52 DR. H. B. GUPPY. 
American Coast Survey, re-echoed the opinion of Chamisso 
in the instance of the reefs of Florida. All the modifications 
which these reefs presented were in his estimation ‘‘ the 
natural consequence of the growth of reef-building corals.”’* 
In the year 1856, Professor Joseph Le Conte, after an examina- 
tion some years before of the same reef-region of Florida, 
advanced a view, in a paper read before the American Asso- 
ciation, which was simply Chamisso’s explanation of atolls 
applied to barrier-reefs. Since corals, as he stated, will not 
grow on muddy shores or in water upon the bottom of which 
sediment is collected, the favourable conditions can only be 
found at some distance from the shore, where, ‘limited on 
one side by the muddiness and on the other by the depth of 
the water,’ a barrier-reef would ultimately be formed.+ 
After a long investigation of the reefs of the Pelew Islands, 
Professor Karl Semper published in 1863 the results of his 
researches.{ Additional notes were appended to his original 
paper from time to time, by which his observations were 
brought into accordance with the results of the deep-sea 
explorations of the Challenger expedition, and with the facts 
brought to light by Agassiz, Pourtales, and others, in the 
Florida seas; and in 1881 the whole of his views on coral 
reefs were included in his interesting work entitled The 
Natural Conditions of Existence as they affect Animal Life 
(vol. xxxi. Internat. Scient. Ser.). This naturalist attributed 
the formation of the different classes of reefs to the following 
causes :—the strength and direction of the constant currents, 
the repressive influence of sand and sediment in the interior, 
the tidal scour, and the boring action of plants and animals. 
He showed that, wherever strong currents impinging on a 
reef run parallel with the coast, the corals grow perpen- 
dicularly, and the reef has an abrupt and perpendicular fall 
along the line of the current, but that, in eddies and places 
where the current 1s weak, the corals grow irregularly in all 
directions. The growth of an atoll, as he pointed out, is 
well illustrated in the growth of a large mass of porites, 
which while below the surface possesses a rounded summit, 
but when exposed at low tide becomes arrested in its upward 
growth and obtains a flat top. Whilst growing at its margin, 
where it is exposed to the tidal currents, its centre dies from 
the accumulation of sand ; and in course of time the actien of 
boring-molluses, echinoderms and sponges, and the scour of 
* Bull. Mus. Compar. Zool., vol. i. p. 363. 
t+ Amer. Journ. of Science, 2nd series, vol. xxiii. p. 46. 
t Zeitschr. Wissen. Zool., vol. xiii. p. 563. 
