312 EVOLUTION OF LIFE, 
and the Horse, the Hippopotamus and the Pig, the Whale 
and the Seal, the Reptiles and Birds, the Ganoid fishes and 
Batrachia, etc. Not only are the fossils invaluable, there- 
fore, to the Evolutionist without reference to their age, but 
the order in which they have appeared, and their relative 
age so far as it is possible to determine it, are in perfect 
harmony with the conclusions we have drawn from the 
structure of living plants and animals. Remembering the 
uncertainty attached to the absolute and relative age of 
rocks, let us examine the Primary, Secondary, and Tertiary 
Ages through which North America has probably succes- 
sively passed, without reference to the relation these Ages 
bear in time to the corresponding parts of Europe, etc. 
Geologists subdivide the Primary, Secondary, and Tertiary 
Ages into periods (epochs) more or less characterized by 
their fossils. 
AGE OF MOLLUSCA AND ALGE. 
Passing from the Azoic rocks, in the northern part of the 
State of New York, through the Potsdam region, to Tren- 
ton Falls, southwardly to the Helderberg Mountains near 
Albany, and eastwardly to Niagara, the immense number 
of fossil shells, particularly Brachiopods (Fig. 145), attracts 
the attention of the traveler. The Brachiopods of the 
present seas are few and far between, whereas the sea of 
that most ancient period was characterized by shells of 
this order; the remains of other Mollusca are found, but 
much less abundantly as compared with those of Brachio- 
pods. The seas of this period must have swarmed with 
Crinoids, from the great number of them found petrified, 
their broken stems being known as Lily Stones (Fig. 146) 
and St. Cuthbert beads. The young of the Comatula, long 
supposed to be a distinct animal, the Pentacrinus (Fig. 42), 
is the only known representative of the Crinoids at the 
presenttime. With the Crinoids are also found abundantly 
