BOTH by the publication of his book, “Coral Reefs and Atolls”, and by a recent contribution to NATURE,1 Prof. J. Stanley Gardiner has materially added to the debt which all interested in problems of corals and of coral reefs already owe him. As a result, however, of work on the physiology of corals carried out during the course of the Great Barrier Reef Expedition, I am unable entirely to agree with Prof. Gardiner's conclusions in one important matter. He suggests that corals obtain supplies of carbohydrate from their contained zooxanthellæ, and also that the oxygen produced by the latter as a result of photosynthetic activity is of vital importance to the animals. My own conclusions as to the significance of the relationship between corals and zooxanthellæ are somewhat different, and, since this work—the result of the combined labours of Mrs. Yonge, Mr. A. G. Nicholls, and myself—is now largely completed, I am able to put them forward with confidence and no longer tentatively as in previous contributions to NATURE. 2,3,4 Moreover, it has proved necessary to publish this work in a series of six large reports, 5, 6, 7, 8, 9, 10 and a short summary of the results obtained is, therefore, not without some justification.