty of separating the two classes. It is 

 scarcely less difficult to draw the line of distinction be- 

 tween minerals and vegetables. 



Notwithstanding the striking peculiarities pertaining to 

 each of the kingdoms, when we reach beings sufficiently 

 developed to represent to any certain degree the class to 

 which they belong, there are very many characteristics 

 which are commou beside the material which enters into 

 their composition. 



The first common characteristic which we observe, aside 

 from material, is a tendency on the part of the ultimate 

 particles or molecules of each of the classes to arrange 

 themselves in certain definite, specific forms ; and if the 

 circumstances surrounding any being be favorable, it will 

 invariably assume the form peculiar to its class. 



Any variation from this law is the result of opposing in- 

 fluences which the molecules cannot overcome. And as 

 here is one of the first fundamental structural laws common 

 to all beings, so here we observe the first indication of 

 a line of distinction upon which any classification can be 

 founded. 



In minerals, where a specific form is assumed, the mole- 

 cules unite in crystals, they are usually bounded by 

 straight lines and angles, while vegetable and animal 

 structures are built of cells ; the prevailing idea is a sphere, 

 and the parts are bounded by curved lines and surfaces. 



Here, it would seem, that there is an absolute line of 

 separation were it not for some exceptions which we may 

 have occasion to mention. 



