198 AGE OF CORAL REEFS. 
and extent than other stony Polyps. But the 
true Reef-Building Polyps follow each other on 
the Reef in the same order as prevails in their 
structural gradation and their geological succes- 
sion ; and whether we classify them according to 
their position on the Reef, or their introduction 
upon the earth in the course of time, or their 
relative rank, the resul@is the same. 
It would require an amount of details that 
would be tedious to many of my readers, were I 
to add here the evidence now on record proving 
that the embryological development of these ani- 
mals, so far as it is known, and their geographical 
distribution over the whole surface of our globe, 
show the same correspondence with the other 
three series. But this recurrence of the same 
thought in the history of animals of the same 
Type, showing that, from whatever side we con- 
sider them, their creation and existence seem to 
be guided by one Mind, is so important in the 
study of Nature, that I shall constantly refer to it 
in the course of these papers, even though I may 
sometimes be accused of unnecessary repetition, 
or of extending my conclusions beyond the facts. 
What is the significance of these coincidences ? 
They were not sought for by the different inves- 
tigators, who have worked quite independently 
while ascertaining all these facts, without even 
knowing that there was any relation between 
