f the astonishing
tricks which she plays upon her admirers!--I say before you can be safe
in dealing with Nature, you must get two or three kinds of cross proofs,
so as to make sure not only that your hypothesis fits that particular
set of facts, but that it is not contradicted by some other set of facts
which is just as clear and certain. And it so happens, that in this case
Mr. Darwin supplied the cross proofs as well as the immediate evidence.
You have all heard of volcanoes, those wonderful vents in the surface of
the earth out of which pour masses of lava, cinders and ashes, and
the like. Now, it is a matter of observation and experience that all
volcanoes are placed in areas in which the surface of the earth is
undergoing elevation, or at any rate is stationary; they are not placed
in parts of the world in which the level of the land is being lowered.
They are all indications of a great subterranean activity, of a
something being pushed up, and therefore naturally the land either gives
way and lets it come through, or else is raised up by its violence. And
so Mr. Darwin, being desirous not to merely put out a flashy hypothesis,
but to get at the truth of the matter, said to himself, "If my notion of
this matter is right, then atolls and encircling reefs, inasmuch as they
are dependent upon subsidence, ought not to be found in company with
volcanoes; and, 'vice versa', volcanoes ought not to be found in company
with atolls, but they ought to be found in company with fringing reefs."
And if you turn to Mr. Darwin's great work upon the coral reefs, you
will see a very beautiful chart of the world, which he prepared with
great pains and labour, showing the distribution on the one hand of the
reefs, and on the other of the volcanoes; you will find that in no case
does the atoll accompany the volcano, or the volcano burst up among the
atolls. It is most instructive to look at the great area of the Pacific
on the map, and see the great masses of atolls forming in one region of
it a most enormous belt, running from north-west to south-east; while
the volcanoes, which are very numerous in that region, go round the
margin, so that we can picture the Pacific to ourselves a section of a
kind of very shallow basin--shallow in proportion to its width, with the
atolls rising from the bottom of it, and at the margins the volcanoes.
It is exactly as if you had taken a flat mass and lifted up the edges
of it; the subterranean force w