24 THE DOCTRINE OF DESCENT. 
I. 
The Animal World in its Present State. 
IN order to approach the doctrine of Descent, and to 
prepare for its necessity, we purpose next to pass in 
review a main part of its object,—the present condition 
of the animal world in its general outlines. Organisms, 
as every one may see, are distinguished from animate 
bodies by a certain mutability of existence ; a sequence 
and alternation of phenomena, combined with constant 
absorption and expulsion of matter. These changes, 
which are ultimately molecular motions, and are there- 
fore calculable, definable, and susceptible of investiga- 
tion, take place in particles ina state of saturation—that 
is to say, soaked in water and aqueous fluids; and this 
peculiar, yet purely mechanical condition, suffices for the 
explanation and comprehension of many of the neces- 
sary phenomena of life. Experience shows that this 
capacity for saturation, and this mobility, essentially 
characterize the combinations of carbon; and the sum 
of these motions and displacements, of which a great 
part has already been susceptible of mathematically cer- 
tain investigation, is termed Life. 
Now it is impossible to resist the impression that 
there are simple and composite, lower and higher, living 
beings ; and we likewise feel, more strongly than words 
will express, a certain antithesis between the plant and 
