of drawing consequences by way of deduction
from simple principles of natural science--a power which has served him
in good stead on other occasions. Well, Mr. Darwin, looking at these
curious difficulties and having that sort of knowledge of natural
phenomena in general, without which he could not have made a step
towards the solution of the problem, said to himself--"It is perfectly
clear that the coral which forms the base of the atolls and fringing
reefs could not possibly have been formed there if the level of the sea
has always been exactly where it is now, for we know for certain that
these polypes cannot build at a greater depth than 20 to 25 fathoms, and
here we find them at 50 to 100 fathoms."

That was the first point to make clear. The second point to deal with
was--if the polypes cannot have built there while the level of the
sea has remained stationary, then one of two things must have
happened--either the sea has gone up, or the land has gone down.

There is no escape from one of these two alternatives. Now the
objections to the notion of the sea having gone up are very considerable
indeed; for you will readily perceive that the sea could not possibly
have risen a thousand feet in the Pacific without rising pretty much
the same distance everywhere else; and if it had risen that height
everywhere else since the reefs began to be formed, the geography of
the world in general must have been very different indeed, at that time,
from what it is now. And we have very good means of knowing that any
such rise as this certainly has not taken place in the level of the sea
since the time that the corals have been building their houses. And so
the only other alternative was to suppose that the land had gone down,
and at so slow a rate that the corals were able to grow upward as fast
as it went downward. You will see at once that this is the solution of
the mystery, and nothing can be simpler or more obvious when you come to
think about it. Suppose we start with a coral sea and put in the middle
of it an island such as the Mauritius. Now let the coral polypes come
and perch on the shore and build a fringing reef, which will stop when
they come to 20 or 25 fathoms, and you will have a fringing reef like
that round the island in the illustration. So long as the land remains
stationary, so long as it does not descend so long will that reef be
unable to get any further out, because the moment the polype embryos
try to g