82 
DISTRIBUTION OF CORAL-REEFS. Cn. IV. 
of the Galapagos Islands , 1 to the coldness of the cur- 
rents from the south, but the Gulf of Panama is one 
of the hottest pelagic districts in the world . 2 In the 
central parts of the Pacific there are islands entirely 
free from reefs ; and in some of these cases this appears 
to be due to recent volcanic action : but the existence 
of reefs, though scantily developed, and according to 
Dana, confined to one part of Hawaii (one of the Sand- 
wich Islands), shows that recent volcanic action does 
not absolutely prevent their growth . 3 
' The mean temperature of the surface sea, from observations 
made by the direction of Captain FitzRoy on the shores of the 
Galapagos Islands, between the 16th of September and the 20th of 
October, 1835, was 68° Fahr. The lowest temperature observed was 
58°’5 at the S.W. end of Albemarle Island ; and on the west coast 
of this island, it was several times 62° and 63°. The mean tem- 
perature of the sea in the Low Archipelago of atolls, and near Tahiti, 
from similar observations made on board the Beagle, was (although 
further from the equator) 77 0- 5, the lowest any day being 76 0- 5. 
Therefore we have here a difference of 9 0- 5 in mean temperature, 
and 18° in extremes; a difference doubtless quite sufficient to affect 
the distribution of organic beings in the two areas. 
2 Humboldt’s Personal Narrative, vol. vii. p. 434. 
3 [Mr. S. J. Whitmee (Nature, August 12, 1875, p. 291) states that 
in Savaii (Samoan group), one of four examples of islands which 
Professor Dana brings forward as instances indicating that recent 
volcanic action has prevented the formation of extensive coral-reefs, 
the cause is more probably the depth of water on the coast. More- 
over, parts of Savaii differ in change of level from the rest of the 
island, and it is in these (the upheaved regions) that coral-reefs are 
almost wanting. He also says that after examining 1 a good many 
intertropical islands of the Pacific belonging to the three orders — 
(1) Volcanic islands with fringing coral-reefs, such as Samoa, the New 
Hebrides, Ac. ; (2) Atolls, such as the Low Archipelago, Ellice, Gilbert 
Islands, &c. ; (3) Upraised coral-islands, such as Niu6 or Savage 
Island, part of the Friendly, the Loyalty Islands, &c.’ — he has been 
the more firmly convinced, the further he has gone, of the correctness 
of Mr. Darwin’s theory. Mr. R. Webb (Nature, id. p. 475) disputes 
