THE LIVING WORLD. 
18 
is generally associated with the clay. From this it has been inferred, and not 
without a strong semblance of probability, that animals have mainly contributed 
FOSSII. SEA HEDGE-HOG, FROM THE UPPER STRATA OF TIME FORMATION. 
to the formation of many limestone strata, in the same way as we see them 
now at work forming vast limestone rocks in-the coral reefs of the Pacific Ocean. 
We find in the lowest beds of the series of the secondary strata that the 
organic remains consist chiefly 
of corals and shells; that is, 
of animals having a compara¬ 
tively simple anatomical struc¬ 
ture, and that as we ascend in 
the series, the proportion of 
animals of more complicated 
forms increases, the bones of 
land quadrupeds being almost 
entirely confined to the more 
recent members of the tertiary 
strata. From these circum¬ 
stances, it is a 
received opin¬ 
ion among cer¬ 
tain geologists 
that the first 
animals which 
were created 
were of an ex¬ 
ceedingly sim¬ 
ple structure, 
that they 
gradually be- 
c a m e more 
MODERN SEA HEDGE-HOGS {Echinus delalandi). primary shew.. Complex in 
their frame, 
and that at last the highly complicated mechanism of the human body was the 
completion of those repeated efforts of nature towards perfection. It has been 
