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condition. Professor Dana further believes that many of the lagoon 
islands on the Paumotu or Low Archipelago and elsewhere have recently 
been elevated to a height of a few feet, although originally formed during 
a period of subsidence ; but I shall endeavour to show that lagoon islands, 
which have long remained at a stationary level, often present a false ap- 
pearance of having been slightly elevated.” 
It will readily be seen, from the foregoing quotation, what it is that Mr. 
Darwin attempts to prove in this volume, and also why the present edition 
has been given to the world. We shall refer further on to some of his argu- 
ments, but in the mean time we may explain in a few words the nature of 
his book. In the first place we may state that it is to Mr. Darwin that we 
owe whatever knowledge we possess on the subject of coral reefs. It was he 
who first explained, more than thirty years ago, the peculiar reason why 
those singular islands called Atolls had their particular shape. And we are 
glad to state that the reasons he then gave forth to the world, to explain 
their origin, are the same he now urges with even stronger arguments than 
he first advanced in their favour. What, then, is the meaning of an 
atoll, and how has it arisen from the deep P An atoll is one of those 
peculiar islands which the navigator sees abundantly in the Pacific Ocean. 
Taking one that is completely formed, what is its appearance ? It is that of 
a ring of solid rock about a mile or so wide, covered with cocoa-nut trees, 
and forming a more or less regular circle which is broken at one point. 
Without, the sea is beating against this belt of rocks with all its fury ; 
within, is a basin ten or twelve or more miles wide, and some thirty or 
more miles long, of perfectly still water. Finally there is usually an en- 
trance ,bv which boats may enter into this so-called lagoon. This is the 
constitution of a perfectly formed atoll. Others are not so completly 
walled in, and in some instances there is an island in the centre. Now 
leaving for a while the consideration of fringing-reefs and barrier-reefs, we 
may ask how came about the formation of the Atoll P Well, it is a 
question whose answer depends on our knowledge of coral reefs, their rapidity 
and mode and other conditions of growth. In the first place these corals 
usually commence growing along a shore, for it is found that they cannot 
live if they are placed at a considerable depth, or if they are exposed by 
tidal fall to the air ; again, that they grow with immense rapidity 5 and 
finally, that they will not grow in any position in which fresh water flows. 
Now, admitting these conditions, and admitting also the fact which no 
geologist will deny, viz., that in the Pacific Ocean, as elsewhere, islands are 
constantly sinking and being elevated, we have all the conditions necessary 
to explain that singular formation, an atoll, or circular coral belt. Suppose, 
for example, an island has been thrown up in the ocean — no extraordinary 
thing in a volcanic district — and measures about thirty miles long by ten 
miles wide. Soon there appears to be coral reefs in its neighbourhood, and 
these eventually appear all round the island at a certain distance from 
shore. Here they may go on living and dying for a thousand years or 
more. Eventually this island begins to descend into the ocean from which 
it came. But as it descends the corals must grow up, because they cannot 
live at a great depth. And so on, for thousands of years, this island keeps 
going down, and, pari passu , the coral wall keeps growing upward, till at 
