DARWIN AND WALLACE 31 
coast. On leaving the Galapagos group, Charles Dar- 
win writes in his diary the suggestive observation 
that this little group of rocky islands seems to be one 
of the greatest centers of creative activity. It was 
this interesting resemblance of the animals of these 
islands to each other and to those of the Peruvian 
coast that finally persuaded Darwin that they were 
all related and were all descended from those of Peru. 
For the rest of his life, with an intensity which in- 
creased with each year, Darwin persisted in a patient 
search for the possible agencies by which such change 
could have been brought about. The problem, how- 
ever, was temporarily eclipsed by a pressing geological 
question aroused by his visit to the Keeling Atoll. 
Here his investigation of coral reef formation abso- 
lutely captivated him. In the case of most coral 
islands in the Pacific Ocean the reef exists as a circle 
of coral enclosing a lagoon of water. In the center 
of this lagoon stands commonly a rocky island. It is 
plain that this is the foundation on which the coral 
built. But, in the case of the Atoll, the coral ring was 
present and so was the internal lagoon, but there was 
no rocky island. The key to the solution came with 
an interesting discovery. Darwin began to put down 
a grappling iron on the outer side of the reef and 
to drag up coral. The farther away from the reef he 
went the deeper was the water from whose bottom he 
