92 GRADATION AMONG ANIMALS. 
will, in the most cursory manner, give a sketch 
of the great geological periods, as generally ac- 
cepted now by geologists. 
The first of these periods has been called the 
Azoic or lifeless period, because it is the only one 
containing stratified deposits in which there are 
no remains of organic life, and it is therefore 
supposed that at that early stage of the world’s 
history the necessary conditions for the mainte- 
nance of animals and plants were not yet estab- 
lished. After this, every great geological period 
that follows has been found to be characterized 
by a special set of animals and plants, differing 
from all that follow and all that precede it, till 
we arrive at our own period, when Man, with the 
animals and plants that accompany him on earth, 
was introduced. 
There is, then, an order of succession in time 
among animals; and if there has been any tran- 
sition between types and classes, any growth of 
higher out of lower forms, it is here that we 
should look for the evidence of it. According to 
this view, we should expect to find in the first 
period in which organic remains are Tound at all 
only the lowest type, and of that type only the 
lowest class, and, indeed, if we push the theory 
to its logical consequences, only the lowest forms 
of the lowest class. What are now the facts? 
This continent affords admirable opportunities for 
