38 
HAROLD’S DISCUSSIONS. 
Plant life, too, made its appearance in the lowest 
forms, snch as the fucoids, a species of seaweed 
which later became so 
abundant, and the lit¬ 
tle wheel-shaped rhi- 
zopods whose silicious 
skeletons make up the 
greater part of the 
rock in some places, 
not only in this time 
but in later periods. 
As vegetation is neces¬ 
sary to animal life, the 
sea doubtless contained 
myriads of minute 
plant forms which 
furnished food for the 
primitive animals al¬ 
ready beginning to 
multiply. On the 
other hand, the land 
Fig. 18.-A modern crinoid animal, g^p] barren 
which, like a plant, lives attached ^ . 
to the bottom of the ocean. vegetation except 
where the lowest 
forms of land plants covered the rocks in dark spots, 
as we now often see them on old stones. 
THE SILURIAN STRATA PROPER. 
Continuing our walk southward, we come upon 
other strata, not so different from the Cambrian 
as the Cambrian is from the Archsean, and yet 
