so that the surrounding element penetrates their structure, thus equalizing the pressure of the weight, which is diminished from without in proportion to the quantity of water they can admit into their bodies. Marine animals differ in their ability to sustain this pressure, just as land animals differ in their power of enduring great variations of climate and of atmospheric pressure. Of all air-breathing animals, none exhibits a more surprising power of adapting itself to great and rapid changes of external influences than the Condor. It may be seen feeding on the sea-shore under a burning tropical sun, and then, rising from its repast, it floats up among the highest summits of the Andes and is lost to sight beyond them, miles above the line of perpetual snow, where the temperature must be lower than that of the Arctics. But even the Condor, sweeping at one flight from tropic heat to arctic cold, although it passes through greater changes of temperature, does not undergo such changes of pressure as a fish that rises from a depth of sixty-four feet to the surface of the sea; for the former remains within the air that surrounds our globe, and therefore the increase or diminution of pressure to which it is subjected must be confined within the limits of one atmosphere, while the latter, at a depth of sixty-four feet, is under a weight equal to that of three such atmospheres, which is reduced to one when it reaches the sea-level. The change is even much greater for those fishes that come from a depth of several hundred feet. These laws of limitation in space explain many facts in the growth of Coral Reefs that would be otherwise inexplicable, and which I will endeavor to make clear to my readers. For a long time it was supposed that the Coral animals inhabited very deep waters, for they were sometimes brought up on sounding-lines from a depth of many hundreds or even thousands of feet, and it was taken for granted that they must have had their home where they were found; but the facts recently ascertained respecting the subsidence of ocean-bottoms have shown that the foundation of a Coral wall may have sunk far below the place where it was laid, and it is now proved beyond a doubt that no Reef-Building Coral can thrive at a depth of more than fifteen fathoms, though Corals of other kinds occur far lower, and that the dead Reef-Corals sometimes brought to the surface from much greater depths are only broken fragments of some Reef that has subsided with the bottom on which it was growing. But though fifteen fathoms is the maximum depth at which any Reef-Builder can prosper, there are many which will not sustain even that degree of pressure, and this fact has, as we shall see, an important influence on the structure of the Reef.