Jukes, who was a man of great acuteness as well as independence of mind, concludes his account of the great Australian reefs with the following words:--"After seeing much of the Great Barrier-Reefs, and reflecting much upon them, and trying if it were possible by any means to evade the conclusions to which Mr. Darwin has come, I cannot help adding that his hypothesis is perfectly satisfactory to my mind, and rises beyond a mere hypothesis into the true theory of coral-reefs."
As the result of the clear exposition of the subject by Darwin, Lyell, Dana, and Jukes, the theory of coral-reefs had, by the middle of the present century, commanded the almost universal assent of both biologists and geologists. In 1859 Baron von Richthofen brought forward new facts in its support, by showing that the existence of the thick masses of dolomitic limestone in the Tyrol could be best accounted for if they were regarded as of coralline origin and as being formed during a period of long continued subsidence. The same views were maintained by Professor Mojsisovics in his "Dolomit-riffe von Sudtirol und Venetien," which appeared in 1879.
The first serious note of dissent to the generally accepted theory was heard in 1863, when a distinguished German naturalist, Dr. Karl Semper, declared that his study of the Pelew Islands showed that uninterrupted subsidence could not have been going on in that region. Dr. Semper's objections were very carefully considered by Mr. Darwin, and a reply to them appeared in the second and revised edition of his "Coral-Reefs," which was published in 1874. With characteristic frankness and freedom from prejudice, Darwin admitted that the facts brought forward by Dr. Semper proved that in certain specified cases, subsidence could not have played the chief part in originating the peculiar forms of the coral-islands. But while making this admission, he firmly maintained that exceptional cases, like those described in the Pelew Islands, were not sufficient to invalidate the theory of subsidence as applied to the widely spread atolls, encircling reefs, and barrier-reefs of the Pacific and Indian Oceans. It is worthy of note that to the end of his life Darwin maintained a friendly correspondence with Semper concerning the points on which they were at issue.
After the appearance of Semper's work, Dr. J.J. Rein published an account of the Bermudas, in which he opposed the interpretation of the structure of the islands given by Nelson and other authors, and maintained that the facts observed in them are opposed to the views of Darwin. Although, so far as I am aware, Darwin had no opportunity of studying and considering these particular objections, it may be mentioned that two American geologists have since carefully re-examined the district--Professor W.N.
Rice in 1884 and Professor A. Heilprin in 1889--and they have independently arrived at the conclusion that Dr. Rein's objections cannot be maintained.
The most serious opposition to Darwin's coral-reef theory, however, was that which developed itself after the return of H.M.S. "Challenger" from her famous voyage. Mr. John Murray, one of the staff of naturalists on board that vessel, propounded a new theory of coral-reefs, and maintained that the view that they were formed by subsidence was one that was no longer tenable; these objections have been supported by Professor Alexander Agassiz in the United States, and by Dr. A. Geikie, and Dr. H.B. Guppy in this country.
Although Mr. Darwin did not live to bring out a third edition of his "Coral-Reefs," I know from several conversations with him that he had given the most patient and thoughtful consideration to Mr. Murray's paper on the subject. He admitted to me that had he known, when he wrote his work, of the abundant deposition of the remains of calcareous organisms on the sea floor, he might have regarded this cause as sufficient in a few cases to raise the summits of submerged volcanoes or other mountains to a level at which reef-forming corals can commence to flourish. But he did not think that the admission that under certain favourable conditions, atolls might be thus formed without subsidence, necessitated an abandonment of his theory in the case of the innumerable examples of the kind which stud the Indian and Pacific Oceans.
