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