20 COMPARATIVE ZOOLOGY. 



to organic matter, and viceversd; dead matter which enters 

 the body of organized beings in tlie form of nutriment is 

 changed into living substance, whicli, after serving its pur- 

 pose, passes again as waste to tiie inorganic world. 



(3) 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 relatiotis to one another. The tissues 

 and organs, again, are heterogeneous, consisting mainly of 

 microscopic cells, structures developed only by vital ac- 

 tion. All the parts of an organism are mutually depend- 

 ent, and reciprocally means and ends, while each part of a 

 mineral exists for itself. The smallest fragment of mar- 

 ble is as much marble as a mountain-mass ; but the frag- 

 ment of a plant or animal is not an individual. 



(3) Size and Shape. — Living bodies gradually acquire de- 

 terminate dimensions; so do minerals in their perfect 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 cir- 

 cumscribed by curved surfaces, and rarely assume accurate 

 .geometrical forms.' 



(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 wliich 

 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 cy- 

 cle of changes — growth, development, reproduction, and 

 death. These phenomena are characteristic of living as 

 ■opposed to inorganic bodies. All living bodies grow from 

 within, constantly give up old matter and replace it by 

 new, reproduce tlieir kind, and die; and no inorganic 

 body shows any of these phenomena. 



