ary 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 increased with each
year, Darwin persisted in a patient search for the possible agencies
by which such change could have been brought about. The problem,
however, was temporarily eclipsed by a pressing geological question
aroused by his visit to the Keeling Atoll. Here his investigation of
coral reef formation absolutely 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 pulled the coral. What at
first puzzled him was the fact that so long as he dragged up his coral
from depths of a hundred feet or less the coral was alive. Whenever he
went to depths of much more than a hundred feet, his coral was always
dead, though he was evidently pulling it from situations in which it
had grown. Then Darwin remembered the rising Andes, lifting themselves
out of the bed of the Pacific. Here was the correlated movement. The
bottom of the ocean here was sinking. As it sank it dragged down the
corals with it. But the descent was so slow that new corals could
build on top of the others fast enough to keep the reef up to the
surface of the water. At the rate of growth of coral, this would seem
to mean that the bottom could be sinking at a rate of only a few feet
a century. But while the reef could keep up to the surface, the rocky
island must slowly sink. Darwin inferred that there must be a rocky
summit within the lagoon, below the surface of the water. A little
sounding soon discovered this island, and the verification of Darwin's
theory of coral reef formation was at hand. The descr