20 COMPARATIVE ZOOLOGY. 
a large number of atoms of those elements enter into com- 
bination. Thus, one molecule of fibrine contains 216 atoms 
of carbon, 169 of hydrogen, 68 of oxygen, 27 of nitrogen, 
and 2 of sulphur. Organic molecules are therefore larger 
than the molecules of minerals. Furthermore, combina- 
tions formed under the influence of life are invariably 
characterized by the presence of carbon and water. An 
animal always contains a mixture of solids, liquids, and 
gases. 
(2) Structure.— A mineral is homogeneous, while an or- 
ganized body is heterogeneous; 7. ¢., it is composed of dif- 
ferent parts, called tissues and organs, having peculiar uses 
and definite relations to one another. The tissues and 
organs, again, are heterogeneous, consisting mainly of mi- 
croscopic cells, a structure 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. The particles of 
a mineral are held together by cohesion; the exact nature 
of the force, commonly called “ life,” which presides over 
an organized body remains to be discovered. 
(3) Size and Shape.— Living bodies gradually acquire 
determinate dimensions; so do minerals in their perfect 
or crystal condition. But uncrystallized, inorganic bodies 
have an indefinite bulk. Most minerals are amorphous; 
erystals have regular forms, bounded, as a rule, by plane 
surfaces and straight lines; plants and animals are cir- 
cumscribed by curved surfaces, but never assume accurate 
geometrical forms.° 
(4) Phenomena.—Minerals remain internally at rest, and 
increase by external additions. In contrast, organisms are 
all in motion: they are constantly in a state of composi- 
tion and decomposition, incorporating foreign particles and 
