112 



structure; among the Madrepores we should have the Madrepora jprolifera, 

 with its small, short branches, broken up by very frequent ramifications; 

 the M. cervicornis, with longer and stouter branches, and less frequent 

 ramifications, and the cup-like M. palmata, resembling an open sponge in 

 form. Every species, in short, that lives upon the present reef is found in 

 the more ancient ones. They all belong to our own geological period, and- 

 we cannot, upon the evidence before us, estimate its duration at less than 

 seventy thousand years, dui-ing which time we have no evidence of any 

 change in species, but, on the contrary, the strongest proof of the absolute 

 permanence of those species, whose past history we have been able to 

 trace."* 



Theory of the Formation of Coral Reefs. — M. Darwin, during 

 his voyage as Naturalist to H.M.S. Beagle, between the years 

 1835 and 1840, was the first to explain the manner by which 

 Barrier and Atoll reefs had been formed from ordinary Fringing 

 reefs, on the principle of the depression of the land ; and from 

 that time to the present nearly all geologists have admitted this 

 to be the true explanation of the phenomena, and of which the 

 following account is a brief resume. 



Assuming that the outer edge of a Barrier reef marks the 

 position, or nearly so, of a Fringing reef that was constructed 

 along the shore when the island around which it formed stood 

 at a higher level out of the water than it does now, we should 

 have a condition of things which would be represented by Fig. 3, 

 where (a, a, b) represents the section of an island surrounded by 

 a Fringing reef, (r, r,) rising to the surface of the sea. 



As the sloping shore slowly subsides by the depression of 

 the land, the sea necessarily flows farther over it, and in 

 course of ages the outer margin of the reef (Fig. 3, r) would 



* L. Agassiz, Methods of Study in Natural History, pp. 185-192. 



