Coral Reefs. Darwin Charles

Чтение книги онлайн.

СКАЧАТЬ the theory of coral-reefs would be slow in meeting with general acceptance was certainly not justified by the actual facts. On the contrary the new book was at once received with general assent among both geologists and zoologists, and even attracted a considerable amount of attention from the general public.

      It was not long before the coral-reef theory of Darwin found an able exponent and sturdy champion in the person of the great American naturalist, Professor James D. Dana. Two years after the return of the "Beagle" to England, the ships of the United States Exploring Expedition set sail upon their four years' cruise, under the command of Captain Wilkes, and Dana was a member of the scientific staff. When, in 1839, the expedition arrived at Sydney, a newspaper paragraph was found which gave the American naturalist the first intimation of Darwin's new theory of the origin of atolls and barrier-reefs. Writing in 1872, Dana describes the effect produced on his mind by reading this passage: – "The paragraph threw a flood of light over the subject, and called forth feelings of peculiar satisfaction, and of gratefulness to Mr. Darwin, which still come up afresh whenever the subject of coral islands is mentioned. The Gambier Islands in the Paumotus, which gave him the key to the theory, I had not seen; but on reaching the Feejees, six months later, in 1840, I found there similar facts on a still grander scale and of a more diversified character, so that I was afterward enabled to speak of his theory as established with more positiveness than he himself, in his philosophic caution, had been ready to adopt. His work on coral-reefs appeared in 1842, when my report on the subject was already in manuscript. It showed that the conclusions on other points, which we had independently reached, were for the most part the same. The principal points of difference relate to the reason for the absence of corals from some coasts, and the evidence therefrom as to changes of level, and the distribution of the oceanic regions of elevation and subsidence – topics which a wide range of travel over the Pacific brought directly and constantly to my attention."

      Among the Reports of the United States Exploring Expedition, two important works from the pen of Professor Dana made their appearance; – one on "Zoophytes," which treats at length on "Corals and Coral-Animals," and the other on "Coral-Reefs and Islands." In 1872, Dana prepared a work of a more popular character in which some of the chief results of his studies are described; it bore the title of "Corals and Coral-Islands." Of this work, new and enlarged editions appeared in 1874 and 1890 in America, while two editions were published in this country in 1872 and 1875. In all these works their author, while maintaining an independent judgment on certain matters of detail, warmly defends the views of Darwin on all points essential to the theory.

      Another able exponent and illustrator of the theory of coral-reefs was found in Professor J.B. Jukes, who accompanied H.M.S. "Fly", as naturalist, during the survey of the Great Barrier-Reef – in the years 1842 to 1846. 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.

      A letter written by Darwin to Professor Alexander Agassiz in May 1881 shows exactly the attitude which careful consideration of the subject led him to maintain towards the theory propounded by Mr. Murray: – "You will have seen," he writes, "Mr. Murray's views on the formation of atolls and barrier-reefs. Before publishing my book, I thought long over the same view, but only as far as ordinary marine organisms are concerned, for at that time little was known of the multitude of minute oceanic organisms. I rejected this view, as from the few dredgings made in the "Beagle", in the south temperate regions, I concluded that shells, the smaller corals, etc., decayed and were dissolved when not protected by the deposition of sediment, and sediment could not accumulate in the open ocean. Certainly, shells, etc., were in several cases completely rotten, and crumbled into mud between my fingers; but you will know whether this is in any degree common. I have expressly said that a bank at the proper depth would give rise to an atoll, which could not be distinguished from one formed during subsidence. I can, however, hardly believe in the existence of as many banks (there having been no subsidence) as there are atolls in the great oceans, within a reasonable depth, on which minute oceanic organisms could have accumulated to the depth of many hundred feet."

      Darwin's concluding words in the same letter written within a year of his death, are a striking proof of the candour and openness of mind which he preserved so well to the end, in this as in other controversies.

      "If I am wrong, the sooner I am knocked on the head and annihilated so much the better. It still seems to me a marvellous thing that there should not have been much, and long-continued, subsidence СКАЧАТЬ