94 GRADATION AMONG ANIMALS. 
Now, what does this fossil creation tell us? It 
says this: that, in the Silurian period, taken in 
its most comprehensive sense, the first in which 
organic life is found at all, there were the three 
classes of Radiates, the three classes of Mollusks, 
two of the classes of Articulates, and one class of 
Vertebrates. In other words, at the dawn of life 
on earth, the plan of the animal creation with its 
four fundamental ideas was laid out, — Radiates, 
Mollusks, Articulates, and Vertebrates were pres- 
ent at that first representation of life upon our 
globe. If, then, all the primary types appeared 
simultaneously, one cannot have grown out of 
another, — they could not be at once contempo- 
raries and descendants of each other. 
The diagram on the opposite page represents 
the geological periods in their regular succession, 
and the approximate time at which all the types 
and all the classes of the Animal Kingdom were 
introduced ; for there is still some doubt as to the 
exact period of the introduction of several of the 
classes, though all geologists ate agreed respect- 
ing them, within certain limits, not very remote 
from each other, according to geological esti- 
mates of time. 
If such discussions were not inappropriate here 
from their technical character, I think I could 
show, upon combined geological and zodlogical 
evidence, that the classes which are not present 
