THE PRIMARY CONDITIONS OF ANIMAL LIFE 113 
and to describe those characteristics which are peculiar to 
it, which absolutely distinguish it from inorganic matter, 
we meet with some difficulties. At least many of the char- 
acteristics commonly ascribed to organisms, as peculiar to 
them, are not so. The possession of organs, or the composi- 
tion of the body of distinct parts, each with a distinct func- 
tion, but all working together, and depending on each other, 
is as true of a steam-engine as of a horse. That the work 
done by the steam-engine depends upon fuel is true; but 
so it is that the work done by the horse depends upon fuel, 
or food as we call it in the case of the animal. The oxida- 
tion or burning of this fuel in the engine is wholly compar- 
able with the oxidation of the food, or the muscle and fat 
it is turned into, in the horse’s body. The composition of 
the bodies of animals and plants of tiny structural units, 
the cells, is in many ways comparable with the composition 
of some rocks of tiny structural units, the crystals. But 
not to carry such rather quibbling comparisons too far, it 
may be said that organisms are distinguished from organic 
substances by the following characteristics : Organization ; 
the power to make over inorganic substances into organic 
matter, or the changing of organic matter of one kind, as 
plant matter, into another kind, as animal matter ; motion, 
the power of spontaneous movement in response to stimuli ; 
sensation, the power of being sensible of external stimuli; 
reproduction, the power of producing new beings like them- 
selves; and adaptation, the power of responding to external 
conditions in a way useful to the organism. Through adap- 
tation organisms continue to exist despite the changing of 
conditions. If the conditions surrounding an inorganic 
body change, even gradually, the inorganic body does not 
change to adapt itself to these conditions, but resists them 
until no longer able to do so, when it loses its identity or 
integrity. 
