152 THE ANATOMY OF INVERTEBRATED ANIMALS. 



or more ? Why do they take so generally the circular form ? 

 What is the connection, finally, between fringing reefs and 

 atolls ? The only thoroughly satisfactory answer to these 

 questions has been given by Mr. Darwin, from whose beauti- 

 ful work on " Coral Reefs " I have borrowed most of the fore- 

 going details. Consider for a moment what would be the 

 effect of a slow and gradual submergence of the island of 

 Mauritius — a submergence, perhaps, of a few feet in a century 

 (at any rate, not greater than the rate of upward growth of 

 coral), continued for age after age. As the edge of the fring- 

 ing reef sank, new coral would grow up from it to the sur- 

 face; and, as the most active and important of the reef-build- 

 ers nourish best in the very surf of the breakers, so the margin 

 of the reef would grow faster than its inner portion, and the 

 discrepancy would increase as the latter, sinking deeper and 

 deeper, became farther removed from the region of active 

 growth. Nevertheless, the sea-bottom within the reef would 

 constantly tend to be raised by the accumulation of frag- 

 ments, and by the deposit of fine mud, in its sheltered and 

 comparatively calm waters. On the other hand, on the sea- 

 ward face of the reef, no possible extension could take place 

 by direct growth; and that by accumulation must be exceed- 

 ingly slow, the incessant wash of tides, waves, and currents, 

 tending incessantly to spread any talus over a wider and 

 wider area. 



Thus, then, the edge of the reef unceasingly compensates 

 itself for the depression which it undergoes, while, inside the 

 reef, only a partial compensation takes place, and, outside, 

 hardly any at all. Continue the sinking process until its 

 highest peak was but a few hundred feet above the surface, 

 and all that would be left of Mauritius would be an island 

 surrounded by an encircling reef ; carry on the depression 

 further still, and a circular reef, or atoll, alone would remain. 

 But the region of the coral-reefs is, for the most part, that of 

 constant winds. During the whole process of growth of the 

 reef, therefore, one of its sides — that to windward — has been 

 exposed to more surf than that to leeward. Not only will 

 the greater quantity of debris, therefore, have been heaped 

 up by storms upon the windward side, but the coral-builders 

 themselves will here have been better fed, better aerated, and 

 consequently more active. Hence it is that, other things 

 being alike, there is a probability that the leeward side of 

 the reef will grow more slowly, and repair any damages less 

 easily, than the windward side ; and hence, again, as a result, 



