20 
COMPARATIVE ZOOLOGY. 
to organic matter, and vice versa; dead matter which enters 
the body of organized beings in the form of nutriment is 
changed into living substance, which, after serving its pur¬ 
pose, 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 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. 6 
(4) Phenomena.— Minerals remain internal^ 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 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 their kind, and die; and no inorganic 
body shows any of these phenomena. 
