SHALLOW-WATER FAUNA OF THE TROPICS. 69 



the beach and separated from it by a shallow 

 lagoon which is frequently left as a dry sand- 

 bank at low tides. 



Many forms of reefs are found in different parts 

 of the world, but they may all be looked upon 

 as special modifications of one of these three 

 groups. The facts which Darwin collected about 

 Coral-reefs in his memorable voyage round the 

 world in the Beagle, suggested to this great 

 observer that all the different forms of reef 

 must be related to one another, and he formu- 

 lated an ingenious theory to show how, by the 

 gradual sinking of the crust of the earth, 

 Fringing reefs may have become, in the course 



A 13 C 



FIG. 23.^!, Fringing-reef ; B, Barrier-reef; C, Atoll, 

 a, sea; &, reef; c, rocks of the coast. 



of a long period of time, either Atolls or Barrier- 

 reefs. Some doubts have recently been ex- 

 pressed as to the truth of Darwin's famous 

 " subsidence theory." But, whether it is true 

 or not, to Darwin is due the credit of bringing 

 home forcibly to our minds the fact that Coral- 

 reefs are slowly undergoing changes of growth 

 and of destruction, which must lead to most 

 important and far-reaching alteration in the 

 character of the Tropical seas. It is not in- 



