2l6 COMPARATIVE ZOOLOGY 



changed into living substance, which, after serving its 

 purpose, passes again as waste to the inorganic world. 



(2) Structure. Minerals are homogeneous, while organ- 

 ized bodies are usually heterogeneous, i.e., composed of 

 different parts, called tissues and organs, having peculiar 

 uses and definite relations to one another. The tissues 

 and organs, again, are heterogeneous, consisting mainly 

 of microscopic cells, structures developed only by vital 

 action. All the parts of an organism are mutually 

 dependent, and reciprocally means and ends, while 

 each part of a mineral exists for itself. The smallest 

 fragment of marble is as much marble as a mountain 

 mass ; but the fragment of a plant or animal is not an 

 individual. 



(3) Shape and Size. Living bodies gradually acquire 

 determinate dimensions ; so do minerals in their per- 

 fect or crystal condition. But uncrystallized, inorganic 

 bodies have an indefinite bulk. Most minerals are 

 amorphous ; crystals have regular forms, bounded, as a 

 rule, by plane surfaces and straight lines ; plants and 

 animals are circumscribed by curved surfaces, and 

 rarely assume accurate geometrical forms. 63 



(4) Phenomena. Minerals remain internally at rest, and 

 increase by external additions, if they grow at all. Liv- 

 ing beings are constantly changing the matter of which 

 they are composed, and grow by taking new matter into 

 themselves and placing it among the particles already 

 present. Organized bodies, moreover, pass through a 

 cycle of changes, growth, development, reproduction, 

 and death. These phenomena are characteristic of liv- 

 ing as opposed to inorganic bodies. All living bodies 

 grow from within, constantly give up old matter and 

 replace it by new, reproduce their kind, and die ; and 

 no inorganic body shows any of these phenomena. 



