These are believed to be due to the warpings produced by the cooling of an originally highly heated globe. It is not true, therefore, that the earth's surface is fixed, or that its land and water areas remain always the same. On the contrary, what is land at one time is water at another time, and so, too, water areas may become changed into land areas. For the most part these changes go on so slowly as not to be noticeable in an ordinary lifetime. Indeed, in some cases, they are so extremely gradual that Methuselah himself might have gone to his grave in ignorance of their progress. Let us briefly note a few well-known gradual changes of level. One of the most extensive of these is the sinking of an immense area, over 6,000 miles in diameter, that covers a large part of the bed or floor of the Pacific Ocean. It is an easy matter to observe the gradual changes of level on the coasts, since the old water line can be at once found, but it is very difficult to detect such changes in the bed of the ocean, hidden as it is by a covering of water. Yet many things that seem impossible to the uninitiated are readily solved by those familiar with physical science. Little signs, meaningless to others, are easily read, and these prove beyond doubt the gradual sinking of the ocean's bed. It was once believed that the coral polyps or animalculæ from the hard, bony skeletons of which coral reefs are formed, could live at the greatest depths of the ocean. These minute animals were, therefore, generally credited with filling up the deep ocean, in certain places, and converting it into dry land, and poetic philosophers were pleased to point to their indefatigable labors as an object lesson to the slothful. But these charming, though fallacious, ideas were rudely overthrown by the sounding line and the drag-net. It had long been known that pieces of coral rock were brought up by dredging apparatus from the bottom of the ocean at all depths, but it was eventually shown that such pieces of coral rock never contained living animalculæ, when brought from water at greater depths than from 100 to 120 feet. It puzzled scientific men no little at first to explain this apparent inconsistency. If the coral polyp could not live in water at greater depths than from 100 to 120 feet, how could the presence of coral rock at a depth of thousands of feet be explained? Happily, however, this problem was solved by the great naturalist, Charles Darwin, who showed that coral islands can only be formed in parts of the ocean whose beds are sinking at the same gradual rate at which the coral rock is being deposited. The presence, therefore, of coral islands on the bed of the Pacific, as well as along parts of its coasts, are, to scientific men, as good indications of its gradual sinking as if such facts had been written in the clearest language.