FOURTH JOURNEY TO THE SOUTH SEAS 129 
and not only under the artificial conditions of the laboratory and experi- 
ment station. 
In June, July and the early part of August a final survey was made of 
certain baffling portions of Tahiti, Moorea, Raiatea and Huahine. I 
then passed on to the Cook Islands by a steamer which stopped long 
enough at Mangaia, Moki, Aitutaki and Rarotonga for a survey to be 
made of each place. With the exception of the last named, which is a 
“high” island like Tahiti, the Cook Islands are uplifted coral atolls 

PAGOPAGO HARBOR, TUTUILA, SAMOAN ISLANDS 
Showing part of the surrounding amphitheater of mountains 
composed entirely of limestone. Mangaia, illustrated on page 128, is 
a typical example of such islands, which are relatively infrequent in the 
South Seas. These were originally low coral atolls scarcely rising above 
the surface of the ocean, and they were subsequently lifted by some geo- 
logical power which raised the bottom of the ocean at this place, so that 
what were formerly the lagoons in the center became basin-like valleys. 
Earlier voyagers like Garrett and Cuming had reported certain species 
