Sept. 3, 1874] 



NATURE 



353 



pays best under a given set of circumstances ; and this 

 only shows the wide field open to British farmers for pro- 

 fiting by the laws of selection. 



I look to the development of this great principle as one 

 of the soundest and surest means of promoting the inte- 

 rests of the agricultural classes. 



Thomas Baldwin 



DARWIN'S "CORAL REEFS" 

 TItc Slnic/iirc and Distribution of Coral Reefs. By 

 Charles Darwin, M.A., F.R.S., c&c. Second edition, 

 revised. 1874 ; pp. 268. (Smith, Elder, and Co.) 

 '"T'HE rising generation of naturalists and geologists 

 -L has not had, and most probably will never have, 

 such feelings of intellectual pleasure as fell to the lot of 

 the first readers of Charles Darwin's book on Coral 

 Reefs, which was offered to science more than thirty years 

 since. The recent researches into the nature of the 

 deposits of the deep sea, and the discoveries of the bathy- 

 metrical zones of water of very different temperatures, are 

 certainly full of vast interest, and will afford the data for 

 the development of many a theory ; but the clear exposi- 

 tion of facts, and the bold theory which characterised the 

 book on Coral Reefs, came unexpectedly and with over- 

 powering force of conviction. The natural history of a 

 zoophyte was brought into connection with the grandest 

 phenomena of the globe — with the progressive subsidence 

 of more or less submerged mountains, and with the dis- 

 tribution of volcanic foci. The forces of the organic and 

 inorganic kingdoms were shown to unite in the pro- 

 duction of those circular growths of coral which appeared 

 to rise from profound oceanic depths ; and it was made 

 evident that the existence and persistent growth of fragile 

 Porites and Madrcporcc were dependent upon movements 

 of the crust of the globe, the result of forces acting almost 

 from the beginning — upon movements so vast, equable 

 and slow, that over thousands of square miles the coral 

 grew upwards, whilst the supporting rock, its base, and 

 the mother crust subsided in a wonderful unison. The 

 pristine condition of the globe was in fact brought in 

 relation with the formation of those beautiful islands, 

 the theme of romance and poesy, the delight of the 

 missionary, the dread of the navigator, and which should 

 be, according to Dana, the lu.\urious home of enervated 

 and used-up investigators. 



The theory of the formation of barrier reefs and atolls 

 is stated with Darwin's usual lucidity : — " From the 

 limited depths at which reef-building polypifers can 

 flourish, taken into consideration with certain other cir- 

 cumstances, we are compelled to conclude that both in 

 atolls and barrier reefs the foundation to which the coral 

 was primarily attached has subsided ; and that during this 

 downward movement the reefs have grown upwards." 

 "There is not one point of essential difference between 

 encircling barrier reefs and atolls ; the latter enclose a 

 simple sheet of water ; the former encircle an expanse 

 with one or more islands rising from it. Remove the 

 central land, and an annular reef like that of an atoll in 

 an early stage of formation is left." It was necessary, in 

 order that this theory should be valid, that the depth at 

 which reef-building corals can exist below the surface 

 should be ascertained, and also that direct or indirect 



proofs of subsidence over a vast area should be offered. 

 The nature of the bottom of the sea immediately sur- 

 rounding Keeling atoll was carefully examined, and more- 

 over soundings with the wide bell-shaped lead, with 

 tallow armings, were carefully taken, off the fringing reefs 

 of Mauritius. In Keeling atoll outside the reef it was 

 found, " to the depth of ten or twelve fathoms the bottom 

 is exceedingly rugged and seems formed of great masses 

 of living coral, similar to those on the margin. The 

 arming of the lead here invariably came up quite clean, 

 but deeply indented, and chains and anchors which were 

 lowered in the hopes of tearing up the coral were broken." 

 "Between 12 and 20 fathoms the arming came up an 

 equal number of times smoothed with sand and indented 

 with coral ; an anchor and lead were lost at the respective 

 depths of 13 and 16 fathoms. Out of twenty-five 

 soundings taken at a greater depth than 20 fathoms, 

 every one showed that the bottom was covered with 

 sand." Off the reef at Mauritius, "from 15 to 20 fathoms, 

 the bottom was with few exceptions either formed of sand 

 or thickly coated with Seriatopora (one of the Tabulata). 

 At 20 fathoms one sounding brought up a fragment of 

 Madrepora which I believe to be the same species as that 

 which mainly forms the upper margin of the reef ; if so, it 

 grows in depths varying from o to 20 fathoms. Between 20 

 and 23 fathoms I obtained several soundings, and they all 

 showed a sandy bottom with one exception at 30 fathoms, 

 when the arming came up scooped out as if by the margin of 

 a large Caryophyllia." " The circumstance of the arming 

 having invariably come up quite clean when sounding 

 within a certain number of fathoms off the reef of Mau- 

 ritius and Keeling atoll (8 fathoms in the former case and 

 12 in the latter), and of its having always come up (with 

 one exception) smoothed and covered with sand when the 

 depth exceeded 20 fathoms, probably indicate a criterion 

 by which the limiting of the vigorous growth of coral 

 might in all cases be ascertained.'" Darwin admits that 

 this limit might be exceptionally transgressed, but insists 

 upon the importance of the gradual change, as depth pro- 

 gresses, from living clean coral to a smooth sandy bottom, 

 in endeavouring to fix the depth at which the reef-builders 

 can grow. 



Even at this period of Darwin's life, the import- 

 ance of the struggle for existence had been recognised 

 by him, and had influenced his thoughts. He remarks 

 that " we can understand the gradation only as a pro- 

 longed struggle against unfavourable conditions." All 

 subsequent investigations by many independent observers 

 have proved the correctness of this bathymetrical limit of 

 the flourishing of reef-builders, and of late years the 

 general characters of the coral which can exist at a greater 

 depth and even on oceanic floors have been shown to 

 differ essentially from, those of the forms which hve and 

 flourish amidst the rush of the wave and surf. Darwin 

 notices that where the sea is very shallow, as in the Per- 

 sian Gulf and in parts of the East Indian Archipelago, 

 the reefs lose their fringing character and appear as sepa- 

 rate and irregularly scattered patches, often of consider- 

 able area. Around the Philippines the bottom of the sea 

 is " entirely coated by irregular masses of coral, which, 

 although often of large size, do not reach the surface and 

 form reefs." There are huge clumps of Porites and many 

 sponges on the floor of the sea off Cuba, but although 



